ADUs & backyard cottages in Unincorporated King County
What HB 1337 actually allows in Unincorporated King County — 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 Unincorporated King County
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Unincorporated King County 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 — 1,000sf heated + 1,000sf unheated allowed; basements/attics do not count toward the cap.
| Rule | Unincorporated King County |
|---|---|
| ADUs per lot | 2 |
| Owner occupancy | Not required (abolished by HB 1337) |
| Parking | None required |
| Max DADU size | 1000 sf heated |
| Max AADU size | 1,000sf heated + 1,000sf unheated allowed; basements/attics do not count toward the cap |
| Max height | 24ft |
Pre-approved plans in Unincorporated King County
No pre-approved plan program yet. Every Unincorporated King County 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
Unincorporated King County permit timeline: King County DPER custom review, variable. 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
Permit fees up roughly 49% since January 2025.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Unincorporated King County 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
Septic (OSS) reviewed by PHSKC; shared OSS re-evaluated as multifamily (drainfield expansion).
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.
Unincorporated King County quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Unincorporated King County:
- 2 ADUs in UGA, but only 1 in Rural zones
- Rural DADU requires minimum acreage (RA-2.5 = 1.875 acres)
- 1,000sf heated + 1,000sf unheated allowed
- Permit fees +49% since Jan 2025
- Septic/PHSKC review on OSS properties
Up to 2 ADUs on a single lot inside the Urban Growth Area, but only 1 in Rural zones. HB 1337 set the floor; King County's rural acreage minimums (RA-2.5 needs 1.875 acres for a detached unit) still limit what you can detach.
