ADU Laws in Washington (2026)
Washington state ADU law details, key provisions, preemption status, and recent legislative changes for accessory dwelling units.
Law Name
Washington House Bill 1337 (HB 1337) — Expanding Housing Options Through ADUs · Verified June 25, 2026 · Source
Effective Date
2023-07-23
Preempts Local Ordinances
Yes — state law overrides local restrictions
Has State Law
Yes
Statewide ADU Preemption Applies
State law establishes statewide preemption standards that supersede local regulations. The citation and source below confirm the verified legal basis.
Key Provisions
Requires all Washington cities and counties planning under the Growth Management Act (GMA) to allow at least 2 ADUs per single-family residential lot by-right · Source
Prohibits local owner-occupancy requirements for ADUs — neither the primary dwelling nor the ADU must be owner-occupied · Source
Removes off-street parking requirements for ADUs located within 0.25 miles of a major transit stop · Source
Allows both an attached ADU and a detached ADU on the same single-family lot simultaneously · Source
Prohibits local ordinances that cap ADU floor area below 1,000 sqft or 60% of primary dwelling size · Source
Requires ministerial (administrative) ADU approval — no discretionary design review for ADUs meeting objective standards · Source
Removes minimum lot size requirements specifically targeting ADU prohibition · Source
Local jurisdictions had until July 1, 2024 to update their codes to comply · Source
Legislative History
2023 — HB 1337 · Verified June 25, 2026 · Source
Comprehensive ADU reform legislation signed into law July 23, 2023. Requires all GMA-planning jurisdictions to allow 2 ADUs per residential lot, removes owner-occupancy requirements, reduces parking mandates near transit, sets minimum ADU size allowances (1,000 sqft or 60% of primary dwelling). Effective July 23, 2023; local code compliance required by July 1, 2024. · Source
2023 — HB 1110 · Verified June 25, 2026 · Source
Middle housing legislation (HB 1110, codified at RCW 36.70A.635–.638) with two population tiers. Cities 75,000+ population must allow at least 4 units per lot on all lots zoned predominantly for residential use (and at least 6 units within one-quarter mile of a major transit stop). Cities 25,000–74,999 must allow at least 2 units per lot (and at least 4 units near major transit or when one unit is affordable housing). Significantly expands missing middle housing in Washington alongside HB 1337. · Source
Official Sources
What Does Preemption Mean for Local Cities?
Because Washington law preempts local ordinances, individual cities cannot impose restrictions stricter than the state standard. A city may still have additional administrative requirements, but cannot deny ADU applications that comply with state minimums.
Learn about Washingtonzoning preemption →Source: Washington House Bill 1337 (HB 1337) — Expanding Housing Options Through ADUs. Last verified April 5, 2026. View source