Zoning decides what you may build. Public Health decides whether the site can handle the wastewater, and that answer comes from a different agency working under its own code and timetable. Very often the answer is no.
The whole of this part deals with unincorporated King County outside public sewer. The governing documents are the King County Board of Health Code, Title 13, and a Public Health technical memorandum for accessory dwelling units that took effect on 9 May 2025 and was updated that June. That memo actually relaxed the old rules in one important respect, so guidance written before mid-2025 is more pessimistic than the current position.
How a septic system gets sized, and why a studio counts as a bedroom
An is sized by bedrooms, not occupants or square footage, converted into in gallons per day. King County works at 150 gallons per bedroom per day, with a minimum of three bedrooms for a single family dwelling.
The rule is in the Board of Health code itself: every bedroom in the accessory unit is added to the total the system must serve, in addition to the bedrooms in the main house, and an accessory unit with no bedroom is deemed equivalent to one bedroom. A studio is sized as a one bedroom unit.
Why an office counts as a bedroom
The Board of Health defines a as a room used for sleeping that has a window, a door and a closet, excluding rooms under 70 square feet. Public Health has gone further and stated that dens, offices and accessory rooms that are not common living areas may be counted as a bedroom where the room has a window for ingress and egress and a closet.
Labeling a room "office" or "study" on the plans does not reduce your septic sizing if the room has a window and a closet. If you genuinely need the smaller number, the design decision is to leave the closet out, not to change the label.
The architecture decision that changes the drainfield
There are three legitimate arrangements. Each is sized differently, so the choice affects cost.
- House and cottage on independent systems, each designed for a single family dwelling. Two applications, two fees.
- House and cottage on one system designed using multi-family criteria. One application, one fee.
- The existing house system expanded to multi-family criteria. One application, one fee.
| Unit size | Sharing the house system | On its own system |
|---|---|---|
| 1 bedroom, or under 600 sq ft | 150 gallons per day, 1,000 gallon tank | 300 gallons per day, 1,000 gallon tank |
| 2 bedrooms | 300 gallons per day, 1,000 gallon tank | 300 gallons per day, 1,000 gallon tank |
| 3 bedrooms | 450 gallons per day, 1,500 gallon tank | 450 gallons per day, 1,500 gallon tank |
| Each additional bedroom | plus 150 gallons per day | plus 120 gallons per day |
The top row shows the difference. A one bedroom cottage sharing the house system is sized at 150 gallons a day. The same cottage on its own independent system is sized for a minimum of two bedrooms, which is 300 gallons a day. The building and occupants are the same, but the design flow doubles. Design flow drives how much you have to build. On a tight site that difference alone can decide whether the project fits.
There is also a route called reallocation, where unused design capacity in the existing system is assigned to the cottage. Public Health allows it, but with conditions: the system must meet current code, floor plans for both units are required, a current inspection and an approved record drawing are required, and the main house must retain a minimum of three bedrooms after any reallocation. So you cannot simply relabel a four bedroom house as a two bedroom to free up capacity.
The reserve area requirement
What a reserve area is, and why it limits rural projects
A is ground set aside, soil tested and kept permanently clear so a replacement drainfield can be built when the first one fails. The Board of Health requires it to equal at least 100 percent of the primary drainfield area, with at least two soil log excavations in each designated reserve area.
Public Health's ADU memo adds the requirement that affects older properties: a conforming reserve area must be established for the single family dwelling if not already present.
The reserve must match the existing drainfield and stay clear. Schematic, not to scale.
That clause affects many older properties. A house permitted in 1978 was very likely permitted without a designated reserve, and the question may not have come up since. The ADU application brings it to the county's attention, and the owner then has to find room for a second full drainfield-sized area that meets every setback, on a parcel where the obvious ground is already occupied by the existing drainfield, the well, the driveway and the house.
| From | Drainfield or reserve | Tank or pump chamber |
|---|---|---|
| Private well | 100 ft | 100 ft |
| Drinking water spring or dug well | 200 ft | 200 ft |
| Surface water | 100 ft | 50 ft |
| Seasonal water | 30 ft | 15 ft |
| Property or easement line | 10 ft | 5 ft |
| Building foundation, down-gradient | 15 ft plus the height of the foundation cut, need not exceed 30 ft | 5 ft |
| Unlined stormwater infiltration pond | 100 ft | 50 ft |
| Interceptor, curtain or footing drains, down-gradient | 30 ft | 5 ft |
As the caption notes, these distances apply to the reserve area exactly as they apply to the working drainfield. The reserve is protected ground, not a notional line on a drawing.
Check two more limits early. Systems are not allowed on slopes over 40 percent, and on slopes over 30 percent the drainfield must use pressure distribution with trenches no wider than two feet. And no part of a system may be built in the FEMA floodway.
The record drawing for the existing system
Before approving anything, Public Health requires two documents about your existing system:
- a for the single family dwelling that has been reviewed by public health, and
- a current, satisfactory operation and maintenance inspection, less than one year old, performed by a certified maintainer, with the report submitted for review.
Plenty of systems installed before the 1990s have no as-built on file at all. If yours is one of them, establishing what is in the ground means investigation and sometimes excavation. Occasionally it turns up a system that is undersized or failing. That becomes a separate project, and it has to be dealt with before the cottage is designed.
The upside is that this is cheap to find out. An inspection by a certified maintainer and a records search at Public Health will tell you within a couple of weeks whether you have a project or not, for a tiny fraction of what a set of plans costs. This is the first step to take. It is the most useful early item on the checklist.
Sewer: available inside the line, prohibited outside it
Inside the , public sewer is the default and septic is permitted only on an interim basis, with a certificate of sewer availability and a recorded certificate of future connection that runs with the land. If sewer cannot serve the cottage, Public Health wants a sewer availability letter saying so in the application package.
Outside the urban growth area, sewer service is prohibited, with narrow exceptions for documented health and safety problems. There is no route where you pay to extend a main out to a rural parcel. One more restriction: modifications to existing structures and lots are not allowed where the modification triggers an expansion of sewage capacity above the original approval of a community system, which generally rules out a cottage on a shared community system.
The well has to pass a yield and quality test
Public Health's ADU guidance sets out what a private well must demonstrate before it can serve a second dwelling:
- A pump test less than five years old. Air tests and bailer tests are not accepted. Minimum two hours, extended to four hours where the well produces under 10 gallons per minute.
- Ability to produce 800 gallons per day, expressed as 400 gallons per day per residence or 10 gallons per minute.
- Current water quality results: bacteria, nitrate under 10 milligrams per liter, and arsenic under 10 parts per billion.
- Rainwater catchment is not a permissible water source for an ADU. A cottage planned around catchment needs another water source.
Shared water systems have a fixed number of connections
If your water comes from a shared with neighbors, Public Health treats the accessory unit as an additional water system connection to the property. The system's approval letter specifies how many connections it is allowed. If they are all in use, adding your 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 someone else's timeline, and it needs the agreement of everyone else on the system. Find out early how many connections your system is approved for.
Wells, water rights and the 2026 watershed rules
A draws groundwater without an individual water right. After the Washington Supreme Court decision usually called Hirst and the legislature's response, new connections relying on permit-exempt wells in affected watersheds carry conditions. King County has been collecting a fee reported at $500 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. Those conditions and cannot be removed until authorized through rules adopted by Ecology.
New watershed rules took effect on 27 June 2026 for 7, the Snohomish, and WRIA 8, the Cedar-Sammamish, under WAC 173-507 and 173-508. WRIA 9, the Duwamish-Green, and WRIA 10, the Puyallup-White, remain under the earlier interim regime. Anyone relying on advice about interim post-Hirst rules needs to check which watershed they are in and when the advice was written.
One part of this is genuinely unsettled. Whether an accessory unit served by a pre-existing well, drilled long before the relevant cut-off dates, triggers the fee and the recorded restriction is ambiguous in the published documents. The county bulletin and the administrative code are drafted around wells constructed after a date, while Ecology's policy language is drafted around new building permits, and those two framings do not obviously line up. Get a written determination from King County Permitting rather than an opinion from anyone, including us.
Fire access, sprinklers, and what the exemption covers
King County's residential sprinkler rule has three triggers: there is no approved fire access; the structure exceeds 10,000 square feet of total floor area including basements; or there is no minimum fire flow of 1,000 gallons per minute and no hydrant within 400 or 600 feet.
The exception is broader than many summaries describe. Structures on lots that are 35,000 square feet or larger, outside the urban growth area, with a residential primary use are excepted, and on the face of the code that exception is unqualified: it covers all three triggers, including the fire access one. Since RA-2.5's minimum lot area is 1.875 acres, essentially every conforming RA lot clears the size threshold. So for most rural cottages, sprinklers are probably off the table.
Two conditions still apply. The fire marshal retains authority to impose conditions on permits that are exempt from the fire flow requirements, and those conditions expressly include increased setbacks, fire retardant materials and sprinkler systems. And an exempt permit generally has to record a covenant acknowledging the absence of fire hydrants, agreeing not to protest a future water district annexation, with a carve-out for lots of five acres or more.
Separately, and regardless of sprinklers, fire apparatus access is a requirement in its own right, as well as a sprinkler trigger. It is an engineering standard applied to your driveway:
| Requirement | Standard |
|---|---|
| Width and height | Minimum 20 feet wide, minimum 13 feet 6 inches unobstructed height |
| Surface | Asphalt, concrete or other approved all-weather surface capable of supporting fire apparatus weighing at least 75,000 pounds |
| Turning radius | 20 feet inside and 40 feet outside, or as otherwise approved by the Fire Marshal |
| Dead ends | Over 150 feet in length require an approved cul-de-sac with a 40 foot turning radius, or a hammerhead turnaround |
| Gradient | Shall not exceed 15 percent at any point |
| Gates | Not allowed unless approved by the Fire Marshal, with approved emergency operation |
A long gravel drive up a hillside, which is an ordinary thing to own in rural King County, can fail on grade, on width, on surface load rating, or on the absence of a turnaround. Improving 300 feet of driveway to a 20 foot all-weather standard with a hammerhead turnaround is a significant cost, and it belongs in the budget at the start rather than coming up later in the fire marshal's review.
If sprinklers do end up required, the follow-on problem is water. A residential sprinkler system needs a supply that a 10 gallon per minute well cannot provide, which pushes you toward stored water. King County's code does not specify cistern sizing for this, so it becomes an "other approved" water supply that the fire marshal signs off case by case. That is a conversation to have early, not a line item you can price from a table.
Most other published accounts say the access standard is what puts sprinklers back on a rural cottage. Read against KCC 17.04.560 the exception looks unqualified and covers all three triggers, access included. Confirm it with the King County Fire Marshal for your parcel before deciding whether to spend money on a driveway.
County and Public Health fees
| Item | Fee |
|---|---|
| New accessory dwelling: application review, minimum | $4,556 |
| New accessory dwelling: site review | $3,618, or $2,033 for basic review with standard conditions |
| Permit inspection | $2.46 per square foot |
| Sprinkler review, then inspection | $1,119, then $1,997 |
| Septic design application review | $731 gravity, $1,181 pressurized |
| Septic installation permit | $1,069 gravity, $1,181 pressurized |
| Private well location review | $450 plus hourly over two hours |
| Exempt domestic well | $350 to Ecology plus $150 to King County |
| Code interpretation | $250 |
| Critical area designation, minimum per parcel | $1,643, plus $1,212 per discipline of review |
Add those up for a rural cottage on a new septic system and you are into five figures in agency fees alone, before construction starts. That is arithmetic on published fees rather than a published total, so treat it as a scale check rather than an estimate.
Next: the constraints that apply on both sides of the city line (trees, critical areas and drainage). Part 4.
Sources
- King County Board of Health Code Title 13, in particular 13.28.030 for reserve areas and accessory unit bedrooms, 13.28.070 for design flow, and Table 13.28-2 for setbacks.
- Public Health, Seattle & King County, technical memorandum on accessory dwelling units, effective 9 May 2025, version 2 updated 26 June 2025.
- King County Code Title 17, fire code: 17.04.373 to 17.04.425 for apparatus access, 17.04.560 for sprinklers, 17.08.030 for fire flow exemptions.
- KCC 13.24.134 and 13.24.136 on sewer availability inside and outside the urban growth area.
- Washington Department of Ecology, WAC 173-507 and WAC 173-508, adopted 27 May 2026 and effective 27 June 2026; RCW 90.44.050 on permit-exempt wells.
- King County 2026 fee guides for pre-development services, single family residential construction and land disturbance; King County on-site sewage system service fees through 2026.
Applies to unincorporated King County. Septic, well and fire requirements are site-specific and are administered by Public Health, Seattle & King County and by the King County Fire Marshal rather than by the permitting department. Nothing here substitutes for a licensed designer evaluating your parcel. Fee schedules reset annually. Read in August 2026. General information from Hanco Electric, not design advice.