Washington ADU Law (HB 1337): What Homeowners Need to Know
6 min read
Washington's HB 1337, passed in 2023 and codified at RCW 36.70A.681, is one of the most aggressive ADU laws in the country. If you live in an urban growth area, it dramatically expands what you can build.
Two ADUs per lot
Cities and counties planning under the Growth Management Act must allow at least two ADUs on most single-family lots — for example one detached unit plus one interior or attached unit.
Size: at least 1,000 sq ft
The law requires cities to allow ADUs of at least 1,000 sq ft, larger than many older local codes permitted.
No owner-occupancy, limited parking
Cities cannot require the owner to live on-site, and cannot require off-street parking for an ADU within half a mile of a major transit stop. Both rules remove obstacles that historically killed Washington ADU projects.
What it means for you
In Seattle, Tacoma, Spokane, Vancouver and other Washington cities, an ADU is now a by-right project on most single-family lots. Confirm your specific lot and any design standards with your city, then estimate your cost.
See Washington ADU costs →Find out what your ADU will cost
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Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.