Washington’s 2026 rural detached-ADU law gives covered counties an option. It does not make rural parcels eligible statewide.
Start with the county and the UGA boundary
Engrossed House Bill 1345 (Chapter 231, Laws of 2026) took effect June 11, 2026. It lets Growth Management Act counties authorize detached ADUs outside a UGA if they adopt a compliant route. Confirm adoption and parcel location on the County map.
Apply the rural-route limits
The route allows one ADU per parcel, attached or detached. It requires water-use and septic or sewer documentation, shared access, and location within 150 feet of the principal unit. Size is capped at the attached-unit limit, up to 1,296 square feet. Nonconforming lots under one acre are excluded; other local review remains.
Treat Thurston County’s route as local law
Thurston County reports adopting Resolution 16574 and Ordinance 16575 on December 16, 2025. Its rural summary allows one ADU per lot in residential zones. Detached units are capped at 1,000 square feet (1,200 with transferable development rights), within 100 feet of the home, and must share a driveway and have approved water and septic.
These are the County’s published local conditions, not proof that it adopted every part of the later state option. Confirm the parcel’s rural designation, current County Code, and legal route. The County has separate urban UGA rules.
For the general parcel workflow, see how to screen an ADU property and the Seattle count and zoning screen.