The electrical side of an accessory dwelling unit runs through at least three processes, each with its own agency and its own clock: the building permit from the city or county, the electrical permit from either SDCI or Washington Labor and Industries depending on your address, and the utility's own work. The utility work usually takes the longest, and people usually start it last.
Who issues your electrical permit
Washington lets certain cities run their own electrical program. Everywhere else, L&I does it. The split doesn't follow an obvious pattern, so check the table.
| Authority | Where |
|---|---|
| The city itself | Bellevue, Burien, Des Moines, Federal Way, Kirkland, Mercer Island, Normandy Park, Redmond, Renton, Sammamish, SeaTac, Seattle, Tukwila |
| Washington L&I | Everywhere else, including all unincorporated King County, Maple Valley, Kent, Auburn, Covington, Black Diamond, Enumclaw, Issaquah, Shoreline, Kenmore, Woodinville, Bothell, Snoqualmie, North Bend, Duvall and Newcastle |
Seattle also writes its own code. The 2023 , adopted by Ordinance 127008, took effect on 15 November 2024. It is based on the 2023 with local amendments, and SDCI does the plan review and the inspections. Most electrical permits need three inspections: cover, service and final, plus separate feeder inspections for subpanels.
The 2026 NEC replaces the 2023 on 31 December 2026
Washington adopts the National Electrical Code in WAC 296-46B-010 and is currently on the 2023 edition. The 2026 edition replaces it on 31 December 2026.
Three changes affect a cottage. The load calculation article moves out of Chapter 2 entirely: Article 220 becomes Article 120, sitting in Chapter 1 alongside definitions. The dwelling minimum unit load drops from 3 volt-amperes per square foot to 2, at the new 120.41, and that unit load now absorbs the general-use receptacle outlets so no separate calculation is required for them. Energy management also gets its own article, Article 130, with the load calculation side at 120.7 as a Power Control System, set by a qualified person to no more than 80 percent of the rating of the overcurrent device it monitors.
In practice, dwelling come out materially smaller. If a project is marginal on service capacity and isn't urgent, that alone is a reason to think about timing.
The rest of this page is unaffected. Articles 225 and 250 keep their numbers in 2026, and so do the sections inside them. 225.30, 225.36, 225.39, 225.41 and 250.32 all survive the renumber intact, so the feeder and grounding rules below still hold after the changeover.
Seattle is expected to stay on its 2023-based code past that date, so the two jurisdictions will diverge for a while. There is also a grandfathering rule. Adopted standards apply by electrical permit issue date, except for new one- and two-family dwellings where the building permit issue date came before the adoption date, and for projects whose plans were received and accepted for review beforehand.
Limits on doing your own wiring for a rental
Washington's homeowner exemption, at RCW 19.28.261, does not apply where the electrical work is on the construction of a new building intended for rent, sale or lease. A detached cottage is unambiguously a new building.
The owner-occupancy affidavit and its limits
The statute does provide an exemption route. For a new residential building of up to four units intended for rent, sale or lease, an owner may apply to L&I by signed affidavit stating that they will perform the work and will occupy one of the units as their principal residence. The route is available once every twenty-four months, and the owner must state an intent to occupy for twenty-four months after completion.
Note the phrase one of the units. The unit you must occupy is a unit in the new building. If you are building a detached cottage to rent while you carry on living in the main house, this route may not be open to you at all.
Whether it is open is L&I's call, not ours, so ask them in writing before you plan around it. An attached unit inside the existing house is a different analysis from a new detached building.
The same statute removes the exemption for a second reason too: where the electrical work is on property offered for sale within twelve months of your acquiring it. That applies to anyone who buys a property, adds a cottage and sells.
Seattle adds an administrative layer: any professional applying for electrical permits there must hold a Washington electrical contractor's license and a Seattle business license.
Feeding a detached unit from the house
If the cottage is fed from the house rather than from its own utility service, the run to it is a . A feeder to a second dwelling has requirements that a feeder to a workshop does not.
- One feeder only. NEC 225.30 permits only one feeder or branch circuit to supply an additional building, with narrow exceptions that do not include needing more power.
- A building disconnect of the right type. There has to be a means of disconnecting all ungrounded conductors supplying the cottage, and for a one-family dwelling NEC 225.39(C) sets a rating of not less than 100 amperes, 3-wire. NEC 225.36 says what type of device qualifies: a circuit breaker, molded case switch, general-use switch, snap switch, or other approved means. An ordinary main-lug-only has no disconnecting means at all, so it does not qualify. This is the requirement most often missed.
- A service-equipment rating is usually not required. The disconnect does not have to be marked suitable for use as service equipment as a general rule. NEC 225.36 requires that only where the installation is made under the 250.32(B)(1) exception, which is the legacy arrangement where no equipment grounding conductor was run. When the cottage is wired the current way, with an equipment grounding conductor, that requirement does not apply.
- Where it goes, and what it says. WAC 296-46B-225 treats a feeder disconnect as being in the building if it is on the outside of the building or within sight and within 15 feet of it, and requires an identification plate in letters at least half an inch high naming the building served and identifying the device as that building's main disconnect.
- Four wires, and the neutral floats. The feeder carries an alongside the two ungrounded conductors and the neutral. NEC 250.32(B)(1) puts it directly: any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrodes at the second building. In plain terms, the neutral floats there. 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.
- Its own connection to earth. Under NEC 250.32(A) the cottage needs a and a grounding electrode conductor, installed to Part III of Article 250. The only exception is a building supplied by a single branch circuit (a multiwire branch circuit counts) that includes an equipment grounding conductor. A feeder-fed cottage does not qualify. Washington adds a requirement the national code does not: under WAC 296-46B-250 a concrete-encased electrode is mandatory at a new building on a permanent concrete foundation, with narrow exceptions. Plan the stub-up with the pour rather than adding it later. Washington's six-foot separation rule between adjacent buildings' rod, pipe or plate electrodes is written to services, so read it carefully before applying it to a feeder-fed cottage. Separately, where no 25 ohm resistance test is done, two made electrodes at least six feet apart are required.
Two 2023 code requirements for feeder-fed cottages
An . NEC 225.41, new in the 2023 code, requires one for a one- or two-family dwelling supplied by a feeder: readily accessible, outdoors, 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 on red in letters at least half an inch high on the outside front of the enclosure.
A . NEC 225.42 and NEC 215.18 both require surge protection on a feeder supplying a dwelling unit, Type 1 or Type 2. Most electricians know the service requirement at NEC 230.67. Fewer people apply the feeder equivalent, and a backyard cottage is the situation it covers. On the rating, 215.18(E) and 230.67(E) call for a nominal discharge current of not less than 10 kA, and 225.42 states none. A device rated at 10 kA meets both.
Both are inexpensive to install at rough-in and harder to add later.
A conduit trench on one of our jobs. This one feeds a standby generator, not a cottage, but the trench, the sweeps and the restoration are the same work a detached unit needs.
Some practical points on the trench. Burial depth is measured from the top of the raceway to finished grade, not from the bottom of the trench. Conduit is required under anything permanent, such as a drive, patio or retaining wall. Over a long run to the back of a lot, voltage drop needs to be calculated, not estimated by rule of thumb. A spare conduit laid while the ground is open costs very little and is the least expensive way to allow for future needs. Washington already applies that logic elsewhere: it requires a raceway or a dedicated circuit into unfinished space that could later become living area.
Rules that exist only in Seattle
Several Seattle amendments change how an ADU is designed, on top of how it is permitted.
- Circuits stay in their own unit. Branch circuits in each dwelling unit may supply only loads within that unit or associated only with it. There is a narrow allowance for a sump pump, septic or water well circuit shared by no more than two dwelling units to be supplied from either of them, with the approval of the authority having jurisdiction and the local health department. That allowance expressly does not extend to accessory dwelling units.
- The unit's breakers must be accessible to it. SDCI's guidance is that circuit breakers, fuses and a heat source may be located in the dwelling unit they serve or in common areas accessible to all residents. A panel locked inside the main house does not meet that. The unit also needs heating controls separate from the primary dwelling's.
- An energy management system cannot reduce the size of a new service. Under NEC 220.70 an setpoint may be used as a single value in load calculations. Seattle adds a sentence that is not in the national code: for new service installations, no Article 220 calculation may be reduced by an energy management system. So a load-management device can avoid an upgrade to an existing service in Seattle, but cannot size down a new one. Outside Seattle, under L&I, the unamended rule applies. City Light's 2026 handbook also lists energy management systems (smart panels, in its words) among the projects that often require engineering services. So the smart panel route is a schedule item at the utility as well as a code decision.
- is required, with an exception for service limits. Where new parking is provided for a new building, at least one space per dwelling unit must have a 40 amp 208 or 240 volt circuit terminated ready for a charger. But where the added load would require on-property power transformation or an upgrade to an existing residential service, the Director may reduce it to a 20 amp circuit and, if necessary, reduce the number of spaces. Raise it early in design so it isn't first found at plan review.
Two utility rules that affect the budget
Seattle: adding a meter can require the whole service to be brought up to standard
Section 3.3 of City Light's Requirements for Electric Service Connection lists the elective modifications that oblige a customer to bring their entire existing electric service, meaning all existing service equipment, up to current City Light construction standards. In the 2026 Edition the examples are a new meter base, a change of meter location, any increase to the service entrance capacity, and "additional meter(s)."
That last entry is the one an ADU meets. Metering the cottage separately is adding a meter, so it puts the whole service on the list even though nothing about the house changed.
If the house is fed by legacy direct-buried cable, section 3.3 goes further: City Light will provide a drawing specifying the route of a new conduit run, and all trenching, backfill and restoration must be completed by, and at the expense of, the customer.
This doesn't have to stop a project. Ask City Light what your address triggers before the budget is fixed, because the cost can be large and it won't be in a builder's bid.
The ADU entry that left the list after 2024
The 2024 Edition carried a fifth example: "Legal dwelling units (e.g., ADUs; see Glossary)." That line named ADUs directly, and it is still widely quoted online. It is gone from the 2026 Edition. The four examples above are the whole 2026 list, read from both editions side by side.
Two caveats. City Light's own table of significant changes for 2026 does not mention section 3.3 at all, so whether the entry came out in 2026 or in the 2025 edition in between is not on the record. And the practical effect is limited: a separately metered cottage is still "additional meter(s)," and an ADU that pushes the panel bigger is still an increase in service entrance capacity. Most ADUs land on the list by another route.
Where it may matter is an ADU that adds no meter and no capacity. Under the 2024 language that project was arguably on the list by virtue of being a dwelling unit. Under the 2026 language it is arguably not. That is a question for your City Light representative, in writing, and not one to settle from a web page.
More generally, City Light revises the RESC every year. The 2026 Edition was published in January 2026 and supersedes 2025, which superseded 2024. If a contractor or a web page quotes the RESC at you, ask which edition.
PSE: an allowance for a new dwelling unit, not for an upgrade
PSE's Schedule 85 provides a margin allowance per meter for each new residence unit initially served, reported at $7,880 with effect from 29 January 2026. The schedule then says, in terms, that a modification to an existing residential service location is not eligible for incremental margin allowance unless an additional residential unit is being connected.
For a homeowner, that means a plain service upgrade earns nothing and a cottage with its own earns the allowance. It cannot be applied to service lines, permitting, trenching, backfill or restoration, or to facilities already sufficient to serve the existing load. It is still a real offset, and it isn't widely known. Ask PSE to confirm it for your project.
On timing, expect the utility work to take the longest, and allow more time than older estimates suggest. Table 1.3 of the 2026 Edition gives City Light's own estimated turnaround times per phase on the looped radial system, which is what almost every house in Seattle is on.
| Service type | Application processing | Service design | Service connection |
|---|---|---|---|
| Service upgrade, simple: single-phase, overhead, 200 A or less | 2 weeks | 16 weeks | 10 weeks |
| Service upgrade, complex: three-phase and/or underground | 2 weeks | 20 weeks | 10 weeks |
| New service, simple: single-phase, overhead, 400 A or less | 2 weeks | 20 weeks | 10 weeks |
| New service, complex: three-phase and/or underground | 2 weeks | 20 weeks | 16 weeks, plus street permitting |
The same row of the same table, two editions apart. Application processing and connection are unchanged; the design queue doubled.
Adding up each row, a simple overhead service upgrade for a cottage is roughly twenty-eight weeks, and a new overhead service is roughly thirty-two. An underground service, which many detached cottages use, falls in the complex rows: thirty-two weeks for an upgrade and thirty-eight for a new service, plus up to five further weeks for non-arterial street permitting or fifteen for arterial. Those street permitting adders are footnoted to the complex new service row, not applied across the table.
Turnaround times increased substantially between 2024 and 2026
The same table in the 2024 Edition read ten weeks of engineering for a simple service upgrade and ten for a simple new service. The 2026 Edition reads sixteen and twenty, and City Light renamed that column from "Engineering" to "Service Design." A simple new residential service now shows twice the design time it did two editions ago.
If your schedule came off an older edition, a contractor's memory, or any web page written before January 2026, move it out by roughly two to three months. And read the table the way City Light qualifies it: these times exclude the work of the City Light representative and the electrical reviewer, your own contractors, the inspecting authority's process, and any corrections. Treat them as minimums.
A service feeding more than one building, residential lot or unit-lot is on City Light's list of projects that often require engineering services, so an ADU is almost always an engineered project. Two newer entries on that list are easy to miss: electric vehicle chargers and energy management systems, which the handbook glosses as smart panels. If your plan for avoiding a service upgrade is a smart panel, note that the smart panel itself triggers engineering at the utility. It can still be the right answer, and it is usually still cheaper than a service upgrade. Include it in the schedule from the start.
PSE tells customers to allow a minimum of four to twelve weeks for possible engineering, right of way permits and easement review. Where no engineering was required, services are typically installed and energized three to five days after a passed inspection. PSE adds that engineering and right of way permits can extend that by eight weeks or more.
Early Design Guidance, and what it now costs
New in the 2026 Edition: City Light runs Early Design Guidance consultations, which replaced what used to be called a pre-application consultation. You can bring design ideas, ask about system limitations at your address, and get service requirements before you have committed to anything. Projects on parcels already served by underground infrastructure qualify, as do projects developing three or more units and projects requesting 600 A and up.
The handbook says appointments are complimentary during the pilot period, but that consultations become subject to a fee from 1 April 2026, per City Light's DPP 417. That date has passed, so expect to pay. The handbook also warns that the meeting fee is not refundable even if you cancel or the project does not go ahead. The request address given in the 2026 Edition is SCL_EarlyDesignGuidance@seattle.gov.
The 2026 Edition also adds an instruction that was not there before: customers are strongly advised to contact City Light during the planning phase to verify the available service point and requirements, in order to avoid costly mistakes and service delays. It cites Seattle Electrical Code 230.1(B) and Seattle Municipal Code 21.49.110(D).
One service or two
The first question most people ask is whether the cottage shares the house's electrical service or gets its own. It is a real choice more often than people expect, and neither Seattle City Light nor PSE requires a separate service for an accessory unit in the ordinary case. Many people, including some in the trade, assume otherwise.
That question has its own page, with the two utility handbooks quoted directly: powering an ADU, one service or two.
We don't publish a combined figure for the house plus the cottage on one service. Two dwelling units on one service are calculated by their own method, with demand factors that do not apply to a single dwelling. Which method governs is a question the code does not answer as cleanly as anyone would like. On a real example the two defensible readings differ by enough to be the difference between a 100 amp service and a 200 amp one. A single published number could lead to an upgrade you do not need, or to an undersized service feeding somebody's home. It needs calculating properly, on your actual property.
If the cottage is getting its own service, that part is settled: an ADU is a dwelling unit in its own right, so applies to it exactly as to any house, and our service size calculator will size it. Enter the cottage on its own, not the whole property. Most small units land on the 100 amp minimum.
The next part covers separate metering, and who pays when the tenant does not. Part 7.
Sources
- WAC 296-46B, Washington's electrical safety standards, including 296-46B-010 on adopted editions, 296-46B-225 on feeder disconnects and 296-46B-250 on grounding electrodes.
- 2023 Seattle Electrical Code, adopted by Ordinance 127008, effective 15 November 2024; SDCI electrical permits.
- Washington L&I, cities that issue their own electrical permits.
- RCW 19.28.261, the homeowner exemption and its limits.
- NFPA 70, National Electrical Code, 2023 edition: Articles 220 and 225, and 250.32, 230.67 and 230.85.
- Seattle City Light, Requirements for Electric Service Connection, 2026 Edition, published January 2026 by the City Light Standards Engineering Organization. Read from the handbook itself on 20 August 2026: section 1.3 and Table 1.3 for the engineering triggers and turnaround times, section 1.6 and 1.6.1 for Early Design Guidance, section 3.3 for the modifications list, and sections 6.8 and 6.11 for metering. This supersedes the 2025 Edition, which superseded 2024.
- PSE Electric Service Handbook, 2026 edition, and PSE Electric Tariff G Schedule 85.
Code sections are given so you know what to ask about, not as a design. Editions change: Washington moves to the 2026 National Electrical Code on 31 December 2026 and Seattle is on its own schedule. Utility standards are revised and should be confirmed for your address before you rely on them. The has the final say on any load calculation. General information from Hanco Electric, not a design, not a permit submittal, and not a substitute for a licensed electrician evaluating your property.