ADU Rules in Seattle, Washington

Seattle is Washington's largest city (population ~737,000) and one of the most ADU-friendly jurisdictions in the nation. Situated in King County on Puget Sound, Seattle faces a severe housing shortage driven by its role as headquarters for Amazon and a major hub for Microsoft, Boeing, and the broader tech economy. ADUs are a key tool for adding housing on existing residential lots.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Seattle and every Washington city. Key provisions:

• Cities must allow up to 2 ADUs per lot on any lot with a single-family home — one attached/internal and one detached — by right (no discretionary approval)
• No owner-occupancy requirement is permitted
• No ADU-specific impact fees allowed
• Cities may not require ADUs to be smaller than 1,000 square feet
• Setback, lot coverage, and height restrictions are limited to prevent cities from effectively blocking ADU construction

Seattle had already adopted progressive ADU rules before HB 1337 passed, making it a pioneer. State law now codifies and strengthens these protections statewide.

Washington HB 1110 (2023) is a companion law requiring cities to allow duplexes and middle housing in residential zones, further expanding housing options alongside ADU production.

Seattle Local ADU Code

Seattle's Land Use Code (Seattle Municipal Code Title 23) governs ADU development locally, within the framework set by HB 1337. Seattle administers permits through the Seattle Department of Construction & Inspections (SDCI).

Permitted ADU Types

• Detached ADU (DADU): A standalone structure in the rear or side yard, separate from the primary home
• Attached ADU: An addition to the primary dwelling, sharing at least one wall
• Internal/Converted ADU: Space converted within the existing home (basement, attic, or main floor)

Lots may have one attached/internal ADU + one detached ADU simultaneously — a total of 2 ADUs.

Size Limits

State law prohibits Seattle from capping ADUs below 1,000 sq ft. Seattle's DADU maximum is 1,000 square feet. Attached ADUs may be larger depending on zone and structure. Confirm current limits with SDCI.

Setbacks and Lot Coverage

State law restricts municipalities from imposing setbacks that effectively prevent ADU construction. Seattle's general standards include:

• Rear setback: 40 feet from rear lot line in some zones; state law prevents overly restrictive standards
• Side setback: Typically 5 feet
• Height: Up to 24 feet for DADUs under Seattle's code (varies by zone)

No Owner-Occupancy Requirement

HB 1337 prohibits owner-occupancy mandates. Neither the state nor Seattle requires the property owner to live on-site to rent an ADU.

Critical Areas Ordinance (CAO)

Seattle's Critical Areas Ordinance protects wetlands, fish/wildlife habitat corridors (including salmon-bearing streams like Thornton Creek), steep slopes, landslide hazard areas, and seismic hazard zones. CAO buffers — often 50–200 feet from wetlands and streams — can significantly reduce the buildable area on a lot and affect ADU siting. Check Seattle's GIS mapping tools for CAO layers before designing an ADU.

Seismic Hazard — Cascadia Subduction Zone

Seattle sits on fill soils and glacial deposits near Puget Sound, presenting significant liquefaction risk in many lowland neighborhoods (SoDo, SODO, Georgetown, South Park, parts of Ballard and West Seattle). The Cascadia Subduction Zone (CSZ) offshore could produce a magnitude 9.0+ earthquake — the largest seismic threat in US history. USGS ShakeMap and Washington DNR seismic hazard maps identify high-risk zones. ADU foundation design must account for local soil conditions; geotechnical investigation is strongly recommended on soft or filled ground.

Permit Process

1. Research property zoning and CAO constraints using Seattle's online permit portal
2. Prepare construction plans meeting SDCI standards
3. Submit building permit application online at seattle.gov/sdci
4. Plan review (ministerial — no design board approval for compliant ADUs)
5. Permit issuance and construction
6. Required inspections during construction
7. Final inspection and Certificate of Occupancy

Contact SDCI: (206) 684-8850 | seattle.gov/sdci

Disclaimer: This guide provides general information based on Washington HB 1337 (2023) and Seattle's municipal code as of April 2026. Zoning regulations change frequently. Always verify current requirements with the Seattle Department of Construction & Inspections before making development decisions. This is not legal advice.