ADU & backyard cottage rules in King County (unincorporated), WA
This page is about unincorporated King County: the areas outside any city, governed by King County Code Title 21A. A lot inside Seattle, Kent, Bellevue or any other city follows that city's code instead — check which one applies before you read the table.
King County does not use a floor-area ratio. What limits a building is the impervious-surface share of the lot, the height limit and the setbacks, set zone by zone in the densities and dimensions table (KCC 21A.12.030). Our engine stores the impervious share as the coverage limit and works out the largest building from it.
The urban residential zones run from R-1, one home per acre on lots of an acre or more, up to R-48 for apartments. Most house lots are R-4 to R-8, where the front setback is 20 feet and the sides are 5 feet.
Whether a detached cottage is allowed on an urban lot is settled by Washington's HB 1337: two ADUs per lot, no owner-occupancy rule. The table below shows whether yours has room. Rural zones are not covered on this page.
The rules our engine reads for King County (unincorporated)
| Zone | Coverage | Height | Smallest lot | Setbacks front / side / rear | Also |
|---|---|---|---|---|---|
| R-1 | 55% | 35 ft | 43,560 sq ft | 30 ft / 5 ft / 5 ft | 1 home per acre |
| R-4 | 65% | 35 ft | 8,000 sq ft | 20 ft / 5 ft / 5 ft | 4 homes per acre |
| R-6 | 70% | 35 ft | 5,000 sq ft | 20 ft / 5 ft / 5 ft | 6 homes per acre |
| R-8 | 75% | 35 ft | 3,750 sq ft | 20 ft / 5 ft / 5 ft | 8 homes per acre |
| R-12 | 80% | 35 ft | 3,000 sq ft | 20 ft / 5 ft / 5 ft | 12 homes per acre |
| R-18 | 85% | 45 ft | 2,400 sq ft | 20 ft / 5 ft / 5 ft | 18 homes per acre |
| R-24 | 85% | 45 ft | 1,800 sq ft | 20 ft / 5 ft / 5 ft | 24 homes per acre |
| R-48 | 85% | 60 ft | 900 sq ft | 20 ft / 5 ft / 5 ft | 48 homes per acre |
Rules checked 2026-10-25. Sources: King County Code 21A.12.030 — densities and dimensions.
What this means for your lot
- Find your zone code on your parcel record or King County (unincorporated)'s zoning map, then read its row above: the setbacks and the lot coverage limit decide how much of the lot a second home can use.
- Whether a cottage fits is a question of room, not permission: setbacks, what is already built, trees and slope leave a buildable area. The 3D check places a home inside it and tells you when it breaks a rule.
- Washington's HB 1337 sets the floor in urban areas — two ADUs per lot, no owner-occupancy rule, and a size cap no lower than 1,000 sq ft — so the local table above is what is left to check.
Common questions
Can I build a detached ADU (backyard cottage) in King County (unincorporated)?
Washington's HB 1337 requires cities and counties that plan under the Growth Management Act to allow two ADUs per lot in urban growth areas, in any mix of attached and detached. Whether one fits on your lot in unincorporated King County depends on its setbacks and coverage, which are in the table on this page.
How big can an ADU be in King County (unincorporated)?
State law does not allow a cap below 1,000 sq ft of floor area. The local cap is not in our tables yet — check the code.
Do I need parking for an ADU in King County (unincorporated)?
State law allows no more than one space per unit on lots under 6,000 sq ft and two on larger lots, and none within a half-mile walk of a major transit stop. The local parking rule is not in our tables yet.
Do I have to live on the property to rent out an ADU in King County (unincorporated)?
No. HB 1337 bars cities and counties from requiring the owner to live on the property or in the ADU.
Check your address in 3D · Washington's ADU law, HB 1337
General information from public sources, not legal advice. Codes change and cities differ — confirm with your permit office before you design or build.