ADU & backyard cottage rules in Snohomish County (unincorporated), WA
This page covers unincorporated Snohomish County, where Snohomish County Code Title 30 applies. Lots inside Everett, Lynnwood, Mukilteo and the other cities follow their own codes.
The county sets building standards in two bulk matrices: SCC 30.23.032 for the urban residential zones, R-9600, R-8400 and R-7200, and SCC 30.23.030 for the rural R-5 zone. The urban zones allow 35% coverage; R-5 allows 15% on lots of five acres. Snohomish County does not use a floor-area ratio, so what limits a building is coverage, height and setbacks.
A detached ADU in unincorporated Snohomish County is capped at 1,200 sq ft (SCC 30.28.010). ADUs do not count against rural density, so on an R-5 lot the legal maximum is one home plus one large ADU.
Our table lists only the zones we read from the county code; other zones, such as the urban multifamily ones, are not on this page.
The rules our engine reads for Snohomish County (unincorporated)
| Zone | Coverage | Height | Smallest lot | Setbacks front / side / rear | Also |
|---|---|---|---|---|---|
| R-9600 | 35% | 30 ft | 9,600 sq ft | 25 ft / 5 ft / 25 ft | 4.5 homes per acre |
| R-8400 | 35% | 30 ft | 8,400 sq ft | 25 ft / 5 ft / 25 ft | 5.2 homes per acre |
| R-7200 | 35% | 35 ft | 7,200 sq ft | 25 ft / 5 ft / 25 ft | 6 homes per acre |
| R-5 | 15% | 35 ft | 217,800 sq ft | 25 ft / 10 ft / 25 ft | 0.2 homes per acre |
Rules checked 2026-10-25. Sources: Snohomish County Code 30.23.030 and 30.23.032 — bulk matrices; Snohomish County Code 30.28.010 — detached ADU size.
What this means for your lot
- Find your zone code on your parcel record or Snohomish 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 Snohomish 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 Snohomish County depends on its setbacks and coverage, which are in the table on this page.
How big can an ADU be in Snohomish County (unincorporated)?
A detached ADU in unincorporated Snohomish County is capped at 1,200 sq ft. State law does not allow a cap below 1,000 sq ft.
Do I need parking for an ADU in Snohomish 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 Snohomish 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.