Building an ADU in Washington: what state law allows

What state law lets you build

Washington's ADU law, passed in 2023 as HB 1337, says a city or county "must allow at least two accessory dwelling units on all lots" in zones within an urban growth area that allow single-family homes. You can arrange them three ways:

• One attached ADU and one detached ADU
• Two attached ADUs
• Two detached ADUs, in one building or two

The city must also allow an ADU on any lot that meets the minimum lot size for the main house. Under the state definition, an ADU can sit on the same lot as a single-family house, duplex, triplex, townhome or other housing unit.

The rules apply only to cities and counties that plan under the Growth Management Act, and only inside urban growth areas. The state ADU requirements don't apply outside those areas. [1][2][3]

What cities can and can't require

State law sets limits that your city's ADU rules can't go past:

• Size: a city "may not establish a maximum gross floor area requirement" for ADUs below 1,000 square feet. Gross floor area means interior habitable area, including basements and attics but not a garage.
• Height: a city may not cap ADU roof height below 24 feet. If the main house has a lower height limit, the ADU limit can't be lower than the house's.
• Setbacks and design: setbacks, yard coverage limits, tree retention rules, entry-door rules, aesthetic rules and design review for ADUs can't be stricter than for the main house.
• Alley lots: a detached ADU must be allowed right at a lot line that abuts a public alley, unless the city routinely plows snow on that alley.
• Owner occupancy: the city may not require you to live in the ADU or the main house.
• Impact fees: ADU impact fees can't be more than 50 percent of the fees for the main house.
• Street improvements: a city "may not require public street improvements" as a condition of an ADU permit.

Cities can still apply the same health, safety, building code and environmental rules that apply to the main house. They can also apply other general development rules, as long as those don't conflict with the ADU law. [1][2][3]

Parking

Under the ADU law, your city may not require off-street parking for an ADU within a half-mile walk of a major transit stop. That includes rail and bus rapid transit stops, and bus stops with service at least every 15 minutes for at least five hours during weekday peak hours.

Elsewhere, the city can require no more than one off-street space per unit on lots smaller than 6,000 square feet, and no more than two per unit on larger lots. These limits don't apply where the state Department of Commerce has certified a city safety study, or within one mile of a commercial airport with at least 9 million annual boardings.

A 2025 law (SB 5184, now RCW 35.21.994) adds a broader rule: cities of more than 30,000 people "may not require any minimum parking requirements" for residences under 1,200 square feet. Cities of 50,000 or more must put this in place within 18 months of the law's July 27, 2025 effective date, and cities of 30,000 to 50,000 within three years. Ask your city which parking rule applies to your lot. [1][3][4][5]

When the rules take effect in your city

Each city or county must write the ADU rules into its code. Under RCW 36.70A.680 as amended in 2025 (SB 5558), the changes take effect "at the same time as the jurisdiction's next periodic comprehensive plan update." The state's update deadlines are:

• King, Kitsap, Pierce and Snohomish counties and their cities: December 31, 2024
• Clallam, Clark, Island, Jefferson, Lewis, Mason, San Juan, Skagit, Thurston and Whatcom counties and their cities: December 31, 2025
• Benton, Chelan, Cowlitz, Douglas, Franklin, Kittitas, Skamania, Spokane, Walla Walla and Yakima counties and their cities: December 31, 2026
• Adams, Asotin, Columbia, Ferry, Garfield, Grant, Grays Harbor, Klickitat, Lincoln, Okanogan, Pacific, Pend Oreille, Stevens, Wahkiakum and Whitman counties and their cities: June 30, 2027

Before 2025 the deadline was six months after the periodic update, which is what the Department of Commerce's ADU guidance (updated June 2024) still says. The 2025 law changed it to the same time as the update. If your city hasn't updated its code, the state ADU rules "supersede, preempt, and invalidate any conflicting local development regulations." [2][6][7][8]

Garage conversions

Cities must let you turn an existing structure, including a detached garage, into an ADU "even if they violate current code requirements for setbacks or lot coverage." The converted space still has to meet building and safety codes that apply to the main house. See our garage conversion guide for the general steps. [1][2]

Middle housing: duplexes to sixplexes (HB 1110)

A separate 2023 law, HB 1110 (RCW 36.70A.635), requires cities to allow more homes on residential lots:

• Cities of 25,000 to 74,999 people: at least two units per lot, or four within a quarter-mile walk of a major transit stop or if at least one unit is affordable housing.
• Cities of 75,000 or more: at least four units per lot, or six near a major transit stop or if at least two units are affordable housing.
• Smaller cities in the same urban growth area as the largest city of a county over 275,000 people: at least two units per lot.

A city may count ADUs toward these unit numbers, but it isn't required to allow ADUs beyond them. It must allow at least six of the nine middle housing types. Within a half-mile walk of a major transit stop, it can't require off-street parking for middle housing. Cities generally must comply by their next periodic plan update, so check whether your city's new rules are in effect. [9]

Where the ADU rules don't apply

State law doesn't force a city to approve an ADU where development is restricted because of a septic system, critical areas, or other "unsuitable physical characteristics of a property." Cities may also:

• Ban ADUs on lots not connected to public sewer
• Ban or restrict ADUs in zones of one home per acre or less that are in wetlands, fish and wildlife habitat, flood plains or geologically hazardous areas

The detailed limits in RCW 36.70A.681 also don't apply to lots with critical areas or their buffers, or to certain watersheds that serve drinking-water reservoirs. A city may cap lots of 2,000 square feet or less at two ADUs plus the main house. [1][2]

Short-term rentals and selling an ADU

State law lets cities restrict ADUs used as short-term rentals. Washington defines those as stays of fewer than 30 consecutive nights. Check your city's rental rules before you list an ADU.

Cities can't block the sale of a condominium unit "solely on the grounds that the condominium unit was originally built as an accessory dwelling unit." Whether your lot can be set up as condominiums depends on local and condo rules, so confirm with your city. [1][2][3]

Where local rules differ

Each city writes its own code within these limits, and many allow more than the state minimum. For example, see our page on building an ADU in Seattle. For more on the general process, see our guides to the ADU permit process, ADU costs and ADU financing.

For state-level help, the Department of Commerce keeps an ADU guidance and resources page. [10]

Washington laws to know

• HB 1337 (2023), RCW 36.70A.680 and 36.70A.681

  Requires cities and counties planning under the Growth Management Act to allow two ADUs per lot in urban growth areas and limits size, height, setback, parking, fee and owner-occupancy rules. [1][2]

• HB 1110 (2023), RCW 36.70A.635

  Requires cities of 25,000 or more people to allow two to six homes per residential lot, depending on city size, transit access and affordable units. [9]

• SB 5184 (2025), RCW 35.21.994

  Limits parking minimums in cities of more than 30,000 people and bars them for residences under 1,200 square feet, with some exceptions. [4][5]

Questions people ask