Washington · WA
ADU laws & builders in Washington
Washington's HB 1110 requires cities with more than 25,000 residents to allow at least two ADUs per lot. Seattle's DADU (Detached ADU) program leads the state with clear permit pathways and growing builder participation.
Last reviewed: 2026-05-29
Are ADUs legal in Washington?
Yes. HB 1110 and related middle-housing laws require larger Washington cities to allow ADUs — generally up to two per lot — and to ease the parking and owner-occupancy rules that used to block them. Seattle's Detached ADU (DADU) program is the most mature in the state, with pre-reviewed plan options and a clear permit path.
Key rules that affect your build
Washington cities are phasing in the new allowances, so exact timelines and standards vary by jurisdiction. Seattle reduced parking requirements and offers pre-approved DADU plans that speed review. Outside Seattle, check whether your city has adopted its HB 1110 code updates yet — a local builder will know.
Cost and timeline in Washington
A permitted detached ADU in the Seattle area typically runs $200K–$420K turnkey. Plan on 9–14 months end to end. Using Seattle's pre-approved DADU plans can shave months off permitting and cut design costs significantly.
Official source
We summarize Washington's ADU rules in plain English. For the authoritative, always-current legal detail, go straight to the source:
WA Dept. of Commerce — Middle HousingAlways verify current rules with the official source before making decisions.
Pre-approved ADU plan programs in Washington
- Seattle — Seattle DADU Program
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