These rules apply to UNINCORPORATED King County - communities like White Center (North Highline), Skyway-West Hill, Fairwood, Cottage Lake, and East Renton Highlands - not Seattle, Bellevue, or other incorporated cities, which run their own ADU programs. Ordinance 19881 (December 2024) implemented Washington's HB 1337 and went further: two ADUs per urban lot in any configuration, no owner-occupancy requirement anywhere in the county, zero ADU parking minimums countywide (urban and rural), and a floor-area trade-up that lets urban ADUs reach 1,500 sq ft of heated space. Standards live in KCC 21A.08.030.B.7 with dimensions in the geography chapters 21A.09H (urban) and 21A.09T (rural).
Zoning & Size Requirements [1][2]
Yes — unincorporated King County allows ADUs. A detached ADU can typically be up to 1,500 sq ft under Washington state law and the County's zoning code.
See what this means for your lot — get 3 free quotes from King County ADU builders →
| Where ADUs Are Allowed | Permitted accessory use in urban residential zones (R-1 to R-48) and rural RA zones per KCC 21A.08.030; attached-only in the Agriculture (A) zone, prohibited in the Forest (F) zone |
| Max Size (Detached) | 1,000 sq ft heated + up to 1,000 sq ft unheated; in the urban area unused unheated allowance trades 1:1 for heated space up to 1,500 sq ft heated (basement floor area doesn't count) |
| Max Size (Attached) | Same 1,000 sq ft heated / 1,000 sq ft unheated limit with the same urban trade-up to 1,500 sq ft - except an ADU wholly inside the home's basement or attic has no size limit at all |
| Max Size (JADU) | No JADU category in county code - the equivalent play is a basement or attic ADU inside the primary home, which is size-unlimited |
| Max Height | Base zone height applies (no ADU-specific cap): 35 ft in urban R-1 to R-8 zones, 40 ft in rural RA zones (KCC 21A.09H.030 / 21A.09T.030) |
| Max Units | Urban area (inside the growth boundary): two ADUs per lot in any mix - two detached, two attached, or one of each. Rural area: one ADU per lot, inside the main home unless the lot meets the zone's minimum area. |
Setback & Parking Requirements [1][2]
Unincorporated King County requires a rear setback of about 5 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.
| Rear Setback | 5 ft interior setback in urban residential zones (base zone standard - no ADU-specific setbacks); rural RA: 5 ft in RA-2.5, 10 ft in RA-5 to RA-20 |
| Side Setback | 5 ft interior in urban zones; 10 ft in most rural RA zones |
| Front Setback | 10 ft street setback in urban zones (20 ft for street-facing garages); 30 ft in rural RA zones. Urban garage conversions may keep nonconforming setbacks |
| Parking Required | None - 'no additional off-street parking spaces are required for accessory dwelling units,' countywide, urban and rural (KCC 21A.08.030.B.7.a.(4)); no street improvements required either |
| Owner Occupancy | Not required - urban or rural. The old owner/family-occupancy rule was struck by Ordinance 19881 in December 2024; only an ADU notice on title must be recorded before permit approval |
| Min Lot Size | None for the first ADU. A rural detached ADU (outside the primary home's building) requires the zone's minimum lot area: 1.875 ac in RA-2.5, 3.75 ac in RA-5, 7.5 ac in RA-10, 15 ac in RA-20 |
Permit Process & Fees [3][4]
An ADU in unincorporated King County is approved ministerially (by-right) within the County's 65-day review target for no-hearing building permits, after a 28-day completeness check; permit fees are roughly $10,000-$12,000 for a typical 800-1,000 sq ft detached ADU per the 2026 fee guide ($4,556 review minimum + $3,618 site review + $2.46/sq ft inspections; most fees rose ~14% on Jan 1, 2026).
| Approval Process | By-right administrative building permit - no public notice or hearing. Apply at MyBuildingPermit.com (King County > Building > Single Family Residential > New Construction > ADU); record the ADU Notice on Title affidavit before approval; septic approval or sewer certificate may be required |
| Review Timeline | the County's 65-day review target for no-hearing building permits, after a 28-day completeness check |
| Permit Fees | roughly $10,000-$12,000 for a typical 800-1,000 sq ft detached ADU per the 2026 fee guide ($4,556 review minimum + $3,618 site review + $2.46/sq ft inspections; most fees rose ~14% on Jan 1, 2026) |
| Impact Fees | School impact fees and septic/critical-areas review can add substantially on top of base permit fees - get a written estimate from Permitting before budgeting |
| Plan Check | Standard plan review through the Department of Local Services Permitting Division; a $126 application screening fee applies since January 2026 |
| Pre-Approved Plans | No county pre-approved ADU plan library as of mid-2026 - custom or designer plans required |
Get Free ADU Quotes in King County
Tell us about your project and we'll connect you with ADU builders serving King County. Free, no obligation — you choose who to work with.
Takes about a minute.
Up to 3 independent local ADU builders — never more, and we tell them nothing you don't submit here.
Compare quotes, verify license & insurance, hire (or don't). No spam either way.
🔒 Your info is secure. We share it only with contractors quoting your project. No spam, no obligation.
Just browsing? Search ADU builders near you by ZIP code.
Construction Costs [5]
Building an ADU in unincorporated King County typically costs $160,000-$340,000 (Seattle-metro market estimate) ($170-$330 (market estimate) per sq ft); a garage conversion runs $80,000-$180,000 (market estimate).
See what this means for your lot — get 3 free quotes from King County ADU builders →
Construction costs in unincorporated King County mirror the surrounding Inland Empire market. Rural parcels can cost more due to septic, well, access, and fire-hazard (WUI) requirements, so treat these as planning ranges and get local bids.
| Average Build Cost | $160,000-$340,000 (Seattle-metro market estimate) |
| Cost per Sq Ft | $170-$330 (market estimate) |
| Garage Conversion | $80,000-$180,000 (market estimate) |
| Prefab ADU (Installed) | $130,000-$280,000 (market estimate) |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
Some links on this page are affiliate links. We may earn a commission at no extra cost to you. Learn more
Rental Income Potential [6]
| Avg Rent (1 Bedroom) | $1,700-$2,400/mo (varies widely: Skyway/White Center below Eastside communities) |
| Avg Rent (2 Bedroom) | $2,100-$3,000/mo |
| Short-Term Rental | The county zoning code has no short-term-rental provisions; the county's ADU bulletin frames ADUs as long-term rentals - confirm with Permitting before planning an STR |
| Est. Annual ROI | 6-8% annual return (market estimate) |
Recent Law Changes
What's Changed Recently
- December 10, 2024: Ordinance 19881 (2024 Comprehensive Plan update) implemented HB 1337 - two ADUs per urban lot, owner-occupancy requirement repealed countywide, ADU parking minimums eliminated, urban heated-area trade-up to 1,500 sq ft, and garage conversions allowed even with nonconforming setbacks
- December 10, 2024: the same ordinance moved dimensional standards into geography-based chapters (21A.09H urban, 21A.09T rural, plus North Highline and Skyway-West Hill chapters)
- January 1, 2026: permit fees rose about 14% and a $126 application screening fee was added (Ordinance 20021)
- March 2026: the Permitting Division reissued its Residential ADU bulletin reflecting the post-Ordinance 19881 rules
Tips for Building an ADU in Unincorporated King County
- White Center, Skyway, Fairwood, Cottage Lake, and East Renton Highlands are unincorporated - these County rules apply there, not Seattle's or Renton's
- Use the urban trade-up: skip the unheated garage/storage allowance and build up to 1,500 sq ft of heated living space instead (e.g., 1,250 heated + 750 unheated also works)
- A basement or attic ADU inside your home has no size cap at all - often the cheapest path to a second unit
- On rural RA lots below the zone's minimum acreage, your ADU must be inside the primary home - check your zone (RA-2.5 needs 1.875 acres for a detached ADU)
- Budget for the county's fee stack (~$10-12K on a typical detached ADU) plus septic review if you're not on sewer
ADU Builders in King County
Looking for an ADU builder in King County? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match King County homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Cities Within King County
If your property is inside one of these incorporated cities, follow that city's ADU code — not the County rules above:
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- King County Code Title 21A Zoning (May 2026 compilation) - KCC 21A.08.030.B.7, 21A.09H.030, 21A.09T.030 Planning Department — Accessed 2026-07-30
- Ordinance 19881 signature report (2024 Comprehensive Plan code update) Government Data — Accessed 2026-07-30
- Residential ADUs information sheet - King County Permitting Division (March 2026) Planning Department — Accessed 2026-07-30
- King County 2026 Fee Guide 02 - Single Family Residential Construction Planning Department — Accessed 2026-07-30
- Angi - ADU Construction Cost Data Market Data — Accessed 2026-07-30
- HUD Fair Market Rents Government Data — Accessed 2026-07-30
- MRSC - Guidance for Accessory Dwelling Units in Washington State (HB 1337 / RCW 36.70A.680) Government Data — Accessed 2026-07-30
Frequently Asked Questions
What are the ADU rules in unincorporated King County?
How much does an ADU permit cost in King County?
Do I need owner-occupancy for an ADU in King County?
Is my property in unincorporated King County or inside a city?
How many ADUs can I build in unincorporated King County?
Building in Unincorporated King County?
Get free, no-obligation quotes from ADU contractors who work in King County.
Get Free Quotes →