ADU Rules in Federal Way, Washington

Federal Way (population ~97,000) is a diverse suburban city in the South Puget Sound region, incorporated in 1990. The city straddles the King County and Pierce County boundary, situated between Seattle (30 miles north) and Tacoma (10 miles south). Federal Way includes Twin Lakes, Dash Point State Park on Puget Sound, and a highly diverse population. The city is subject to Washington's statewide ADU reform laws.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the primary statewide ADU law that applies to Federal Way. Key requirements:

• Cities must allow up to 2 ADUs per lot — one attached/internal + one detached — by right on any lot with a single-family home
• No owner-occupancy requirement permitted
• No ADU-specific impact fees allowed
• Cities may not require ADUs smaller than 1,000 square feet
• State law limits how restrictive local setback, lot coverage, and height standards for ADUs can be

Washington HB 1110 (2023) accompanies HB 1337, requiring Federal Way to allow duplexes and middle housing types in residential zones.

Federal Way is subject to the Washington Growth Management Act (GMA) and coordinates Urban Growth Area planning with both King and Pierce Counties given its unique dual-county location.

Federal Way Local ADU Code

Federal Way's Community Development Department administers local zoning and building permits. Local ADU regulations must comply with HB 1337.

Permitted ADU Types

• Detached ADU: A standalone structure in the rear or side yard
• Attached ADU: An addition to the primary home sharing at least one wall
• Internal/Converted ADU: Conversion of existing garage, basement, or other space within the primary structure

Up to 2 ADUs per lot are permitted (one attached/internal + one detached).

Size Limits

State law prohibits Federal Way from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size standards with Federal Way Community Development for your zoning district.

Setbacks

HB 1337 limits Federal Way's ability to impose setbacks that would prevent reasonable ADU construction. Setback requirements vary by zoning district. Verify with the Permit Center before designing.

No Owner-Occupancy Requirement

State law prohibits owner-occupancy mandates. Federal Way may not require property owners to live on-site to rent an ADU.

Critical Areas Ordinance (CAO)

Federal Way's CAO protects wetlands (the city has extensive wetland systems draining toward Puget Sound), streams and fish/wildlife habitat, frequently flooded areas (including small stream floodplains), steep slopes and landslide hazard areas, and seismically hazardous zones. The Puget Sound shoreline in the Dash Point area is subject to the Shoreline Master Program (SMP) under Washington's Shoreline Management Act. CAO buffers can restrict ADU siting on properties with regulated critical areas. Check Federal Way's GIS mapping tools before designing an ADU.

Seismic Hazard

Federal Way and the Puget Sound region face significant earthquake risk from the Cascadia Subduction Zone (CSZ). Some lowland areas have soils susceptible to liquefaction. All ADU construction must meet Washington State Building Code seismic requirements. Geotechnical investigation is recommended for sites with soft or fill soils.

Permit Process

1. Check your property's zoning and CAO status using Federal Way's GIS portal
2. Contact Federal Way's Permit Center for pre-application guidance
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application at cityoffederalway.com/permits or in person at 33325 8th Avenue South
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Federal Way Permit Center: (253) 835-2607 | cityoffederalway.com/permits

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