ADUs & backyard cottages in Sammamish
What HB 1337 actually allows in Sammamish — 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 Sammamish
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Sammamish that means up to 2 ADUs on a single lot, no owner-occupancy requirement, and parking handled as none required near transit. Detached units (DADUs) can run to 1000 sf of heated space; attached units follow a different rule — aadu up to 50% of the main home's floor area.
| Rule | Sammamish |
|---|---|
| ADUs per lot | 2 |
| Owner occupancy | Not required (abolished by HB 1337) |
| Parking | None required near transit |
| Max DADU size | 1000 sf heated |
| Max AADU size | AADU up to 50% of the main home's floor area |
| Max height | 24ft |
Pre-approved plans in Sammamish
No pre-approved plan program yet. Every Sammamish 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
Sammamish permit timeline: Custom, 12-16 wk. 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
School impact fee of $10,914 per unit, plus park/street fees.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Sammamish 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
Standard utility connection; ADU excluded from FAR.
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.
Sammamish quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Sammamish:
- School impact fee $10,914/unit (high upfront liquidity needed)
- AADU capped at 50% of main home
- ADU excluded from FAR calculation
- HOA covenant battles common
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 Sammamish's old one-ADU limits and removed owner-occupancy.
