Washington State ADU Rules: What Homeowners Can Build in 2026

Washington state passed some of the most permissive ADU legislation in the country in 2023 (HB 1337). The law requires all cities in Washington to allow at least one ADU on any lot that permits a detached single-family home — and in most cases, two.

What the statewide law requires

• Cities must allow at least one ADU (attached or detached) on every single-family lot.
• Maximum size: at least 1,000 sq ft for detached ADUs; cities may allow more.
• Setbacks: no local setback requirement can exceed 5 feet from the rear or side property line.
• Height: detached ADUs may be up to 24 feet or the height of the primary dwelling, whichever is greater.
• Cities may NOT require owner-occupancy of either the primary home or the ADU.
• Cities may NOT require additional off-street parking for an ADU.

What cities can still restrict

Local governments retain authority over design standards, lot coverage limits, and utility connections. They can require ADUs to match the exterior character of the primary dwelling, and they can limit ADU size to no larger than the main house.

Key cities: Seattle, Spokane, Tacoma

Seattle already allowed ADUs before the state law and permits both a detached ADU and a DADU on a single lot. Spokane and Tacoma have updated their codes to comply with the state mandate. Check your specific city's zoning portal for local maximums beyond the state floor.

This article is informational only. Local ordinances and lot-specific conditions affect feasibility — always confirm with your city planning department.