ADU glossary

Accessory dwelling unit. A second, complete home on a lot that already has one, with its own kitchen, bathroom and entrance.

An accessory dwelling unit is a second, self-contained home on a lot that already has a house on it. The test that matters to every agency you will deal with is whether it has its own kitchen, its own bathroom and its own entrance. A spare bedroom with a mini fridge is not an ADU. A converted basement with a stove, a sink, a bathroom and a door to the outside is. The word "accessory" is a zoning term, not a description of size or quality: it means the unit is subordinate to the main house on the same lot, not that it is a lesser building.

Attached accessory dwelling unit. An ADU inside or joined to the main house, such as a basement or over-garage apartment.

An attached accessory dwelling unit shares structure with the main house. A basement apartment, a converted upper floor, an apartment over an attached garage. In King County the attached test is strict: the unit must share at least one common wall with the primary dwelling and appear to be contained within one structure. That distinction is not cosmetic, because in rural King County an attached unit is often allowed where a detached one is not.

Detached accessory dwelling unit, also called a backyard cottage. A free-standing ADU in its own building.

A detached accessory dwelling unit stands on its own, separate from the house. Backyard cottage is the same thing in plain English, and Seattle uses both terms. Seattle City Light also treats a tiny house on a permanent foundation as a DADU. Detached units carry costs that attached ones do not: a separate foundation, a separate roof, a trench for power and water, and in most cases their own grounding electrode system and building disconnect.

A King County use with no kitchen. Not a dwelling unit, but it still counts against your septic design.

Accessory living quarters, abbreviated ALQ, is a distinct King County land use: living space with no kitchen facilities, not used as a separate dwelling unit. One per lot, capped at 1,000 square feet of heated floor area, prohibited in the Forest zone. People sometimes reach for an ALQ to sidestep ADU rules. It works for zoning, but not for Public Health: an ALQ still has its bedrooms counted against your on-site sewage system design, and an ALQ with no bedroom is still deemed equivalent to one bedroom. Source: KCC 21A.08.030.B.7.b and KCC Board of Health 13.28.030.M.

On a permanent foundation it is a DADU. On wheels it is not a permanent residence in Seattle.

A tiny house on a permanent foundation is treated as a detached accessory dwelling unit and has to meet the same code as any other one. A tiny house on wheels is not a dwelling unit and Seattle does not permit it as a permanent residence. In rural King County there is a further trap: Public Health has stated that tiny homes with holding tanks may not discharge waste from the holding tank into a septic system, which removes the workaround most people have in mind.

Floor area ratio. Total building floor area divided by lot area. It caps how much building the lot may hold.

Floor area ratio is the total chargeable floor area of everything on the lot divided by the area of the lot. A 5,000 square foot lot at 0.6 FAR allows 3,000 square feet of building in total, across every structure. It is the single number that decides how much house you are allowed, and it is separate from lot coverage, which limits footprint rather than floor area. The reason FAR matters so much to an ADU in Seattle is that the ADU draws from the same allowance as the house, so a 1,000 square foot cottage can consume a third or more of everything the lot is permitted. A pre-adoption draft of the 2026 code exempted ADU floor area and is still circulating; the signed ordinance does not, and both SDCI and the Office of Planning state that ADUs count toward density, FAR and lot coverage.

The share of the lot that buildings may sit on, measured as footprint rather than floor area.

Lot coverage limits footprint: the outline of the buildings pressed flat onto the lot, as a percentage of lot area. Seattle Neighborhood Residential zones cap it at 50 percent for most development. It is a different constraint from floor area ratio, and a project can pass one and fail the other. A two-story cottage uses the same lot coverage as a one-story cottage of the same footprint but twice the floor area ratio.

The measured size of the unit. What counts and what is excluded varies by jurisdiction, and the exclusions are worth money.

Gross floor area is how a jurisdiction measures the size of your unit against the size cap. The exclusions matter more than the number. State law defines it as interior habitable area including basements and attics but excluding a garage or accessory structure. Seattle additionally excludes up to 250 square feet of attached garage, all underground stories, and 35 square feet dedicated to long-term bicycle parking. King County measures heated and unheated floor area separately, and for a detached unit does not count basement floor area at all. Design to the exclusions and you can gain real living space without breaking the cap.

The minimum distance a structure must sit from a property line.

A setback is the strip along a property line that has to stay clear of structure. Seattle allows a detached ADU in the rear yard no closer than 5 feet to a lot line that does not abut an alley, and zero feet where it does abut an alley. Rural King County setbacks are driven by the zone rather than by the ADU: typically 30 feet from the street and 5 or 10 feet from interior lines, plus a 100 foot setback from any property line adjoining Agricultural, Mineral or Forest zoning.

Built lawfully under old rules, but not compliant with current ones. It may keep its old position.

A legally nonconforming structure was built to the rules of its day and no longer complies with current rules, usually setbacks or coverage. It is not illegal. This matters enormously for garage conversions, because Seattle allows an existing accessory structure to become a DADU notwithstanding current lot coverage and setback provisions. A garage sitting three feet off the line can often become a cottage in that position, where a new building on the same spot could not be permitted at all. In rural King County that relief is not available: the conversion allowance is written into the urban subsections only.

Seattle's zoning family for lower-density residential areas, replacing what used to be called Single Family.

Neighborhood Residential, abbreviated NR, is Seattle's zoning classification for its lower-density residential areas, renamed from Single Family. Most of the city's backyard cottage stock sits in NR-1, NR-2 or NR-3. The One Seattle Plan rewrote these zones substantially, changing height, density and floor area ratio together. If you are reading guidance that says Single Family 5000 or SF 7200, it predates the change.

Seattle zoning for small apartment and townhouse scale development, denser than Neighborhood Residential.

Lowrise, abbreviated LR, covers townhouse, rowhouse and small apartment development. Two things follow for ADUs. Height for an ADU in an LR zone is set by the rowhouse and townhouse standard for that zone rather than by the Neighborhood Residential rule. And the narrow 1,500 square foot ADU allowance is available only in LR zones, and only in combination with two other conditions.

King County Rural Area zoning: RA-2.5, RA-5, RA-10, RA-20. The number is not the minimum lot size.

RA stands for Rural Area, and the number after it describes the intended density, not the minimum lot area. This is the single most expensive misreading in rural King County. RA-5 has a minimum lot area of 3.75 acres, not 5. RA-2.5 is 1.875 acres. RA-10 is 7.5 acres and RA-20 is 15 acres. Because a rural detached ADU requires the lot to meet the minimum lot area for its zone, a great many perfectly legal RA-5 building lots of, say, 3 acres can never have a detached cottage. Only an attached one.

The smallest lot a zone will create. In rural King County it also gates whether you may build a detached ADU.

Minimum lot area is the floor size for creating a new lot in a zone. It normally has nothing to do with what you may build on an existing lot. Rural King County is the exception: a detached ADU outside a rural town is allowed only where the lot meets the minimum lot area for the applicable zone. So the subdivision standard becomes a construction test, and an older, smaller, entirely legal parcel fails it.

The number of dwelling units a lot may hold. In Seattle, ADUs now count toward that number.

Density is the cap on how many dwelling units a lot may contain. In Seattle's Neighborhood Residential zones the base is one unit per 1,250 square feet for attached and detached units and one per 600 for stacked flats. The headline four-unit and six-unit figures carry lot size conditions that run the opposite way to expectation: four units only on a lot smaller than 5,000 square feet, and six only on a lot smaller than 7,500, and then only within a quarter mile walking distance of a major transit stop or, beyond it, with at least two principal units as low-income units under an instrument the City can enforce. Seattle counts ADUs toward the number, so two detached cottages use two of your slots.

The boundary inside which urban development is directed. Most state ADU protections apply only inside it.

The urban growth area is the line drawn under the Growth Management Act separating land intended for urban development from rural and resource land. It is the most important line on the map for an ADU, because the state ADU law applies only inside it. Outside the urban growth area you are governed by county rules alone, sewer is generally prohibited, and the state guarantees on unit count, size and height simply do not reach you.

A defined transit location that switches off parking requirements within half a mile and can raise allowed density.

A major transit stop is a defined category under the Growth Management Act that includes rail and ferry terminals and bus service running at 15 minute headways or better for at least five hours during weekday peak periods. Proximity to one changes several rules at once: no off-street parking may be required for an ADU within half a mile walking distance, and in Seattle a quarter mile radius raises allowed density. Whether your address qualifies is worth confirming rather than assuming, because the definition turns on scheduled frequency.

Splitting a developed site into separately owned lots for each building. Seattle does not allow it to separate an ADU.

A unit lot subdivision carves a developed site into individually owned lots, one per building, so that each can be sold in fee simple. It is how townhouse rows are sold separately. Seattle does not permit it as a way to separate an accessory dwelling unit from the house: the code requires the ADU to remain on the same lot as the principal dwelling, and SDCI states an ADU may not be separated from the principal unit through subdivision. The available route to separate ownership is a condominium, which divides the buildings without dividing the land.

Selling the ADU separately by recording a condominium regime over the lot instead of splitting the land.

A condominium divides ownership of buildings and airspace while the land stays a single lot held in common. State law bars jurisdictions from prohibiting the sale of an ADU condominium unit independently of the principal unit, and Seattle's code carries the same language. It is a title exercise, not a permit: you record a declaration and a survey map under the Washington Uniform Common Interest Ownership Act. It also creates a common interest community with a declaration, a budget, a reserve study and governance obligations, and condominium lending rules then apply to both units. Price the conversion before you build a project whose economics depend on it.

A charge for public facilities needed by growth: streets, parks, schools, fire. Capped at half rate for ADUs.

An impact fee is a Growth Management Act charge collected by a city or county for public facilities that new growth requires. State law defines the category by a closed list: public streets and multimodal facilities, publicly owned parks, open space and recreation, school facilities, and fire protection facilities. Impact fees on an ADU may not exceed 50 percent of what the principal unit would pay. Water and sewer are not on that list and the cap binds only cities and counties, which is why the 50 percent rule does not touch a utility system development charge, a district general facilities charge, a water tap fee, or the King County capacity charge. Source: RCW 36.70A.681(1)(a) and RCW 82.02.090.

A ministerial Seattle permit decision made by staff, with no public comment period and no hearing.

A Type I decision is made administratively by SDCI staff against fixed standards, rather than through a discretionary review with public participation. For a backyard cottage this is good news: SDCI has stated there is no public comment period and no appeal opportunity to the Hearing Examiner or City Council for a DADU. A neighbor who objects has no procedural route to stop it. It also means the standards are applied literally, because the reviewer has no discretion to waive them.

A mapped hazard or habitat area. Its presence removes your state-law ADU protections entirely.

Environmentally critical area, abbreviated ECA, is Seattle's term for the mapped categories that trigger extra review: landslide-prone areas, steep slope erosion hazard areas, liquefaction-prone areas, peat settlement-prone areas, abandoned landfills, flood-prone areas, wetlands and riparian corridors. The consequence people miss is legal rather than technical. RCW 36.70A.681(4) says the state ADU provisions do not apply to lots designated with critical areas or their buffers. Your two-ADU right, the 1,000 square foot minimum size allowance and the 24 foot minimum height allowance all disappear as a matter of state law. Seattle also reduces allowed density proportionally on such lots, and disqualifies them from the pre-approved plan fast track.

King County's equivalent term: wetlands, streams, steep slopes, aquifer recharge areas and their buffers.

Critical areas in King County include wetlands, aquatic areas and streams, steep slopes and landslide hazards, and critical aquifer recharge areas, each with a buffer. Buffers outside the urban growth area are wider than inside it: 200 feet on a Type S or F water rurally against 180 feet urban, and buffers double where they overlap a steep slope or landslide hazard. Structures must also sit 10 feet back from the outside edge of any critical area or buffer. Where no geological report is submitted for a steep slope, the buffer defaults to 50 feet.

A protective strip around a critical area where building is restricted. It is part of the critical area for legal purposes.

A buffer is the protected margin around a wetland, stream or slope. Two points are easy to miss. First, a buffer counts as designated critical area for the purpose of the state law carve-out, so a lot can lose its state ADU protections without containing a wetland at all, simply by being within someone else's wetland buffer. Second, King County requires structures to sit an additional 10 feet outside the buffer, so the effective exclusion is the buffer plus ten.

Diameter at standard height. How Seattle measures a tree trunk to decide which protection tier it falls into.

DSH is the trunk diameter measured at a standard height above ground, and it is the number that assigns a tree to a protection tier in Seattle. Tier 2 is 24 inches DSH and above, plus tree groves and certain listed species below that size. Tier 3 is 12 inches up to 24. Tier 4 is 6 inches up to 12. Tier 1 is heritage trees regardless of size. Measure your trees before you draw anything, because the tier decides both whether the tree can come out and how much unbuildable ground it generates.

A circle around a protected tree, one foot of radius per inch of trunk diameter. In Seattle NR zones it is not buildable area.

The tree protection area is a circle centered on the trunk with a radius of one foot for every inch of trunk diameter. A 30 inch tree therefore generates a 30 foot radius, a circle roughly 60 feet across. In Seattle Neighborhood Residential zones that circle is not counted as buildable area, and neither is any strip less than 10 feet wide left between a protection area and a setback. Where there are several trees the areas are analyzed in combination rather than one at a time. On an ordinary 40 by 100 foot back yard, one mature conifer plus the narrow-strip rule can eliminate the entire building envelope, and you cannot simply move the cottage elsewhere because the excluded ground stays excluded.

Ground that water cannot soak through: roofs, driveways, patios. Rural King County caps it as a percentage of the lot.

Impervious surface is anything that stops rain reaching the soil, principally roofs and paving. In rural King County it is capped as a percentage of lot area: RA-2.5 25 percent, RA-5 20, RA-10 15, RA-20 12.5. There is also a floor that often decides the answer, since lots of half an acre or more are allowed at least 10,000 square feet regardless of the percentage, which on a conforming rural lot is usually generous rather than binding. The much tighter 8 percent figure people quote is narrower than its reputation: it applies to subdivisions and short subdivisions in R-1 and RA zones within named subbasins of Issaquah Creek and the Patterson Creek portion of Grand Ridge. Exceeding whichever cap applies requires a conditional use permit.

Stormwater engineering triggered by area thresholds. In Seattle 750 square feet is usually enough to trigger it.

Drainage review is the stormwater engineering step, and it adds design cost, a physical stormwater facility and review time. Seattle triggers it at more than 750 square feet of new or replaced hard surface, more than 750 square feet of building measured by roof outline, more than 5,000 square feet of land disturbance, or any grading permit. Most real cottages clear that comfortably once you add a walkway. Unincorporated King County triggers at 2,000 square feet of new impervious surface. Seattle's 2026 Stormwater Code and Manual took effect on 1 July 2026, so plans priced under the 2021 rules need repricing.

A septic system. Abbreviated OSS in King County code, and the usual reason a rural ADU cannot be built.

On-site sewage system, abbreviated OSS, is the code term for a septic system. Connecting an ADU to a system serving a single-family residence may be permitted only where public health and groundwater quality are not affected and the system is designed for the anticipated increased flow. Every bedroom in the ADU is added to the bedroom count the system must serve, and an ADU with no bedroom is deemed equivalent to one bedroom. Source: KCC Board of Health 13.28.030.M.

The buried trenches that disperse treated effluent into soil. Called the soil dispersal component in code.

The drainfield, called the subsurface soil absorption system or soil dispersal component in King County code, is the network of buried trenches that releases treated effluent into the soil. Its size is driven by design flow in gallons per day and by how fast your soil accepts water. It carries most of the setbacks that constrain a rural site: 100 feet from a private well, 100 feet from surface water, 200 feet from a drinking water spring, 10 feet from a property or easement line.

A second, undeveloped drainfield location held in reserve. It must equal 100 percent of the primary, and older sites often lack one.

A reserve area is ground set aside, tested and kept clear so a replacement drainfield can be built when the first one fails. King County requires it to equal at least 100 percent of the primary drainfield area, with at least two soil log excavations in each designated reserve. Public Health also requires that a conforming reserve area be established for the existing house if one is not already there. This is the quiet killer on rural ADU projects: the house was permitted decades ago without one, the ADU application is what forces the issue, and once you subtract the well setback, the surface water setback and the driveway there is often nowhere left for it to go. No reserve area, no ADU.

The reviewed as-built of your existing septic system. Public Health requires one on file before approving an ADU.

A record drawing is the as-built plan of the installed septic system, reviewed and held on file by Public Health. Before approving an ADU connection, Public Health requires a reviewed record drawing for the existing house plus a satisfactory operation and maintenance inspection less than a year old performed by a certified maintainer. Many systems installed before the 1990s have no as-built on file at all. Establishing what is actually in the ground then means investigation, sometimes excavation, and occasionally full replacement, all before the cottage has a drawing of its own.

Gallons per day a septic system is sized to handle, calculated per bedroom rather than per person.

Design flow is the daily volume, in gallons, that a septic system is sized for. King County calculates it at 150 gallons per bedroom per day, with a minimum of three bedrooms for a single-family dwelling. An ADU sharing the house system adds 150 gallons per bedroom. An ADU on its own separate system is sized for a minimum of two bedrooms, which is 300 gallons per day, even if the unit is a studio. That difference in architecture, shared system against independent system, changes how much drainfield you have to build.

For septic sizing, a room with a window, a door and a closet. Your office may qualify whether you call it one or not.

King County Board of Health defines a bedroom as a room used for sleeping that includes a window, a door and a closet, excluding rooms smaller than 70 square feet. Public Health has stated that dens, offices and accessory rooms that are not common living areas may be counted as a bedroom if the room has a window for ingress and egress and a closet. Design flow is then calculated at a maximum of two people per bedroom. The practical effect is that naming a room an office on the plans does not reduce your septic sizing if the room has a window and a closet.

A small shared water system serving a limited, approved number of connections. An ADU consumes one of them.

A Group B public water system serves a small number of connections, typically a handful of rural households sharing a well. The approval letter specifies how many connections are allowed. Public Health treats an ADU as an additional water system connection to the property, so if your system's allotted connections are already used, adding the cottage means engaging a professional engineer and filing a design application with the Department of Health to expand the system. That is a separate project on its own timeline.

A small domestic well drilled without a water right. Since the Hirst decision it comes with fees and recorded restrictions.

A permit-exempt well is a small domestic well drawing groundwater without an individual water right, allowed under RCW 90.44.050. After the Washington Supreme Court decision known as Hirst and the legislative response in 2018, new connections relying on permit-exempt wells in affected watersheds carry conditions. King County has been collecting a fee and recording conditions on the title that limit the property to a stated daily volume per connection, require on-site stormwater infiltration and require connection to public water if it ever becomes available, with removal of those conditions only through rules adopted by Ecology. Whether an ADU served by a well drilled before the relevant cut-off triggers any of this is genuinely ambiguous in the published documents. Get a written determination rather than an opinion.

Water Resource Inventory Area. Washington's watershed numbering. Which one you are in changes the well rules.

A Water Resource Inventory Area is one of Washington's numbered watersheds, and rural water rules are written per watershed. King County spans several. WRIA 7 is the Snohomish and WRIA 8 the Cedar-Sammamish; new watershed rules for both, WAC 173-507 and 173-508, took effect on 27 June 2026. WRIA 9, the Duwamish-Green, and WRIA 10, the Puyallup-White, were not part of that rulemaking and have no watershed rule implementing the 2018 legislation at all; they are governed directly by statute plus instream resource protection programs dating from the 1980s. Advice written before mid-2026 may no longer describe your watershed.

The private pipe from your building to the public sewer main. You own it and you maintain it.

The side sewer is the privately owned pipe running from your building to the public main in the street. A cottage may often connect to the existing side sewer rather than getting its own, which saves real money. It also creates an ownership problem worth thinking about: a single private pipe serving two units is the owner's maintenance responsibility, a backup affects both units at once, and the landlord habitability duty under RCW 59.18.060 runs to the tenant in the second unit while you deal with it.

A King County wastewater charge on new connections, billed to the property owner quarterly for 15 years.

The King County capacity charge is a growth-cost recovery charge, not a metered utility bill. It applies to properties whose sewage flows to a King County treatment plant and which connect on or after 1 February 1990, and it is triggered by new sewer service, reconnections after five years, additions and changes of use. It is billed quarterly for 15 years and is expressly the responsibility of the current property owner. An ADU is assessed at 0.59 residential customer equivalents, which at the 2026 rate of $77.99 per RCE per month works out at about $46 a month, roughly $8,300 over the full term. A lump sum payoff at a discount is available on request. It is enforced under RCW 35.58.570 through the water-sewer district lien machinery in RCW 57.08.081.

Residential customer equivalent. King County's unit for the capacity charge. An ADU counts as 0.59.

A residential customer equivalent is King County's standardised unit of wastewater demand. A single-family house between 1,500 and 2,999 square feet is 1.00; under 1,500 square feet is 0.81; 3,000 square feet and over is 1.16. An accessory dwelling unit, attached or detached, is 0.59, and a house plus an ADU together is 1.62. The RCE figure is what the monthly capacity charge is multiplied by, so it is the number that turns a cottage into a fifteen year obligation.

Seattle Public Utilities' connection charge, assessed by meter size rather than per dwelling unit.

A system development charge is what Seattle Public Utilities charges for buying into existing water and wastewater capacity. The structural point is that it is assessed by meter size, not per dwelling unit, and the water charge is triggered by a development that requires a new or larger water connection. Share the existing meter without upsizing and the water and wastewater charges are not triggered. Add a second meter and they are: on the 2026 schedule a three-quarter inch meter carries $6,900 of water and $2,600 of wastewater charge, $9,500 in total, plus a drainage charge of $1,225 per 1,000 square feet of new hard surface. Wastewater and drainage charges in this form began on 1 January 2026, so budgets built on 2025 figures are materially wrong.

A water or sewer district's connection charge. Often assessed per structure or per unit, even on a shared meter.

A general facilities charge is the district equivalent of Seattle's system development charge, and the difference in how it is assessed costs real money. Districts commonly charge per structure or per equivalent residential unit rather than per meter size, which means Seattle's share-the-meter-and-skip-the-charge logic does not carry across. Covington Water District charges a detached ADU the full future facilities and existing capital facilities charge per structure even on a shared meter, while waiving it for an attached unit unless the meter needs upsizing. King County Water District 90 assesses an ADU up to 1,000 square feet at one full equivalent residential unit. Cedar River charges 50 percent of one ERU. Ask your district before you assume anything.

Equivalent residential unit. A water or sewer district's standard unit of demand, used to size connection charges.

An equivalent residential unit is the demand of one typical single-family household, used by water and sewer districts as the unit for general facilities charges. Districts differ sharply in how they rate an ADU: King County Water District 90 has assessed an ADU up to 1,000 square feet at a full ERU, while Cedar River Water and Sewer has charged ADUs built after October 2019 at 50 percent of one ERU. The word is doing the same job as King County's residential customer equivalent but the numbers are set independently.

A claim attaching to the property itself for unpaid utility charges, regardless of whose name is on the account.

Washington's municipal utility statutes create liens against the premises rather than against the customer, which is why a tenant's name on the account does not by itself protect the owner. City water and municipal electric carry a lien capped at four months of charges under RCW 35.21.290, extended after a governor's emergency declaration. City sewer under RCW 35.67.200 is superior to all other liens and encumbrances except general taxes and local and special assessments, meaning it outranks a mortgage; RCW 35.67.210 makes it automatically effective for up to six months of delinquent charges and RCW 35.67.215 lets a city extend that to a year by ordinance. Water-sewer district charges are a lien subject only to general taxes under RCW 57.08.081, with no stated time limit. An investor-owned utility such as PSE is outside all of these statutes and has no lien at all.

A recorded obligation that binds every future owner of the property, not just the person who signed it.

An obligation that runs with the land is recorded against the title and binds whoever owns the property afterwards, whether or not they agreed to it or knew about it. Several ADU obligations work this way: King County's accessory-unit notice on title, the water use conditions recorded for new permit-exempt well connections, and the covenant acknowledging the absence of fire hydrants required for some rural fire-flow exemptions. It is also why buyers should read what is recorded rather than relying on the seller's description.

A King County document recorded against your property identifying the unit as accessory. It runs with the land.

King County requires an ADU applicant to file a notice, on a form approved by the Records and Licensing Services Division, identifying the dwelling unit as accessory. It is recorded against the title, it runs with the land, and proof of filing has to be submitted before the department approves any permit, so it happens at the front of the process rather than the end. The county form states the notice shall not be removed except upon specific written authorization by King County recorded upon the title, so it is not something you clear by finishing the project or by selling. Practically it is a disclosure device: it tells every future owner, lender, title company and appraiser that the second unit on this parcel is legally accessory to the first and cannot be treated as an independent house. Source: KCC 21A.08.030.B.7.a.(5).

A private meter you install downstream of the utility meter, to bill a tenant for actual use. It transfers cost, not liability.

A submeter is your own meter, installed after the utility's meter, measuring what the second unit actually uses so you can bill the tenant fairly. It is legal in Washington and it costs a few hundred dollars rather than tens of thousands. What it does not do is change anything about the utility's claim: the utility's customer is still you, and its lien against your property is untouched. Submetering answers the fairness question, not the liability question. Seattle's third party billing ordinance regulates the practice at three or more units, so a house plus one ADU sits below the threshold.

Ratio utility billing: allocating one bill by formula with no measurement. Legal today, but under active attack.

Ratio utility billing system, abbreviated RUBS, allocates a master utility bill among units by formula, typically by occupant count, square footage or bedroom count, with no measurement of actual use. It is legal in Washington today. Two risks are worth weighing before you build a rental around it. The Seattle Renters' Commission has formally recommended banning it outright and folding unmetered utilities into base rent. And under the statewide rent increase cap in HB 1217, whether variable utility charges paid directly to a landlord count as rent has been characterized as an unsettled area, which would mean a pass-through could later be counted against the cap.

The physical supply from the utility: the drop or lateral, service conductors, meter base and main disconnect.

The service is the physical supply from the utility to the property: the overhead drop or underground lateral, the service entrance conductors, the meter base and the main disconnect. It is not the same thing as a meter, and the distinction decides the budget. Two meters can sit behind one service. What matters for sizing is that the load calculation follows the service, so where both dwellings are supplied from one service, that service has to carry both however the billing is arranged.

The utility's measuring device, and the basis for billing. Not the same thing as a service.

A meter measures energy for billing. A service delivers it. You can have one service and two meters, which is the usual arrangement for a house plus a cottage. PSE will run a single service line to an enclosure holding two meters where both are on one parcel billed to the same owner. Seattle City Light requires at least one meter for each legal dwelling unit on a multi-use service. Where an accessory unit has no meter of its own it has no account of its own, so its energy is billed to whoever holds the account for the property.

The conductors carrying power from the main panel to a subpanel or a second building, downstream of the service.

A feeder is the set of conductors running from the service equipment to a downstream panel. Feeding a detached cottage from the house is the feeder route, as against giving the cottage its own utility service. NEC 225.30 allows only one feeder or branch circuit to supply an additional building, with narrow exceptions that do not include "the cottage needs more power." A feeder to a dwelling is four-wire: two ungrounded conductors, a neutral, and an equipment grounding conductor.

A downstream panelboard fed from the main panel. At a second building it must be more than an ordinary subpanel.

A subpanel is a panelboard fed by a feeder rather than directly by the service. At a detached dwelling an ordinary main-lug-only subpanel is not sufficient, because it provides no disconnecting means at all. The building disconnecting means has to be rated not less than 100 amperes, 3-wire, for a one-family dwelling under NEC 225.39(C), and NEC 225.36 lists what qualifies: a circuit breaker, molded case switch, general-use switch, snap switch, or other approved means. It does not generally have to be suitable for use as service equipment; 225.36 requires that only where the installation relies on the 250.32(B)(1) exception, the legacy case where no equipment grounding conductor was run. Under WAC 296-46B-225 it also needs an identification plate in half inch lettering naming the building it serves and identifying itself as that building's main disconnect.

The wire that carries fault current back to the source. It must be run with the feeder to a detached building.

The equipment grounding conductor is the dedicated fault-current path bonding metal enclosures back to the source. At a second building fed by a feeder, NEC 250.32(B)(1) requires it to be run with the supply conductors and connected to the building disconnecting means and to the grounding electrodes there, sized to 250.122. The same paragraph then says any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrodes. In plain terms, the neutral floats at the cottage. The old three-wire feeder that used the neutral as the fault path survives only as an exception for certain pre-2008 installations. There is no three-wire feeder to a new cottage.

The connection to earth at a building. A detached dwelling needs its own, and Washington adds two requirements.

The grounding electrode system is the set of connections to earth at a building: ground rods, metal water piping, and reinforcing steel in the foundation. Under NEC 250.32(A) a building supplied by a feeder or branch circuit needs a grounding electrode system and a grounding electrode conductor installed to Part III of Article 250. The one exception is where a single branch circuit, including a multiwire branch circuit, supplies the building and includes an equipment grounding conductor. Washington adds a rule of its own in WAC 296-46B-250: a concrete-encased electrode, often called a Ufer ground, is mandatory at a new building on a permanent concrete foundation, with narrow exceptions, so the connection has to be coordinated with the pour rather than added later. Washington also requires rod, pipe or plate electrodes of structurally separate adjacent buildings to sit at least six feet apart, but that rule is written to services, so read it against your own installation before applying it to a feeder-fed cottage.

The NEC method for sizing a service or feeder from the actual loads, with demand factors applied.

A load calculation sizes a service or feeder by applying the National Electrical Code's demand factors to the connected loads, rather than by adding up nameplates. It is the difference between an accurate answer and an expensive guess. Article 220 offers a standard method and an optional method for dwellings, and a separate method for existing dwellings taking on new load. Two dwelling units on one service are calculated by their own method with their own demand factors, so the combined figure is not the house plus the cottage added together, and it needs running properly before anyone concludes the existing service is too small.

NEC 220.82. The simplified dwelling calculation: 100 percent of the first 10 kVA, 40 percent of the rest, plus heating or cooling.

NEC 220.82 is the optional calculation for a dwelling unit whose total connected load is served by a single 120/240 volt or 208Y/120 volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater. It takes 100 percent of the first 10 kVA of general load and 40 percent of the remainder, then adds a heating and air conditioning component which is the largest of six listed selections, the familiar ones being 100 percent of air conditioning, 100 percent of a heat pump without supplemental heat, and 65 percent for various supplemental and space heating arrangements. A cottage on its own service is a dwelling unit in its own right, so this is the method that sizes it, and most small units land at the 100 amp minimum service size required by NEC 230.79(C).

A device that limits total current so a smaller service can serve more load. Seattle will not let you use one to size a new service.

An energy management system, and its electric-vehicle-specific relative sometimes called EVEMS, limits the current a feeder or service can draw so that a smaller supply can serve more equipment. Under NEC 220.70 the maximum setpoint may be used as a single value in load calculations, which is often the cheapest alternative to a service upgrade. Seattle adds a sentence that is not in the national code: for new service installations, no branch circuit, feeder or service load calculation required under Article 220 may be reduced by an energy management system. So in Seattle the device can save an existing service but cannot shrink a new one. Outside Seattle the unamended rule applies. Note the 2026 code rebuilds this: energy management moves to Article 130, and the load calculation route becomes 120.7, a Power Control System set to no more than 80 percent of the rating of the overcurrent device it monitors.

Whole-building surge protection, required on services and on feeders supplying dwelling units since the 2023 code.

A surge protective device clamps voltage transients before they reach the equipment downstream. The 2023 code requires one on all services supplying dwelling units under NEC 230.67, and, in the change most often missed on detached cottages, on feeders supplying dwelling units under NEC 225.42 and NEC 215.18. A backyard cottage fed from the house is a dwelling unit supplied by a feeder, so it needs its own. It must be Type 1 or Type 2, and both 215.18(E) and 230.67(E) call for a nominal discharge current of not less than 10 kA.

An outdoor disconnect for firefighters, marked in white on red in half inch letters. Feeder-fed cottages need one.

The emergency disconnect is an outdoor, readily accessible means of killing power to a dwelling, intended for first responders. NEC 230.85 applies it to dwellings supplied by a service and NEC 225.41, new in the 2023 code, applies it to one- and two-family dwellings supplied by a feeder or branch circuit. It has to be on or within sight of the dwelling, rated for the available fault current, grouped if there is more than one, and marked EMERGENCY DISCONNECT in white text on a red background in letters at least half an inch high on the outside front of the enclosure.

The National Electrical Code, NFPA 70. Washington is on the 2023 edition; the 2026 edition replaces it on 31 December 2026.

The National Electrical Code, published as NFPA 70, is the model electrical standard adopted by reference in each state. Washington adopts it in WAC 296-46B-010 and is on the 2023 edition, with the 2026 edition replacing it on 31 December 2026. The 2026 edition moves load calculations out of Chapter 2: Article 220 becomes Article 120, the dwelling minimum unit load drops from 3 volt-amperes per square foot to 2 at the new 120.41, and energy management systems get their own Article 130 with the calculation side at 120.7. Articles 225 and 250 keep their numbers, so the rules for feeding a detached dwelling do not move. Seattle is expected to remain on its own 2023-based code past that date, so the two jurisdictions will diverge.

Seattle's own amended electrical code. The 2023 edition took effect 15 November 2024 and SDCI inspects to it.

Seattle writes and enforces its own electrical code rather than deferring to the state. The 2023 Seattle Electrical Code, adopted by Ordinance 127008, took effect on 15 November 2024 and is based on the 2023 National Electrical Code with local amendments folded in. SDCI issues the permits and performs the inspections. Several amendments bite on ADUs specifically: branch circuits in a dwelling unit may supply only loads within that unit, shared well or septic circuits may not be fed from an accessory dwelling unit, and load calculations for new services may not be reduced by an energy management system.

Authority having jurisdiction. The inspector or agency whose reading of the code is the one that counts.

The authority having jurisdiction is whoever actually enforces the code on your job: SDCI in Seattle, Washington Labor and Industries in unincorporated King County and most of the surrounding cities, the fire marshal for access and sprinklers, Public Health for septic and plumbing. Where a code section is genuinely ambiguous, the AHJ decides, and their decision is the operative answer regardless of what any published guide says. Getting an interpretation in writing before design is finished is far cheaper than getting one at inspection.

A 40 amp, 208 or 240 volt circuit run to a parking space and terminated ready for a charger.

EV-ready in the Seattle Electrical Code means a minimum 40 ampere dedicated 208 or 240 volt branch circuit, giving 32 amps of output, terminated at a junction box or receptacle close to a parking space. When a new building is constructed, at least one parking space for each dwelling unit in a private garage, carport or surface parking area has to be EV-ready. There is a relief valve: where the added load would require on-property power transformation or an upgrade to an existing residential service, the Director may reduce the requirement to a 20 amp circuit, and if necessary reduce the number of spaces.

Seattle's Rental Registration and Inspection Ordinance. Rented ADUs must register, and the penalties compound daily.

The Rental Registration and Inspection Ordinance, SMC 22.214, requires rental housing in Seattle to be registered and periodically inspected, and rented accessory dwelling units are explicitly covered. Registration on the 2026 schedule is about $126 for the first unit and $31.50 for each additional, valid two years, with a city inspection at about $241.50 for the first unit. The first inspection comes within five years of registration. The reason to take it seriously is the penalty structure: $150 per day for the first ten days and $500 per day after that. SDCI may also block a rent increase if it determines the unit fails RRIO standards.

Seattle rule requiring a listed legal reason to end a tenancy. You cannot simply decline to renew.

Seattle's just cause eviction ordinance requires a landlord to have one of a defined list of reasons to end a tenancy. Letting a lease lapse because you would rather have the space back is not on the list by itself. The ADU-specific provisions carried into the current code include relocation assistance of $2,000 or two months' rent, depending on tenant income, where an owner terminates an ADU tenancy after receiving a development standard violation. Note also that the ADU-specific just cause provisions are written around a permitted accessory dwelling unit, which can make an unpermitted unit's tenant harder rather than easier to remove.

Seattle rule requiring you to rent to the first applicant who meets your stated criteria.

Seattle's first-in-time rule requires a landlord to offer the unit to the first applicant who meets the screening criteria the landlord published, rather than choosing among qualified applicants. It was challenged and upheld by the Washington Supreme Court. For a large building it is an administrative rule. For a cottage twenty feet from your kitchen window, occupied by someone who shares your driveway, it is a genuinely different proposition, and it is worth understanding before you build rather than when the first application arrives.

Economic displacement relocation assistance. A rent increase of 10 percent or more can trigger a payment to the tenant.

Economic displacement relocation assistance is Seattle's rule that a rent increase at or above a threshold, reported as 10 percent, entitles a qualifying tenant who chooses to move to relocation assistance of three months' rent, advanced to the tenant and billed back to the landlord. It sits alongside Seattle's 180 day written notice requirement for any rent increase, which in practice means deciding in June what you will charge in January.

A dwelling-fire policy written for a rented building. A standard homeowner policy usually will not cover a rented cottage.

A landlord or dwelling-fire policy is written for a building occupied by a tenant rather than the owner. It matters because Washington's Office of the Insurance Commissioner states plainly that a homeowner policy usually will not provide coverage for other buildings on your property that renters occupy, and that the other structures coverage is typically only about 10 percent of the dwelling limit in any case. On a home insured for $800,000 that is roughly $80,000 of coverage against a cottage that may have cost $400,000 to build, before you reach the occupancy exclusion that may void it entirely. Call your carrier before the tenant moves in, not after.

Insurance covering a structure while it is under construction, plus materials on site. Your homeowner policy does not.

Builder's risk covers a building during construction, along with materials stored on site and theft from the job site. A standard homeowner policy does not cover a structure under construction on your property. The gap opens the day the first delivery arrives, which is usually well before anyone has thought about insurance. Confirm what your carrier requires and what it costs before that delivery, not after.

Revised Code of Washington. State statute, passed by the legislature.

The Revised Code of Washington is the compiled body of state statute. A citation like RCW 36.70A.681 reads as title 36, chapter 70A, section 681. State statute sits above city and county code, which is why the ADU preemption statute can override a local ordinance. Full text is published free at app.leg.wa.gov.

Washington Administrative Code. Agency rules that carry the force of law, written under authority of a statute.

The Washington Administrative Code is the body of rules written by state agencies under authority delegated by statute. It carries the force of law. WAC 296-46B is the electrical chapter, written by Labor and Industries, and it is where Washington adopts the National Electrical Code and amends it. WAC 173-507 and 173-508 are Ecology's watershed rules for two of King County's watersheds.

Seattle Municipal Code. City law. Title 23 is land use, Title 25 covers trees and critical areas.

The Seattle Municipal Code is the city's own body of law. Title 23 is the Land Use Code and contains the ADU standards, now at SMC 23.42.022 after Ordinance 127211 repealed the old SMC 23.44.041. Title 25 contains the environmentally critical areas code at 25.09 and the tree protection code at 25.11. Title 22 contains rental registration and tenant protections. Any guidance citing SMC 23.44.041 is describing a section that no longer exists.

King County Code. Title 21A is zoning; a separate Board of Health Title 13 governs septic.

The King County Code is the county's own law. Title 21A is the zoning code and contains the ADU standards at 21A.08.030.B.7. Title 17 covers fire code and access. Title 16 covers addressing. Septic is governed separately by the King County Board of Health Code, Title 13, which is administered by Public Health rather than by the permitting department. Three different agencies, three different codes, three different clocks.

A single piece of city or county legislation. It amends the code; the code is the standing law.

An ordinance is one act of legislation by a city council or county council. It amends the standing code. This distinction matters when reading ADU guidance, because the current Seattle rules arrived through two ordinances in quick succession: Ordinance 127211, effective 30 June 2025, which moved the ADU rules into SMC 23.42.022, and Ordinance 127376, effective 21 January 2026, which rewrote that section again as part of the One Seattle Plan. Guidance written between them is already out of date.