ADUs & backyard cottages in Seattle
What HB 1337 actually allows in Seattle — 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 Seattle
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Seattle that means up to 2 ADUs on a single lot, no owner-occupancy requirement, and parking handled as none required. Detached units (DADUs) can run to 1000 sf of heated space; attached units follow a different rule — aadu governed by far (floor-area ratio), not a flat cap.
| Rule | Seattle |
|---|---|
| ADUs per lot | 2 |
| Owner occupancy | Not required (abolished by HB 1337) |
| Parking | None required |
| Max DADU size | 1000 sf heated |
| Max AADU size | AADU governed by FAR (floor-area ratio), not a flat cap |
| Max height | 24ft |
Pre-approved plans in Seattle
ADUniverse is live and free to use. Picking a pre-reviewed plan instead of a custom design is the single biggest lever on your permit timeline in Seattle — ADUniverse. The plans are already vetted against city code, so review collapses to weeks instead of months.
Permit timeline
Seattle permit timeline: 2-6 wk with pre-approved plan; 12+ wk for custom. A pre-approved plan is the fastest route — custom designs add review cycles. Run the numbers for your own lot in the feasibility tool before you commit.
Fees & what you'll pay
King County sewer capacity charge applies.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Seattle 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
Shoreline overlay within 200ft of major water adds +4-8 wk and +$5-15K in consulting.
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.
Seattle quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Seattle:
- ADUniverse pre-approved plan library (free)
- Shoreline Management Act overlay within 200ft of water
- Short-term rentals limited to primary residence
- Height ranges 18-24ft by configuration
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 Seattle's old one-ADU limits and removed owner-occupancy.
