King County rewrote its accessory dwelling unit standards in 2024 through 19824 and 19881, amending 21A.08.030.B.7. Several PDFs still circulating online describe the repealed pre-2024 rules: one unit per lot everywhere, a mandatory owner-occupancy condition, a required extra parking space, and removal of the unit if the owner stopped living there. All of that is gone.
The new section is divided in two. Subsection (7) is headed "For lots in the urban area" and subsection (8) is headed "For lots in the rural area or on natural resource lands." The generous provisions almost all sit in (7), and that was deliberate. The state ADU law only applies inside an , so the county wrote the state's requirements into the urban subsection and left the rural area to its own older logic.
The split, side by side
| Question | Urban area | Rural area and resource lands |
|---|---|---|
| How many ADUs | Two per lot: one attached plus one detached, two attached, or two detached in one or two structures | One per lot. None at all in the Forest zone |
| Detached allowed? | Yes | Only where there is no more than one primary dwelling on the lot and the lot meets the for its zone, or is 3,200 sq ft or more in a rural town. The baseline is that a rural ADU is allowed only in the same building as the primary dwelling; detached is the carve-out |
| Maximum size | 1,000 sq ft heated and 1,000 sq ft unheated, with unused unheated allowance convertible to heated up to 1,500 sq ft | 1,000 sq ft heated and 1,000 sq ft unheated. No trade-up |
| Convert a nonconforming garage? | Yes, even if the existing structure is as to setbacks or | No such relief |
| Street improvements | None required | No exemption written in |
| Parking | No additional off-street parking required, anywhere in unincorporated King County. Broader than the state floor | |
| Owner occupancy | Not required. Dropped countywide, not just inside the urban growth area | |
| Required, recorded, and it runs with the land | ||
Size is measured differently from Seattle. King County counts heated and unheated floor area separately, and for a detached unit does not count basement floor area at all. If the unit sits wholly within the basement or attic of the main house, the size limit does not apply.
Height follows the zone rather than the unit: a unit not wholly contained within an existing dwelling may not exceed the base height for the applicable zone, which is 40 feet in the RA zones and 35 feet in the A, F and M zones and in the R-1 through R-8 urban residential zones. There is no ADU-specific setback at all. You take the zone's ordinary setbacks, which rurally means 30 feet from the street and 5 or 10 feet from interior lines. Two exceptions apply. A residence must sit at least 100 feet from any property line adjoining Agricultural, Mineral or Forest zoning or an existing extractive operation, and RA lots between one and 2.5 acres take R-1 setbacks while RA lots under an acre take R-4 setbacks.
The rural minimum lot area
RA-5 does not mean five acres
This is a common and costly misreading in rural King County, and it can rule out a detached unit entirely.
The number in an name describes intended density, not minimum lot size. The actual minimum lot areas are set out in KCC 21A.09T.030 and they are three quarters of what the zone name suggests.
| Zone | Minimum lot area | Base height | Street setback | Interior setback | Max impervious |
|---|---|---|---|---|---|
| RA-2.5 | 1.875 acres | 40 ft | 30 ft | 5 ft | 25% |
| RA-5 | 3.75 acres | 40 ft | 30 ft | 10 ft | 20% |
| RA-10 | 7.5 acres | 40 ft | 30 ft | 10 ft | 15% |
| RA-20 | 15 acres | 40 ft | 30 ft | 10 ft | 12.5% |
| A-10 | 10 acres | 35 ft | 30 ft | 10 ft | 15% / 35% |
| F (Forest) | 80 acres | 35 ft | 50 ft | 100 ft | ADUs prohibited |
Two notes on the impervious column, which is often misquoted. There is a floor as well as a percentage: lots of half an acre or more are allowed at least 10,000 square feet of impervious surface regardless of the percentage, so on a conforming rural lot the cap is usually generous rather than binding. And the much tighter 8 percent figure people repeat 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. It is not a general rural cap. Exceeding whichever limit applies to you requires a conditional use permit, which is a longer process than a building permit.
Worked example: the three acre RA-5 parcel
You own three acres zoned RA-5. It is a legal building lot with a house on it, created lawfully years ago, and nothing about it is unusual.
RA-5's minimum lot area is 3.75 acres. The rural detached ADU test requires the lot to meet the minimum lot area for the applicable zone. Three is less than 3.75. A detached cottage is not allowed on this parcel unless you acquire more land.
You can still have an attached unit. That is a real option and often a better one financially. It shares the foundation, the roof, the water heater and the electrical service, and it does not need its own trench. But it is a different building. If the plan was a cottage at the back of the property, expect a redesign.
A breezeway does not make a detached unit attached
The obvious workaround is to connect a detached building to the house with a covered walkway and call the result attached. The code addresses this directly. Under KCC 21A.08.030.B.7.a.(3) an attached unit must share at least one common wall with the primary dwelling and appear to be contained within one structure, and connection through a breezeway or covered pathway does not make it attached unless the connection is:
- less than ten feet in length, and
- sharing a common wall with both the ADU and the primary residence, and
- completely enclosed, and
- heated space.
The four conditions in KCC 21A.08.030.B.7.a.(3), and the three usual ways to fail them.
All four conditions must be met. If any one fails, the unit is detached and goes back to the minimum lot area test. An open-sided covered walk would fail, and so would a twelve foot connection or an enclosed but unheated one. If you are designing around this, design to all four conditions from the start. Adding heat and glazing to a walkway later to get a permit approved is expensive.
One more rural-only rule applies at the drawing stage. Where the primary and accessory units are in the same building, or in buildings connected by a breezeway, only one entrance may front a street.
What the notice on title is
Many references mention this requirement without explaining how it works. This section covers it.
KCC 21A.08.030.B.7.a.(5) requires the applicant to file a notice, on a form approved by the county's Records and Licensing Services Division, identifying the dwelling unit as accessory. The code says the notice , and that the applicant must submit proof the notice was filed before the department approves any permit. So this step comes at the front of the process, before approval, not after.
What it means in practice
What it is. A recorded document, filed with King County Recording against your parcel, stating that the second dwelling unit on this property is accessory to the first. It joins your deed, your easements and any covenants in the chain of title.
Who sees it. Everyone who ever looks the property up. Title companies, lenders, appraisers, buyers, and their attorneys. It works as a disclosure device rather than a penalty. It keeps a future owner from discovering that what looks like two houses is legally one house plus an accessory unit.
Effect on the unit. It fixes the unit's legal character. The cottage cannot be treated as an independent residence, cannot be sold on its own without a separate legal process, and comes with the county's accessory-unit conditions attached wherever the property goes.
Removal. In general, it can't be removed. The county form states the notice shall not be removed except upon specific written authorization by King County recorded upon the title. It stays in place after the project is finished and after the property is sold. Removing the kitchen might open a discussion with the county, but there is no simple form for it.
Cost. Ordinary county recording fees, which are small. King County's own permitting fee schedule also lists a review category for notice on title or easement review only at $483, so the paperwork can carry a review fee separate from the recording itself.
None of this prevents you from building. Tell your lender about the notice early so it doesn't come up unexpectedly in underwriting.
A detached rural unit affects future subdivision
This is a long-range issue, and it rarely comes up at design stage. A detached rural ADU is counted as a separate dwelling unit for the purpose of lot calculations at the time of any proposed subdivision. And if a detached unit in an RA zone is later converted into a primary unit on a separate lot, neither the original lot nor the new one may add another detached ADU unless it is at least twice the minimum lot area for the zone.
If your land might one day be divided, or if the cottage is part of a longer plan to hand a parcel to family, work that sequence out on paper before you build. The order in which you do things changes what remains possible.
Two more rural issues
Accessory living quarters still count for septic
are a separate, lesser King County use: living space with no kitchen. One per lot, capped at 1,000 square feet heated, prohibited in the Forest zone, and in the urban area or a rural town only on lots of 3,200 square feet or more.
Some owners use it to avoid the ADU rules, and for zoning purposes it can work. Public Health is another matter: an accessory living quarter still has its bedrooms counted against your septic design, and one with no bedroom is still deemed equivalent to one bedroom. So it avoids the zoning limit but not the septic limit, which is the more likely of the two to stop a project. The same goes for a : on a permanent foundation it is simply a detached unit, and every rule on this page applies to it.
The Agriculture zone: the code and the county disagree
On its face KCC 21A.08.030.B.7.a.(8)(b) would permit a detached ADU on an A-10 lot of ten acres or more. King County's own published March 2026 bulletin says the opposite: only attached ADUs may be permitted in the Agriculture zone, and they must be accessory to a primary use of commercial agriculture on the site. The code basis for the county's position sits elsewhere in the same chapter, which limits A-zone residences to farm residences accessory to active, ongoing agricultural use, with its own recorded occupancy notice.
We don't take a position on which reading is correct. If you are on agricultural land, buy a written code interpretation before anyone draws a detached unit. King County's published fee for one is $250, which is small next to the other costs on this page.
Costs of building without a permit
King County's 2026 fee guide lists a work-without-permit investigation fee for a new accessory dwelling unit at $4,556. You pay that on top of the permit fee itself. Application review for a new accessory dwelling starts at $4,556 and site review at $3,618, with inspection charged at $2.46 per square foot. Plan resubmittals after the second are charged again.
Legalising an unpermitted rural cottage also brings in the whole analysis at once: septic capacity, well yield, fire apparatus access, critical areas and impervious surface, all in a single application, with no option to address them in stages. Property tax assessment doesn't depend on permit status either. State guidance is that new construction value goes on the rolls where a building permit was issued or should have been issued, so an unpermitted unit is taxed whether or not it is legal.
On septic, wells and fire access, which is where most rural projects run into limits, carry on to Part 3.
Sources
- King County Code Title 21A, in particular KCC 21A.08.030.B.7 as amended by Ordinances 19824 and 19881 (2024), and KCC 21A.09T.030 for rural density and dimensional standards. Published by the Clerk of the Council at kingcounty.gov.
- King County Department of Local Services, Permitting Division, Residential Accessory Dwelling Units bulletin, March 2026, and the 2026 fee guides for pre-development services and single family residential construction.
- RCW 36.70A.680, whose subsection (2) is the provision limiting the state ADU requirements to urban growth areas, and RCW 36.70A.681.
- King County ADU Notice on Title affidavit form, published by the Permitting Division.
Applies to unincorporated King County only, not to Seattle or to any incorporated city. Read from primary documents in August 2026; county fee schedules reset annually. Where the code text and the county's published bulletin conflict, both are shown. Confirm with King County Permitting for your specific parcel. General information from Hanco Electric, not legal or design advice.