Federal Way, WA Accessory Structures: ADU Rules (2026)
ADU rules in Federal Way, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Max ADUs per lot
- 2, plus the primary dwelling
- Max ADU size
- 1,000 sq. ft. living area
- Max bedrooms
- 2 per ADU
- Parking
- 1 space per ADU, 0 near transit
- Review process
- None required for the ADU use
- Height limit
- 30 ft. above average building elevation
Summary
Federal Way allows up to two accessory dwelling units per single-family lot under FWRC 19.200.180, each capped at 1,000 square feet and two bedrooms unless the ADU is attached and built inside the existing home's footprint. One parking space per unit is required, dropping to zero within a half mile of a major transit stop.
1. An accessory dwelling unit (ADU) shall meet the definition outlined in FWRC 19.05.040. Minimum lot size for ADUs is based upon the minimum lot size for the existing primary dwelling unit as set forth in FWRC 19.200.010... 2. Up to two ADUs may be created per subject property. The ADUs, accessory structures and the primary dwelling unit together shall not exceed the maximum lot coverage and other development standards prescribed in FWRC 19.200.010... 9. Each ADU, excluding any garage, workshop and similar nonliving areas, shall contain no more than 1,000 sq. ft. An attached ADU may exceed 1,000 sq. ft., up to a maximum of the largest floor area of the primary dwelling unit if the creation of the ADU does not necessitate an exterior alteration of the residence... Each ADU shall have no more than two bedrooms.
Official source re-checked September 8, 2026: no newer edition of the code had been published (publisher’s edition: current through Ordinance 26-1049, passed August 11, 2026).
Full Breakdown
180 governs ADUs in Federal Way's single-family residential (RS) zones. 0 zones. " Minimum lot size for an ADU tracks the minimum lot size already required for the primary dwelling on that lot, and an ADU is still allowed on a lot that is legally nonconforming in size. Each ADU is capped at 1,000 square feet, not counting a garage, workshop or other nonliving space, and no more than two bedrooms. An attached ADU may grow past 1,000 square feet, up to the primary dwelling's largest floor area, if adding it does not require altering the outside of the house, aside from work needed to make the ADU functionally separate.
Detached ADUs may sit on the property line if that line runs along a public alley, and ADUs are permitted inside existing structures even where those structures are already legally nonconforming for setback or lot coverage violations. Parking is one space per ADU, dropping to zero within a half mile of a major transit stop. Side yard setback is five feet, or 10 feet on the street side of a corner lot, and maximum structure height is 30 feet above average building elevation.
Violations & Fines
Building or occupying an ADU that exceeds the 1,000-square-foot cap, adds a third bedroom, or skips the required parking violates FWRC 19.200.180 and is enforced as a Title 19 zoning violation under FWRC 1.15.040: a notice and order to correct, then an accruing daily penalty of $100 for a first violation up to $500 for repeated or continuing violations of the same kind.
Frequently Asked Questions
How many ADUs can a Federal Way homeowner build?
How big can a Federal Way ADU be?
Does an ADU need its own parking space in Federal Way?
Sources & Official References
Other rules in Federal Way
How Federal Way compares: Easiest Cities to Build an ADU·Compare Federal Way to another location·View the Washington accessory structures overview
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ADU Rules in Nearby Cities
How other cities in King County handle adu rules.