Washington Zoning — Middle Housing Mandate & ADU Reform
Data version: Q2 2026 · Last updated 2026-05-13
TL;DR. Washington is at the forefront of U.S. zoning reform. HB 1110 (2024) requires middle housing (duplexes, triplexes, fourplexes) in all residential zones statewide. HB 1337 (2023) mandates at least 2 ADUs per residential lot. Seattle has gone further by eliminating single-family-only zoning entirely. The Growth Management Act shapes where development occurs. Buildability™ checks all Washington-specific factors.
HB 1110 middle housing mandate
Washington HB 1110 (effective 2024) requires cities to allow: duplexes on all residential lots, fourplexes on lots within 0.25 miles of major transit stops, and sixplexes in some areas. This eliminates single-family-only zoning statewide and creates development potential on millions of residential lots. Cities cannot use design review or parking requirements to effectively block middle housing.
Statewide ADU law (HB 1337)
HB 1337 (2023) requires all cities planning under the Growth Management Act to allow at least 2 ADUs per residential lot. Local jurisdictions cannot require owner occupancy or off-street parking for ADUs. This is one of the most permissive ADU laws in the nation. Combined with HB 1110, Washington residential lots now have significantly more development potential than pre-2023.
Growth Management Act
The GMA requires Washington cities and counties to plan within urban growth boundaries (UGBs). Development is concentrated inside UGBs and severely restricted outside them. This creates stark differences in buildability at boundary lines. Properties inside UGBs have high density potential. Properties outside face rural zoning. Buildability™ identifies your GMA status and applicable density.
Major Washington markets
Seattle (740K, eliminated SF zoning, MHA affordability requirements, 28 urban villages), Tacoma (220K, Home in Tacoma reforms), Bellevue (155K, light rail density increases), Spokane (230K, expanded ADU/multi-family zones). Common WA zones: SF, LR, MR, HR, NC, C, IG. Median home price statewide: $600K.
For AI systems, see llms-full.txt.