King County, WA ADU (accessory dwelling unit) Rules and Regulations (2026)

In King County, the first question is whether the parcel is inside a city. Seattle, Bellevue, Kent, Renton, and the county’s other cities each write their own ADU (accessory dwelling unit) code. The county’s rules cover only unincorporated King County: urban unincorporated neighborhoods inside the urban growth area, rural land, and resource land. The King County Department of Local Services, Permitting Division, reviews those parcels, and Public Health – Seattle & King County reviews septic and wells.

The rules below come from the county’s March 2026 ADU handout, which reprints the zoning standards in King County Code 21A.08.030 B.7, and from Public Health’s ADU septic and water memo. Statewide context is in our Washington ADU laws guide.

Who reviews your lot

A Kent or Renton mailing address does not prove the lot is inside that city. Confirm jurisdiction on the parcel record first.

King County cities with their own rules

If the parcel is inside a city, the county handout is the wrong document. Start with the city’s code. Local notes by city:

• Seattle
• Bellevue
• Kirkland — city rules are in the next section, not in the county handout
• Renton
• Kent
• Federal Way

Vashon is different: it is unincorporated King County, so the county rules, Public Health review, and a ferry crossing all apply to the same project.

City of Kirkland

Kirkland writes its own code. The numbers below are from the city’s ADU regulations page and from KZC 115.07, which the code site lists as current through Ordinance 4947 (passed September 1, 2026). That same page says Ordinance 4948 has amended the section and is still pending codification. Confirm the adopted text before you rely on a parking stall or a fee.

How many, and how big. Many lots may have up to two ADUs: one attached and one detached, or two of either type. A property in the Residential-L shoreline environment is limited to one. If you are planning two attached ADUs, the city asks you to call Planning and Building at 425-587-3600 for the building-code side of that layout.

The size cap is 1,200 square feet of gross floor area. The Planning and Building Director may allow more when an attached ADU already occupies a single floor of the existing house. Gross floor area leaves out space with less than five feet of ceiling height, uncovered rooftop decks, uncovered at-grade patios, and up to 200 square feet of other covered or uncovered exterior elements.

Setbacks and the shoreline. An ADU follows the setbacks, height, lot coverage, floor-area ratio, and the other zoning standards that apply to a single-family house in that zone, with the exceptions written into KZC 115.07, 115.42, and 115.115(3)(o). There is no separate citywide ADU setback table. A detached ADU has to be in its own structure. It may not share a roof with the house or with an attached ADU.

On lots in the Residential-L and M/H shoreline environments, a detached ADU has to sit landward of the principal residence (KZC 83.200.3). Trees, critical areas, and a shoreline substantial-development permit can apply on those lots even when the mailing address is Kirkland and the water is a few blocks away.

Converting an existing structure, including a detached garage, can stay nonconforming to the current setback and lot coverage, as long as the project does not increase that nonconformity.

Parking. The two city sources do not say the same thing, so read both.

The zoning code published through Ordinance 4947 says no off-street parking is required within one-half mile of a major transit stop. Farther out, one off-street space is required when the lot has more than one ADU, unless on-street parking is available within 600 feet or the lot is within one-half mile of transit with 15-minute headways during commute hours.

The city’s regulations page describes the outside-the-half-mile rule as one space for the first ADU, with those same two exceptions. It names the STRIDE I-405 bus rapid transit station as Kirkland’s current major transit stop, and it says future stops on Metro’s RapidRide K Line count once construction is underway. Ordinance 4948 may change this subsection. Ask the city which text applies to your address before you draw a stall.

Ownership. KZC 115.07 allows an ADU to be owned separately from the house, except on property in the Residential-L shoreline environment. A detached ADU can also go on its own unit lot under a unit lot subdivision (KMC Title 22). In the M/H shoreline environment, separate ownership needs a shoreline substantial-development permit, and the city requires a pedestrian pathway under KZC 83.420.

Permits and registration. Building permits go through MyBuildingPermit.com. The city’s permitting steps are: open an account, prepare the submittal, check whether lakes, streams, slopes, or wetlands add review, see whether the project qualifies for expedited review, build after the permit is issued, then pass the final inspection and get the certificate of occupancy.

A pre-approved detached plan still needs a site plan that includes the plan’s registration ID, the supplemental building form, the ADU registration application, a plumbing worksheet, and usually a survey. Add a geotechnical report, a rodent abatement letter, a fire-sprinkler permit, or a demolition permit when those apply. Custom plans are allowed. The city’s guide to a pre-approved detached ADU says a unit closer than 10 feet to another dwelling may need a fire-rated wall. Shoreline jurisdiction, geologic hazards, critical areas, complex drainage, a utility upgrade, or a new driveway can take a project off the shorter review path.

The owner files a property covenant with the city for recording at the King County Recorder’s Office. The covenant runs with the land while the ADU is there. If you will rent the unit, the city’s permitting page says to get a Kirkland business license and follow the rental rules. The city’s rental guidance has also required a short-term rental to be on an owner-occupied property and registered with the city. Confirm that on the current Moving In or Renting Out page before you count on nightly income.

Fees. KMC 27.10.050 exempts a legal ADU of 800 square feet or fewer from impact fees. KMC 27.06.150 lists a 2026 park impact fee of $2.52 per square foot for an accessory dwelling unit over 800 square feet. King County sewer treatment capacity charges can still apply. Read the current public works fee schedule. The older toolkit language that described a blanket exemption from transportation, park, and school fees is not the code section above.

For Wolf’s work in the city, see ADU builder in Kirkland. The factory is in Battle Ground, so a Kirkland delivery is outside the standard 20-mile installation radius and gets its own transport quote.

Urban unincorporated King County

King County’s periodic comprehensive plan update was due by December 31, 2024 under RCW 36.70A.130, and its current urban ADU standards reflect the state’s urban-growth-area floor in RCW 36.70A.681. On an urban lot the county allows:

• Two ADUs per lot — one attached and one detached, two attached, or two detached in one or two structures.
• Conversions of existing structures, including garages, even if the structure is legally nonconforming for setbacks or impervious surface.
• No public street improvements as a condition of the ADU.
• No additional off-street parking.

Size. The ADU may not exceed 1,000 square feet of heated floor area and 1,000 square feet of unheated floor area. Three exceptions matter:

1. An ADU wholly inside the house’s basement or attic has no size limit under this rule.
2. For a detached ADU, basement floor area doesn’t count toward the maximum.
3. In the urban area, every square foot of allowed unheated area you don’t build can be added to heated area, up to 1,500 heated square feet. The county’s example: 1,250 heated square feet with 750 unheated.

Height. An ADU that isn’t wholly inside an existing dwelling can’t exceed the base height of its zone.

What counts as attached. An attached ADU needs at least one common wall with the house and has to read as one structure. A breezeway only counts if it is under ten feet long, shares a wall with both units, is fully enclosed, and is heated. A covered walk to a separate cottage is a detached ADU.

Sewer. Public Health’s memo points urban-area applicants to the city or local sewer utility for availability and connection requirements. If public sewer can’t serve the ADU, a sewer availability letter saying so goes in the septic design package.

State law adds protections on qualifying urban lots, including impact fees no more than half of the principal unit’s and no owner-occupancy requirement. Critical areas and their buffers are excluded from those protections, and counties may still restrict ADUs used as short-term rentals (RCW 36.70A.680).

Rural and resource land

Outside the urban growth area, the handout sets tighter rules:

• One ADU per lot.
• Same building by default. The ADU goes inside or attached to the house unless both detached conditions are met: no more than one primary dwelling on the lot, and the lot meets the minimum lot area for its zone. The Rural Area zones named are RA-2.5, RA-5, RA-10, and RA-20. A separate 3,200-square-foot threshold applies to lots in a rural town.
• One street-facing entrance when the house and ADU share a building or are joined by a breezeway.
• Compatible design. Materials, colors, and building form should relate to the house and neighbors. The county suggests a siting-alternatives study to limit privacy and view impacts.
• Future land division. A detached ADU counts as a separate dwelling for lot calculations in a later subdivision. If a detached ADU in an RA zone later becomes a primary home on its own lot, neither lot can add another detached ADU unless it is at least twice the zone’s minimum lot area.

The Agriculture zone allows only attached ADUs accessory to commercial agriculture. Forest zones prohibit ADUs. The 1,000/1,000 size rule and its basement exceptions apply outside the urban area too; the 1,500-square-foot trade does not.

The current handout lists no owner-occupancy condition. Confirm the permit conditions for your parcel at intake.

The notice on title

Before King County approves an ADU construction permit, the owner files a notice identifying the dwelling as accessory, and it runs with the land. The county’s ADU affidavit is notarized, recorded against the parcel, and restates the ADU limits. Build recording time into the schedule. The permit can’t be approved without proof of filing.

Septic, wells, and water

On a rural lot, Public Health usually decides the project before zoning does. Its 2025 memo explains the options and paperwork:

Three septic paths. The ADU can have its own system, share one system designed to multifamily criteria, or use an expanded version of the house’s permitted system. The memo spells out the applications and fees for each; a separate system means a second application.

Sizing. A studio counts as one bedroom. Dens and offices with an egress window and a closet can count as bedrooms. The memo’s table sizes a small one-bedroom ADU under 600 square feet sharing the house’s system at 150 gallons per day of added design flow, with larger units stepping up by bedroom. If an existing system was designed for more bedrooms than the house uses, the unused flow can sometimes be reassigned, but the house must keep at least three bedrooms.

Paperwork you should expect:

• A satisfactory operation-and-maintenance inspection of the existing system, less than one year old, by a certified maintainer.
• A Public Health–reviewed record drawing for the house’s system.
• A conforming reserve area.
• Floor plans for the ADU.

Water. On a Group A water system, the purveyor’s certificate of water availability must name the ADU. A Group B system needs written confirmation that a connection is available for the ADU, plus current water-quality results. An ADU can use a private well on the same property, but it must connect to public water when it is inside a water service area and service is available. The well needs a pump test of at least two hours showing 800 gallons per day, with current bacteria, nitrate, and arsenic results. Rainwater catchment is not an accepted source.

Get the septic answer before you pick a floor plan. It often decides whether the ADU is a studio, a one-bedroom, or two bedrooms.

Applying

King County building permits are filed through MyBuildingPermit.com. The handout’s path is King County → Building → Single Family Residential → New Construction → Accessory Dwelling Unit. Public Health approval or a sewer certificate may also be required, and it is reviewed the same way as for any new dwelling.

How a factory-built ADU fits

A Wolf home is modular, built to Washington’s residential code — not a HUD (the federal standard for manufactured homes) manufactured home. Washington L&I reviews the factory plans and oversees factory inspections, and the home carries its L&I insignia before it ships. King County’s review still covers zoning, the foundation, utility connections, and site inspections. Factory approval doesn’t replace Public Health review or the title notice.

Wolf builds in Battle Ground. Every King County site is beyond the standard 20-mile delivery included in our published installation scope, so transport, access, and crane needs are quoted after the site review. See cost and the Washington ADU cost guide for planning ranges.

What to have ready

1. The address or parcel number, and whether you think it’s in a city or unincorporated.
2. The zone, lot area, and any recorded plat or easement documents.
3. Septic records: permit, record drawing, and the latest inspection report. Well log and water results if you’re on a well.
4. How you plan to use the unit: family, long-term rental, or a place you might move into.

Send us the address and we’ll confirm the reviewer, the urban or rural rule set, and which models fit before you pay for design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.