These rules apply only to UNINCORPORATED King County - places like White Center and the rest of North Highline, Skyway-West Hill, Fairwood, East Renton Highlands, Vashon-Maury Island, Fall City, the Snoqualmie Valley and the Enumclaw plateau - not to Seattle, Bellevue, Kirkland, Redmond, Sammamish, Bothell, Woodinville or any other city, each of which runs its own ADU code. King County Code 21A.08.030.B.7 allows an ADU of up to 1,000 sq ft of heated floor area plus 1,000 sq ft unheated; inside the urban growth area you may build two ADUs per lot (any mix of attached and detached) and trade unused unheated area for heated area up to 1,500 sq ft, while rural and resource lots get one ADU that must be inside the main house unless the lot meets the zone's minimum lot area (1.875 acres in RA-2.5, 3.75 in RA-5, 7.5 in RA-10, 15 in RA-20). No parking, no owner-occupancy and no street improvements are required anywhere in the county, and ADUs are exempt from the county's school impact fee. The county fee stack for a typical new detached ADU is about $10,000-$12,000 (2026 Fee Guide 02), plus $126 screening, a recorded notice on title, and Public Health septic review if you are not on sewer; ADU permits applied for since January 2025 took a median of about 64 days from complete application to approval.
Zoning & Size Requirements [1][2][3][4]
Yes — unincorporated King County allows ADUs. A detached ADU can typically be up to 1,000 sq ft under Washington state law and the County's zoning code.
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| Where ADUs Are Allowed | Permitted as a residential accessory use (P7) wherever a dwelling is allowed: urban UR and R-1 through R-48 zones, rural RA-2.5 to RA-20, and the NB/CB/RB/O commercial zones (KCC 21A.08.030.A). Agriculture (A-10/A-35) zones only as accessory to a farm residence - the Permitting Division's March 2026 ADU bulletin treats these as attached-only. Prohibited in the Forest (F) zone (KCC 21A.08.030.B.7.a.(6)) |
| Max Size (Detached) | 1,000 sq ft heated floor area plus up to 1,000 sq ft unheated (garage, storage). In the urban area, every square foot of unheated allowance you skip can become heated space, up to 1,500 sq ft heated (e.g., 1,250 heated + 750 unheated). A detached ADU's basement does not count toward the cap (KCC 21A.08.030.B.7.a.(1)) |
| 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 heated - except an ADU wholly contained in the main house's basement or attic has no size limit at all (KCC 21A.08.030.B.7.a.(1)(a)). To count as attached it must share a common wall; a breezeway only qualifies if it is under 10 ft long, enclosed, heated and shares a wall with both units |
| Max Size (JADU) | King County has no junior-ADU category. The nearest equivalents are a basement or attic ADU inside the main house (no size cap) or 'accessory living quarters' - a guest suite with no kitchen, capped at 1,000 sq ft heated, one per lot, on lots of 3,200 sq ft or more in the urban area or a rural town (KCC 21A.06.010, 21A.08.030.B.7.b) |
| Max Height | The base height of the zone - there is no ADU-specific cap: 35 ft in UR and R-1 through R-8 (60 ft in R-12 to R-48), 40 ft in RA-2.5 to RA-20, 35 ft in A zones. An ADU wholly inside an existing dwelling is exempt (KCC 21A.08.030.B.7.a.(2); tables at 21A.09.030, 21A.09D.030, 21A.09H.030, 21A.09T.030) |
| Max Units | Urban area (inside the Urban Growth Area boundary): two ADUs per lot in any configuration - one attached plus one detached, two attached, or two detached in one or two buildings (KCC 21A.08.030.B.7.a.(7)). Rural area and natural resource lands: one ADU per lot; when it shares the building with the main house only one entrance may face the street (B.7.a.(8)). A detached rural ADU counts as a separate unit if the lot is ever subdivided (B.7.a.(8)(f)) |
Setback & Parking Requirements [1][2][3][4]
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 every urban residential zone (UR, R-1 to R-48), including North Highline and Skyway-West Hill. Rural: 5 ft in RA-2.5, 10 ft in RA-5, RA-10 and RA-20 - but RA lots between 1 and 2.5 acres use the R-1 standards and RA lots under 1 acre use R-4 (5 ft). No ADU-specific setbacks (KCC 21A.09H.030, 21A.09T.030.B.5) |
| Side Setback | 5 ft interior setback in urban zones; 5 ft in RA-2.5 and 10 ft in RA-5 to RA-20 (same lot-size step-downs as the rear). Rural residences must also stay 100 ft from a property line that adjoins an A, M or F zone (KCC 21A.09T.030.B.5.a) |
| Front Setback | Urban: 10 ft street setback in R-4 through R-48 (20 ft for a garage, carport or fenced parking, measured along the driveway), 20 ft in R-1, 30 ft in UR. Rural RA: 30 ft street setback (R-1 or R-4 standards on RA lots under 2.5 acres). An urban garage or other existing structure may be converted to an ADU even if it sits inside today's setbacks (KCC 21A.08.030.B.7.a.(7)(b)) |
| Parking Required | None. 'No additional off-street parking spaces are required for accessory dwelling units' - urban and rural, any distance from transit (KCC 21A.08.030.B.7.a.(4)). No public street improvements can be required either (B.7.a.(7)(c)). This is stricter than the state floor in RCW 36.70A.681(2), which would still allow one or two spaces away from major transit |
| Owner Occupancy | Not required anywhere in the county. Ordinance 19881 (Dec. 10, 2024) struck the old rule that the owner or an immediate family member live in one of the units. What remains is a recorded Notice on Title identifying the unit as accessory, which runs with the land and must be recorded before the permit is issued (KCC 21A.08.030.B.7.a.(5)). State law (RCW 36.70A.681(1)(k)) also bars the county from blocking the separate sale of an ADU as a condominium unit; county code is silent on it - ask Permitting and a real-estate attorney before planning a sale |
| Min Lot Size | Urban area: none - two ADUs are allowed on any lot with a dwelling. Rural area and resource lands: an ADU inside the main house needs no minimum, but a DETACHED ADU requires no more than one primary dwelling on the lot and a lot at or above the zone's minimum lot area - 1.875 acres in RA-2.5, 3.75 acres in RA-5, 7.5 acres in RA-10, 15 acres in RA-20, 10 acres in A-10, 35 acres in A-35 - or at least 3,200 sq ft inside the Vashon, Fall City or Snoqualmie Pass rural towns (KCC 21A.08.030.B.7.a.(8)(b); lot areas in 21A.09T.030) |
Permit Process & Fees [5][6][7][8][9][10][11][12][13][14]
An ADU in unincorporated King County is approved administratively — no public hearing. Typical timeline: Code targets since Jan. 1, 2025: completeness decision within 28 days of submittal (KCC 20.20.050.B.2) and a final decision within 65 days of completeness for no-notice permits (KCC 20.20.100.A.1, Ord. 19872). Actual data from the county's issued-permit reports: the 32 ADU permits applied for after Jan. 1, 2025 and issued through Aug. 2026 took a median 64 days from complete application to approval (middle half 36-89 days), and a median 114 days from intake to issuance for all 42 ADU permits issued in 2025-26 (the gap is mostly applicant time: corrections, notice on title, fee payment). Permit fees: 2026 Fee Guide 02 (Ord. 20021, in effect through Dec. 31, 2026): $4,556 building/zoning/site plan review + $3,618 drainage, right-of-way and critical-area review + $2.46 per sq ft inspection + $126 application screening + $6.50 state surcharge. An 800 sq ft detached ADU is about $10,275; 1,000 sq ft about $10,767; 1,500 sq ft about $11,997. Add $1,119 review + $1,997 inspection if fire sprinklers are required. A basement or garage conversion is billed as 'addition or conversion to living space': $3,109 review + $2.46/sq ft + site review of $483-$3,618 depending on complexity. Most fees rose about 14% on Jan. 1, 2026.
| Approval Process | Type 1 administrative building permit - no public notice, no hearing (KCC 20.20.020). Apply online at MyBuildingPermit.com under King County > Building > Single Family Residential > New Construction > Accessory Dwelling Unit. Record the ADU Notice on Title affidavit with the Recorder's Office (Permitting can send it for you and add the recording fee to your invoice) - proof of recording is required before the permit issues. Lots on septic need a Public Health building-permit clearance (see impact_fees); sewered lots may need a sewer availability certificate from the local district. Rural applicants are asked to design the unit to match the main house and consider a siting study for neighbors' privacy and views (KCC 21A.08.030.B.7.a.(8)(d)-(e)) |
| Review Timeline | Code targets since Jan. 1, 2025: completeness decision within 28 days of submittal (KCC 20.20.050.B.2) and a final decision within 65 days of completeness for no-notice permits (KCC 20.20.100.A.1, Ord. 19872). Actual data from the county's issued-permit reports: the 32 ADU permits applied for after Jan. 1, 2025 and issued through Aug. 2026 took a median 64 days from complete application to approval (middle half 36-89 days), and a median 114 days from intake to issuance for all 42 ADU permits issued in 2025-26 (the gap is mostly applicant time: corrections, notice on title, fee payment) |
| Permit Fees | 2026 Fee Guide 02 (Ord. 20021, in effect through Dec. 31, 2026): $4,556 building/zoning/site plan review + $3,618 drainage, right-of-way and critical-area review + $2.46 per sq ft inspection + $126 application screening + $6.50 state surcharge. An 800 sq ft detached ADU is about $10,275; 1,000 sq ft about $10,767; 1,500 sq ft about $11,997. Add $1,119 review + $1,997 inspection if fire sprinklers are required. A basement or garage conversion is billed as 'addition or conversion to living space': $3,109 review + $2.46/sq ft + site review of $483-$3,618 depending on complexity. Most fees rose about 14% on Jan. 1, 2026 |
| Impact Fees | None for ADUs. The county's only impact fee is the school impact fee (KCC 21A.43) and accessory dwelling units are expressly excluded (KCC 21A.43.070.A.9; 2026 Fee Guide 03) - better than the 50% cap in RCW 36.70A.681(1)(a). Two other charges are real: (1) if your sewage flows to a King County treatment plant, the Wastewater Treatment Division capacity charge for an ADU is 0.59 RCE x $77.99 = $46.01/month for 15 years (about $8,280; 2026 rate; payable early at a discount) on top of the local sewer district's connection fee; (2) on septic, Public Health - Seattle & King County must approve the building application ($338 review + $450 inspector review = $788 for a new building with plumbing, 2026 OSS fee schedule) and the drainfield must be sized for every bedroom on the property at 150 gallons/day per bedroom - a separate ADU system may be designed for a 2-bedroom minimum (OSS memo, Apr. 15, 2019). New or expanded septic: $731-$1,181 design review plus $1,069-$1,181 installation permit |
| Plan Check | Full plan review by the Department of Local Services Permitting Division (206-296-6600). Expect a critical-areas check (KCC 21A.24: wetlands, streams, steep slopes, landslide and flood hazards) with the site review; the Public Health septic memo notes a Critical Area Designation review ($1,643 minimum per site, Fee Guide 01) is required with septic work. Optional pre-application conference $1,452 per staff reviewer, creditable against permit fees if you apply within 180 days. Fire sprinklers are priced separately 'if required' ($1,119 review + $1,997 inspection) - ask Permitting whether your lot's fire district requires them |
| Pre-Approved Plans | No. King County publishes no pre-approved or standard ADU plan set (none on the Permitting Division's forms and resources pages as of Sept. 2026); you need custom or designer plans. The county's 'registered plan' fee category is for builders re-using their own approved basic-home plans |
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Construction Costs [15][16][17][18]
Building an ADU in unincorporated King County typically costs $250,000-$550,000 all-in for a new detached ADU in the Seattle metro (2026 builder guides; design, permits, utilities and construction); $100,000-$250,000 for a basement or attached conversion. County permit valuations tell a lower story: the 42 ADU permits issued in unincorporated King County in 2025-26 carried a median declared construction value of $168,000 (middle half $121,000-$232,000), which excludes design, fees and site work ($350-$650 per sq ft all-in for new construction, per Seattle-area 2026 guides (the low end is a simple one-story unit on a flat, sewered lot; the high end is a two-story unit with septic, a long utility run or steep-slope work) per sq ft); a garage conversion runs $80,000-$200,000 (Seattle-area 2026 guides); more if the slab, foundation or roof must be rebuilt. On septic, the Public Health review for a garage-to-living-space conversion is $788.
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Construction costs in unincorporated King County track the surrounding metro market. Rural parcels can cost more because of septic, well, access and fire-hazard requirements, so treat these as planning ranges and get local bids.
| Average Build Cost | $250,000-$550,000 all-in for a new detached ADU in the Seattle metro (2026 builder guides; design, permits, utilities and construction); $100,000-$250,000 for a basement or attached conversion. County permit valuations tell a lower story: the 42 ADU permits issued in unincorporated King County in 2025-26 carried a median declared construction value of $168,000 (middle half $121,000-$232,000), which excludes design, fees and site work |
| Cost per Sq Ft | $350-$650 per sq ft all-in for new construction, per Seattle-area 2026 guides (the low end is a simple one-story unit on a flat, sewered lot; the high end is a two-story unit with septic, a long utility run or steep-slope work) |
| Garage Conversion | $80,000-$200,000 (Seattle-area 2026 guides); more if the slab, foundation or roof must be rebuilt. On septic, the Public Health review for a garage-to-living-space conversion is $788 |
| Prefab ADU (Installed) | $200,000-$400,000 installed. Factory units are advertised from roughly $100,000-$250,000 (Abodu from $278,800), but the unit is typically only 40-60% of the finished cost once foundation, utilities, crane, delivery and permits are added (2026 industry guide) |
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Rental Income Potential [19]
| Avg Rent (1 Bedroom) | $1,850-$2,600/mo gross (HUD FY2026 Small Area Fair Market Rents, 40th percentile incl. utilities): $1,870 in White Center 98146, $2,020 in Skyway 98178, $2,200 in Fairwood 98058, $2,310 in East Renton 98059, $1,850 on Vashon 98070, $2,570 in the Woodinville-area 98077. Builder guides put market 1BR DADU rents at $1,800-$2,500 |
| Avg Rent (2 Bedroom) | $2,160-$3,000/mo gross (HUD FY2026 SAFMR): $2,180 White Center, $2,360 Skyway, $2,560 Fairwood, $2,690 East Renton, $2,160 Vashon, $2,990 Woodinville-area 98077, $3,570 Snoqualmie 98065 |
| Short-Term Rental | Not addressed. Title 21A contains no short-term-rental rules (Sept. 9, 2026 code compilation) and the county's ADU bulletin describes ADUs as long-term rentals or family housing. State law lets the county restrict STR use of ADUs (RCW 36.70A.680(5)(a)) but King County has not adopted such a rule - confirm with Permitting before planning an STR |
| Est. Annual ROI | Roughly 5-9% gross annual rent yield: a unit renting for $2,200/mo brings in $26,400/yr against a $300,000-$500,000 new-build cost, before taxes, insurance, vacancy and the sewer capacity charge. Basement and garage conversions at $100,000-$200,000 yield far more (market estimate) |
Recent Law Changes
What's Changed Recently
- June 11, 2026: EHB 1345 (2026 c 231) took effect, creating a state framework for counties that allow detached ADUs outside urban growth areas (one ADU per parcel, 1,296 sq ft cap, within 150 ft of the main house, metered wells). Section 1(4)(a) preserves county ADU ordinances adopted before that date, so King County's Ordinance 19881 rural rules still govern; the Sept. 9, 2026 code compilation shows no change to KCC 21A.08.030.B.7
- March 2026: the Permitting Division reissued its Residential ADU information sheet reflecting the post-Ordinance 19881 rules (two ADUs per urban lot, one per rural lot, attached-only in the A zone)
- January 1, 2026: Ordinance 20021 raised most permit fees about 14% (ADU review $3,987 to $4,556; site review $3,166 to $3,618; inspection $2.16 to $2.46/sq ft), added a $126 application screening fee and a 3.5% permit-software surcharge; Ordinance 20016 reset school impact fees, from which ADUs remain exempt. WTD's sewer capacity charge rose to $77.99 per RCE/month (ADU = 0.59 RCE)
- January 1, 2025: Ordinance 19872 set the permit clocks now in KCC 20.20 - 28 days to a completeness decision and 65 days to a decision on no-notice permits such as ADU building permits (county said this cut the average from 119 days)
- December 10, 2024: Ordinance 19881 (2024 Comprehensive Plan update) implemented HB 1337 and went further - two ADUs per urban lot, owner/family-occupancy requirement repealed countywide, zero parking countywide, urban heated-area trade-up to 1,500 sq ft, garage conversions allowed inside nonconforming setbacks, the old 3,200 sq ft urban minimum for detached ADUs deleted, and dimensional standards moved into geography chapters (21A.09 North Highline, 21A.09D Skyway-West Hill, 21A.09H other urban, 21A.09T rural)
Tips for Building an ADU in Unincorporated King County
- First confirm you are unincorporated: use the county's Property research and maps page to check jurisdiction and zoning. White Center, Skyway-West Hill, Fairwood, East Renton Highlands, Vashon and the rural valleys are county; if your address is inside Seattle, Bellevue, Kirkland, Redmond, Sammamish, Bothell or Woodinville, use that city's page instead
- Urban lot? Use the trade-up: skip the unheated garage/storage allowance and build up to 1,500 sq ft of heated space, or build two units - a basement ADU inside the house (no size cap) plus a detached cottage
- Rural RA lot below the zone minimum (e.g., 2 acres in RA-2.5, which needs 1.875 acres - fine; 3 acres in RA-5, which needs 3.75 - not fine)? Your one ADU must be inside the main house, and only one entrance may face the street
- On septic, get the Public Health bedroom count sorted before you design: the system must handle every bedroom on the property at 150 gallons/day each, and older drainfields often need a new or separate system ($731-$1,181 design review plus $1,069-$1,181 permit)
- Budget the real fee stack: about $10,000-$12,000 in county permit fees, $788 Public Health clearance if on septic, and on sewer the $46.01/month WTD capacity charge for 15 years plus your district's connection fee - but no impact fees and no parking
- Rural impervious-surface caps (25% in RA-2.5, 20% RA-5, 15% RA-10, 12.5% RA-20; lots of half an acre or more always get at least 10,000 sq ft) are what usually limit a detached ADU plus its driveway - check the total before buying plans
ADU Builders in King County
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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 (compilation updated Sept. 9, 2026) - KCC 21A.06.010, 21A.06.350, 21A.08.030.A and .B.7, 21A.09.030, 21A.09D.030, 21A.09H.030, 21A.09T.030, 21A.43.070 Municipal Code — Accessed 2026-09-19
- Ordinance 19881 signature report (2024 King County Comprehensive Plan code update, passed Dec. 10, 2024) - Section 160 amending KCC 21A.08.030 Government Data — Accessed 2026-09-19
- RCW 36.70A.680 and 36.70A.681 - Accessory dwelling units, local regulation and limitations (HB 1337, 2023 c 334; as amended 2025 c 148) Government Data — Accessed 2026-09-19
- EHB 1345, Chapter 231, Laws of 2026 - detached accessory dwelling units outside urban growth areas (effective June 11, 2026) Government Data — Accessed 2026-09-19
- Residential Accessory Dwelling Units (ADUs) information sheet - King County Permitting Division (March 2026) Planning Department — Accessed 2026-09-19
- 2026 Fee Guide 02 - Single Family Residential Construction (King County Permitting, Dec. 2025) Planning Department — Accessed 2026-09-19
- 2026 Fee Guide 03 - School Impact Mitigation Measures (ADU exemption) and 2026 Fee Guide 01 - Pre-Development Services Planning Department — Accessed 2026-09-19
- Permit fees - Building and land use (2026 changes, Ordinance 20021 and 20016) - King County Permitting Planning Department — Accessed 2026-09-19
- King County Code Title 27 Development Permit Fees (updated Mar. 2, 2026) - KCC 27.02.202, 27.10.015, 27.10.035 Municipal Code — Accessed 2026-09-19
- King County Code Title 20 Planning (updated May 20, 2026) - KCC 20.20.020, 20.20.050, 20.20.100 permit time periods Municipal Code — Accessed 2026-09-19
- Accessory Dwelling Unit Notice on Title Requirements - King County Permitting Division Planning Department — Accessed 2026-09-19
- Issued Permits Reports, monthly Jan. 2025 - Aug. 2026 (Dwelling-ADU permit type) - King County Permitting reports page Government Data — Accessed 2026-09-19
- On-site sewage system memo: Accessory Dwelling Units (Apr. 15, 2019) and OSS fee schedule effective Jan. 1, 2026 - Public Health Seattle & King County Government Data — Accessed 2026-09-19
- About the capacity charge (2026 rate, ADU = 0.59 RCE) - King County Wastewater Treatment Division Government Data — Accessed 2026-09-19
- Seattle ADU Cost 2026 guide - Kolmo (Jan. 8, 2026) industry — Accessed 2026-09-19
- ADU Cost in Seattle 2026, all 4 types compared - Remodel Right Seattle (Feb. 2026) industry — Accessed 2026-09-19
- Cost to Build a DADU in Seattle (2025-2026 update) - Ballard Backyard Cottages industry — Accessed 2026-09-19
- Prefab Modular ADU: 2026 costs - Dwelling Index (updated May 27, 2026) industry — Accessed 2026-09-19
- HUD FY2026 Fair Market Rents and Small Area FMRs - Seattle-Bellevue, WA HUD Metro FMR Area (King County) Government Data — Accessed 2026-09-19
- Property research and maps (confirm jurisdiction, zoning, critical areas) - King County Local Services Planning Department — Accessed 2026-09-19
- Council approves permitting update (Ordinance 19872) - King County Council news release, Dec. 3, 2024 Government Data — Accessed 2026-09-19
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?
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