ADUs & backyard cottages in Federal Way
What HB 1337 actually allows in Federal Way — how many ADUs you can build, how big, how tall, the permit timeline, the fees, and the local quirks that decide whether your backyard cottage pencils out.
What HB 1337 lets you build in Federal Way
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Federal Way that means up to 2 ADUs on a single lot, no owner-occupancy requirement, and parking handled as 1 space (waived near transit). Detached units (DADUs) can run to 1000 sf of heated space; attached units follow a different rule — up to 1,000sf.
| Rule | Federal Way |
|---|---|
| ADUs per lot | 2 |
| Owner occupancy | Not required (abolished by HB 1337) |
| Parking | 1 space (waived near transit) |
| Max DADU size | 1000 sf heated |
| Max AADU size | Up to 1,000sf |
| Max height | 16ft (city code — likely preempted) |
Pre-approved plans in Federal Way
No pre-approved plan program yet. Every Federal Way ADU goes through a custom permit review. You (or your designer) submit a full plan set, so budget for the longer timeline below and the design work up front.
Permit timeline
Federal Way permit timeline: Custom, ~60 days. Because there is no pre-approved track here, plan for a full custom review. Run the numbers for your own lot in the feasibility tool before you commit.
Fees & what you'll pay
Standard fee schedule.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Federal Way layers its own fees and exemptions on top. The line item that surprises people most here is in the fee note above; price it before you fall in love with a floor plan.
Sewer & septic
Fire sprinklers required if nearest hydrant flows under 1,750 gpm.
If your property is on septic (an on-site system, or OSS), that's the single biggest hidden risk in an ADU budget — many older septic systems are at or past their design life. The feasibility tool asks about your utilities so the estimate reflects connection or drainfield costs.
Federal Way quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Federal Way:
- Code height 16ft likely preempted by state 24ft
- 10ft separation from primary structure, or fire-rated wall
- Fire sprinklers if hydrant flow < 1,750 gpm
Up to 2 ADUs on a single residential lot — that can be an attached unit (AADU/MIL) plus a detached backyard cottage (DADU). HB 1337 (effective June 30, 2025) preempted Federal Way's old one-ADU limits and removed owner-occupancy.
