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Building an ADU in unincorporated King County

City code and Washington law · Last reviewed October 10, 2026

This page covers only unincorporated King County, the areas outside any city. If you live in a city, such as Seattle or Kirkland, your city's rules apply instead. In the county's urban areas you can build two accessory dwelling units (ADUs) on a lot. In rural areas you can build one. An ADU is generally limited to 1,000 square feet of heated floor area, no extra parking is required, and you apply through MyBuildingPermit.com. [1][2][3]

King County at a glance

Where this page applies
Unincorporated King County only Not inside city limits [2]
ADUs per lot (urban area)
Two: one attached and one detached, two attached, or two detached [1][3]
ADUs per lot (rural area and resource lands)
One Detached only if the lot has no more than one main home and meets the zone's minimum lot area (3,200 sq ft in a rural town) [1][3]
Forest (F) zone
ADUs not allowed [3]
Max size
1,000 sq ft heated plus 1,000 sq ft unheated Urban ADUs can trade unheated area for heated area up to 1,500 sq ft [3]
Not counted in size
Basement or attic ADUs inside the house have no size cap Basements of detached ADUs don't count [3]
Height
No taller than the base height for the zone Unless wholly inside an existing home [3]
Parking
No additional off-street parking required [1][3]
Owner occupancy
Cannot be required State law [4]
School impact fees
ADUs are exempt [7]
2026 permit application review fee (ADU)
$4,556 minimum, plus inspection fees of $2.46 per sq ft and other fees [6]
Notice on title
Required before the county approves the permit [3]
On this page
  1. At a glance
  2. Which areas this page covers
  3. What you can build
  4. Size and height
  5. Parking, street improvements and conversions
  6. Fees
  7. Septic, sewer and the notice on title
  8. How this fits state law
  9. Permit steps
  10. Who to call
  11. Questions people ask

Which areas this page covers

This page covers unincorporated King County only. That means land outside the limits of any city, where King County Permitting is the permit office. If your home is inside a city, the city's code applies and you get your permit from the city. See our pages on Seattle and Kirkland.

To check, King County suggests entering your address in King County's Parcel Viewer and seeing what is listed as the jurisdiction. King County's permits page also has a map where the green layer shows unincorporated King County. [2]

What you can build

King County's ADU rules are in King County Code 21A.08.030, last amended by Ordinance 19881 in 2024. What you can build depends on whether your lot is in the urban area or the rural area.

Urban area. "Two accessory dwelling units are allowed per lot" in these combinations:

Rural area and natural resource lands. One ADU is allowed per lot. It must be in the same building as the main home, unless the lot has no more than one main home and meets a lot size test. In a rural town the lot must be at least 3,200 square feet. Elsewhere in the rural area it must meet the minimum lot area for its zone. The county's info sheet says only attached ADUs are allowed in the Agriculture (A) zone, as an accessory to commercial farming. ADUs are not allowed in the Forest (F) zone.

An attached ADU must share at least one wall with the main home. A breezeway counts only if it is less than 10 feet long, shares a wall with both units, is fully enclosed and is heated. [1][3]

Size and height

An ADU "shall not exceed one thousand square feet of heated floor area and one thousand square feet of unheated floor area," with these exceptions:

An ADU that isn't wholly inside an existing home can't be taller than the base height for the zone. Base heights and setbacks are set by zone in KCC Title 21A, and some areas have their own tables, so confirm the numbers for your lot with King County Permitting.

State law says a county can't cap ADU roof height below 24 feet (or below the main home's limit, if that is lower), and ADU setbacks and design rules can't be stricter than those for the main home. See our Washington ADU law page. [3][4]

Parking, street improvements and conversions

"No additional off-street parking spaces are required for accessory dwelling units." In the urban area, "No public street improvements are required" either.

Also in the urban area, you can convert an existing structure, such as a garage, into an ADU "even if the existing structure is legally nonconforming with respect to setbacks or maximum impervious surface percentage." See our garage conversion guide for the general steps. [3]

Fees

King County Permitting sets fees each year. For 2026, most permit fees rose about 14 percent, and a $126 application screening fee applies to most applications when you submit them.

The 2026 single-family fee guide lists these fees for a new accessory dwelling:

ADUs are on the list of housing types exempt from school impact fees. Sewer, septic and health department reviews can add their own fees. Check the county's permit fees page for current amounts. For budgeting, see our ADU cost guide. [1][5][6][7]

Septic, sewer and the notice on title

An ADU may need public health approval or a sewer certificate, reviewed the same way as any home. If your lot is on a septic system, check King County's on-site sewage system requirements early.

Before the county approves a permit to build an ADU, you must file a notice that identifies the unit as accessory, using a form approved by the county's Records and Licensing Services Division. The notice "shall run with the land," and you must show proof that it was filed. [1][3]

How this fits state law

Washington's ADU law, RCW 36.70A.681, requires counties to allow at least two ADUs on lots in urban growth areas where single-family homes are allowed. It also bars owner-occupancy rules, caps ADU impact fees at 50 percent of the main home's, and says a size cap can't be less than 1,000 square feet. King County's urban rules allow two ADUs and set a 1,000-square-foot heated floor area limit.

The state ADU requirements apply only inside urban growth areas. King County's one-ADU limit in the rural area is a local rule. State law says counties must adopt these requirements "to take effect at the same time as the jurisdiction's next periodic comprehensive plan update." For King County, that update was due "on or before December 31, 2024." [3][4][8][9]

Permit steps in King County

  1. Confirm you're in unincorporated King County

    Look up your address in King County's Parcel Viewer and check the jurisdiction. If it's a city, apply to that city instead. [2]

  2. Check your zone and lot

    Find your zone and whether the lot is urban or rural with the county's property research tools. Rural lots need to meet the zone's minimum lot area for a detached ADU. [1][3]

  3. Sort out water and sewer or septic

    Find out whether your ADU needs public health approval, a sewer certificate or both. The county reviews these the same way as for any home, and its info sheet points to its on-site sewage (septic) system requirements. [1]

  4. Record the notice on title

    File the county's notice identifying the unit as accessory (the form is approved by the Records and Licensing Services Division), and submit proof that it was filed before the permit is approved. [1][3]

  5. Apply on MyBuildingPermit.com

    Apply online at MyBuildingPermit.com by selecting King County, Building, Single Family Residential, New Construction, Accessory Dwelling Unit. You'll get a confirmation email, then screening; the county tells you through the portal if anything is missing. [1][10]

  6. Pay fees and pass inspections

    Pay invoices through the portal, then schedule inspections as the work goes on. [2][10]

Who to call

Questions people ask

How many ADUs can I build in unincorporated King County?

Two on a lot in the urban area: one attached and one detached, two attached, or two detached. In the rural area and on natural resource lands, one ADU per lot. None are allowed in the Forest zone. [3]

How big can an ADU be in unincorporated King County?

Up to 1,000 square feet of heated floor area and 1,000 square feet of unheated floor area. In the urban area you can trade unused unheated area for heated area, up to 1,500 square feet heated. An ADU entirely inside the main home's basement or attic has no size cap. [3]

Do I need parking for an ADU in King County?

No. The county code says no additional off-street parking spaces are required for ADUs. [1][3]

Do these rules apply in Seattle, Kirkland or other cities?

No. This page covers only unincorporated King County. Cities set their own ADU rules and issue their own permits; see our pages for Seattle and Kirkland. [2]

Does an ADU pay school impact fees in King County?

No. King County's 2026 school impact fee guide lists ADUs among the housing types exempt from school impact fees. [7]

Sources (11)
  1. Residential Accessory Dwelling Units (ADUs) information sheet, March 2026, King County Department of Local Services, Permits. Accessed October 10, 2026.
  2. Permits, King County Department of Local Services. Accessed October 10, 2026.
  3. King County Code Title 21A Zoning, 21A.08.030 Residential land uses (development condition B.7.a, accessory dwelling units), King County Council. Accessed October 10, 2026.
  4. RCW 36.70A.681 Accessory dwelling units — Limitations on local regulation, Washington State Legislature. Accessed October 10, 2026.
  5. Permit fees - Building and land use, King County Department of Local Services, Permitting Division. Accessed October 10, 2026.
  6. 2026 Fee Guide 02, Single Family Residential Construction, King County Department of Local Services, Permitting Division. Accessed October 10, 2026.
  7. 2026 Fee Guide 03, School Impact Mitigation Measures, King County Department of Local Services, Permitting Division. Accessed October 10, 2026.
  8. RCW 36.70A.680 Accessory dwelling units — Local regulation, Washington State Legislature. Accessed October 10, 2026.
  9. RCW 36.70A.130 Comprehensive plans — Review procedures and schedules — Amendments, Washington State Legislature. Accessed October 10, 2026.
  10. Single family permit forms and application materials, King County Department of Local Services, Permitting Division. Accessed October 10, 2026.
  11. Permit customer services, King County Department of Local Services, Permitting Division. Accessed October 10, 2026.

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