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
Effective Date
2023-07-23
Preempts Local Ordinances
Yes — state law overrides local restrictions
Has State Law
Yes
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
Prohibits local owner-occupancy requirements for ADUs — neither the primary dwelling nor the ADU must be owner-occupied
Removes off-street parking requirements for ADUs located within 0.25 miles of a major transit stop
Allows both an attached ADU and a detached ADU on the same single-family lot simultaneously
Prohibits local ordinances that cap ADU floor area below 1,000 sqft or 60% of primary dwelling size
Requires ministerial (administrative) ADU approval — no discretionary design review for ADUs meeting objective standards
Removes minimum lot size requirements specifically targeting ADU prohibition
Local jurisdictions had until July 1, 2024 to update their codes to comply
Legislative History
2023 — HB 1337
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.
2023 — HB 1110
Companion middle housing legislation requiring cities with 25,000+ population to allow at least 4 units per lot in areas with frequent transit service, and at least 2 units per lot in all other single-family zones. Alongside HB 1337, significantly expands missing middle housing in Washington.
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