If your property is in unincorporated King County — outside the limits of Seattle, Bellevue, Renton, Kent, and the county's other cities — your ADU rules come from King County Code Title 21A and are administered by King County DPER (Department of Permitting and Environmental Review), not Seattle DCI.
The rules differ significantly depending on whether you're inside an Urban Growth Area (UGA) or in a rural area. This guide covers both.
Who Has Jurisdiction
All permitting in unincorporated King County goes through King County DPER. This is a different agency, code, and process than Seattle DCI — fees, timelines, and submittal requirements all differ.
If you're not certain whether your address is in the city or the county, check the King County Parcel Viewer or our Seattle vs King County comparison.
Source: King County DPER, KCC Title 21A
Zoning Districts That Allow ADUs
Urban Residential Zones (R-1 to R-12)
King County's residential zones run from R-1 (lowest density, ~1 unit/acre) up to R-12, R-18, and R-24 (highest density). The higher the number, the more density allowed. Most lots in R-4 through R-12 qualify for an ADU under Title 21A.
Rural Area (RA) Zones
RA-2.5, RA-5, and RA-10 designate rural areas where minimum lot sizes are measured in acres. ADUs are allowed but rural parcels carry larger lot-size expectations and almost always rely on septic and a private well.
Two ADUs Per Lot
Consistent with HB 1337, unincorporated King County permits up to two ADUs per qualifying lot in urban zones, in attached or detached configurations.
No Owner-Occupancy
King County has removed owner-occupancy requirements in line with HB 1337. You do not have to live on the property to build or rent an ADU.
Source: KCC Title 21A (zoning)
Urban Growth Area vs. Rural
Inside the UGA
UGA parcels allow more density, smaller minimum lot sizes, and frequently have access to public sewer. These lots behave most like a city lot and carry the lowest infrastructure cost.
Best case for ADU feasibility in the county.
Rural Area
Rural parcels face larger minimum lot sizes, septic and well requirements, and more frequent critical-areas review. ADUs are allowed but the path is longer and more expensive.
Possible, but budget for septic and a longer timeline.
Lot Size and ADU Size
Minimum lot size and the maximum ADU size both vary by zone under Title 21A. In urban zones (roughly R-4 through R-12) most standard lots qualify. In rural RA zones you generally need a larger parcel, especially for a detached ADU.
HB 1337's statewide protections — including the 1,000 sq ft minimum size cap — apply to King County just as they do to cities, so the county cannot restrict ADU size below the state floor in areas where HB 1337 applies.
Source: KCC Title 21A, RCW 36.70A.681
Septic Systems: The Rural Cost Variable
For rural parcels not on public sewer, the septic system is the single biggest cost and feasibility variable. Adding an ADU adds bedrooms and load — which often means upgrading or replacing the existing system. Expect $15,000–$30,000, plus Public Health – Seattle & King County review.
- RA zones require larger parcels — detached ADUs generally need acreage rather than a standard city lot
- Septic capacity is the single biggest cost variable — a new or upgraded system runs $15,000–$30,000
- A private well or water-association connection must serve the ADU
- Critical-areas review (steep slopes, wetlands, streams, aquifer recharge) is common and adds time
Permit Process Through DPER
Applications are submitted to King County DPER. Typical review runs 8–14 weeks, and longer when critical-areas review (steep slopes, wetlands, streams, aquifer protection) is triggered or septic design is involved.
UGA Timeline
Faster, especially with public sewer — closer to the lower end of the range.
Rural Timeline
Longer — septic design and critical-areas review push toward the upper end or beyond.
Source: King County DPER
Why Being In or Out of City Limits Matters
The difference between a Seattle parcel and an unincorporated King County parcel can change your project's cost, timeline, and complexity dramatically — different permitting agency, different code, often sewer vs. septic, and different critical-areas exposure.
Annexation and UGA boundaries can also place very similar-looking lots under entirely different rules. See our full Seattle vs. King County comparison before you assume which rules apply.
County permitting can be complex. Let our permit specialist handle it — your investment is applied as a build deposit. See full permitting cost details.
Official Sources
- King County DPER: kingcounty.gov
- King County Code Title 21A (zoning): King County Code
- King County Parcel Viewer (check your jurisdiction): Parcel Viewer
Last updated: March 2026. Not legal advice. Verify current regulations with King County DPER.
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