Can I build a backyard home in King County (unincorporated)?
Since 2023, Washington law lets most city lots hold up to two more homes. King County (unincorporated) writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.
What King County (unincorporated) allows.
The headline standards. Open any card for the full wording and the code section.
ADUs per lotUrban growth area: up to 2. Rural: 1. Forest: noneFull wording
Urban growth area: up to 2. Rural: 1. Forest: none.
Max size1,000 sq ft heated + 1,000 sq ft unheatedFull wording
1,000 sq ft heated plus 1,000 sq ft unheated (KCC 21A.08.030 B.7.a). In the urban area heated area may rise to 1,500 sq ft where the unheated allowance is not fully used. DADU basement area is excluded.
Max heightThe zone’s base height: 35 ft in R-4 / R-6 / R-8Full wording
Base height of the zone — an ADU not wholly within an existing dwelling 'shall not exceed the base height for the applicable zone' (KCC 21A.08.030.B.7.a.(2)); 35 ft in R-4/R-6/R-8 (KCC 21A.09H.030), 40 ft in RA-2.5 through RA-20 (KCC 21A.09T.030)
Front setback10 ft in R-4 / R-6 / R-8 (20 ft to a garage)Full wording
R-4/R-6/R-8: minimum street setback 10 ft; 20 ft for garages, carports or fenced parking (KCC 21A.09H.030 table). RA-2.5 through RA-20: 30 ft (KCC 21A.09T.030; RA lots of 1–2.5 ac use R-1 setbacks, under 1 ac use R-4)
Side setback5 ft in R-4 / R-6 / R-8Full wording
R-4/R-6/R-8: minimum interior setback 5 ft (KCC 21A.09H.030). RA-2.5 5 ft; RA-5/10/20 10 ft (KCC 21A.09T.030). One 'interior setback' applies to all non-street lot lines (KCC 21A.12.110.A); converted structures may remain 'legally nonconforming with respect to setbacks' (21A.08.030.B.7.a.(7)(b))
Rear setback5 ft in R-4 / R-6 / R-8Full wording
Same interior setback as the side: R-4/R-6/R-8 5 ft (KCC 21A.09H.030); RA-2.5 5 ft, RA-5/10/20 10 ft (KCC 21A.09T.030)
Lot coverageImpervious: 55% R-4, 70% R-6, 75% R-8Full wording
No building-coverage standard; maximum impervious surface R-4 55%, R-6 70%, R-8 75% (KCC 21A.09H.030; R-4–R-6 lots under 9,076 sq ft use the nearest R-6/R-8 standard). RA-2.5 25%, RA-5 20%, RA-10 15%, RA-20 12.5%; RA lots of one-half acre or more get at least 10,000 sq ft (KCC 21A.09T.030 note 12)
ParkingNo additional off-street parking requiredFull wording
No additional off-street parking required
Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.
Living there, selling it, plans, time and fees.
Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.
Are backyard homes allowed?
Yes in urban growth areas and rural zones; none in Forest zones
Minimum lot size
Rural (RA-2.5/5/10/20): a DADU is allowed only if the lot meets the zone minimum. Rural towns: 3,200 sq ft.
Do you have to live there?
No — KCC 21A.08.030.B.7.a has no owner-occupancy standard; a notice 'that identifies the dwelling unit as accessory' must be recorded before permit approval and 'shall run with the land' (21A.08.030.B.7.a.(5))
Can it be sold on its own?
Not addressed in KCC 21A.08.030.B.7.a; the RCW 36.70A.681 floor applies inside the UGA — confirm the process with the county
Pre-approved plans
None found
How long the city takes
Permitting Division '2025 County Review Timelines': building permits with no public notice (example: home remodel) 65 county review days; land use with public notice 100; public hearing 170. Completeness determination within 28 days; the clock counts only time the application is with the county (https://kingcounty.gov/en/dept/local-services/certificates-permits-licenses/permits/permits-inspections-codes-buildings-land-use/permit-forms-application-materials/timelines)
Impact fees
School impact fees: ADUs exempt — 'Accessory dwelling units as defined in K.C.C. 21A.06.350 and K.C.C. 21A.08.030.B.7.a.' (KCC 21A.43.070.A.9). No county transportation or parks impact-fee chapter found in Titles 14 or 21A (2026-10-08). Public-health approval or a sewer certificate may be required (county ADU bulletin, Mar. 2026)
The code is half the answer. Your parcel is the other half.
Lines and easements
Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.
Critical areas and slope
Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.
Sewer, septic, water, trees
Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.
What King County (unincorporated) allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.
Where this page comes from.
Read from the King County (unincorporated) municipal code and the city’s own pages on October 9, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the county.
- cdn.kingcounty.gov/-/media/king-county/depts/local-services/permits/building-land-use-permits/r/residential-accessory-dwelling-units-information.pdf
- aqua.kingcounty.gov/council/clerk/code/24-30_Title_21A.htm (KCC 21A.08.030, 21A.09H.030, 21A.09T.030, 21A.12.100/.110, 21A.18.030, 21A.43.070)
- kingcounty.gov/en/dept/local-services/certificates-permits-licenses/permits/permits-inspections-codes-buildings-land-use/permit-forms-application-materials/timelines
- RCW 36.70A.681 — the state floor for accessory dwelling units
Find out what your property could become.
Type your address. Within a few hours: what King County (unincorporated) allows, what could fit, and a planning cost range. Free, no obligation.