Washington ADU and DADU Rules by City: HB 1337 and the Two-ADU Rule

Washington is now the gold standard for ADU preemption in the United States. With the signing of House Bill 1337 in May 2023 and its effective date of July 23, 2023, the state legislature stripped cities and counties of most of the regulatory levers they had used for decades to slow accessory dwelling unit construction. Inside an urban growth area, every residential lot now has the right to two accessory dwelling units — attached, detached, or one of each — with a minimum allowed size of 1,000 square feet, no owner-occupancy requirement, no parking requirement near major transit, and capped impact fees. Seattle had already moved in this direction in 2019, and Portland-style permissive ADU regimes were spreading across the Puget Sound, but HB 1337 made what had been a Seattle-and-Bellevue story a statewide one. This guide walks through the state framework codified at RCW 36.70A.681 and 36.70A.682, the related middle-housing reform in HB 1110, and the specific code updates Seattle, Bellevue, Tacoma, Spokane, Vancouver, Bellingham, Olympia, Federal Way, Renton, and Kent have adopted to comply.

The Growth Management Act backdrop

To understand HB 1337, you have to understand the Growth Management Act, codified at RCW 36.70A. Washington has organized its land-use planning around urban growth areas (UGAs) since 1990. Every county that meets a statutory population and growth threshold — which is most of Western Washington and the Spokane and Tri-Cities corridors — must designate UGAs around its incorporated cities, inside which urban-density development is concentrated. Land outside the UGA is preserved for rural and resource uses. This UGA framework is the geographic scaffolding HB 1337 hangs from: the bill's preemptive force applies inside UGAs and to cities planning under the GMA, while unincorporated rural land outside a UGA is largely unaffected.

The GMA also requires periodic plan updates, with the most recent comprehensive plan deadlines hitting King, Pierce, Snohomish, and Kitsap counties on December 31, 2024, and other counties on subsequent dates running through 2027. HB 1337 and HB 1110 were both written to slot into this comprehensive-plan cycle, with deadlines tied to each city's next required update. For most large Western Washington cities, that meant new ADU code on the books in 2024 or early 2025.

HB 1337 in detail

House Bill 1337 (Chapter 332, Laws of 2023), codified primarily at RCW 36.70A.681 and 36.70A.682, is structured as a series of mandates that cities and counties planning under the GMA must incorporate into their development regulations. The statute defines an "accessory dwelling unit" as a dwelling unit located on the same lot as a primary dwelling, used for housing purposes, and meeting the regulatory standards in the statute. It expressly recognizes both attached ADUs — which Washington practitioners shorten to AADU — and detached ADUs, or DADUs.

The mandates run through ten substantive provisions. First, every city and county must allow at least two ADUs on any lot that allows a single-family dwelling, in any combination of attached and detached. Second, the maximum gross floor area a jurisdiction may cap an ADU at is no smaller than 1,000 square feet — cities cannot cap below that, though they may allow larger. Third, jurisdictions may not impose a minimum lot size for ADUs beyond what is already required for the primary dwelling. Fourth, owner-occupancy requirements — the rule that the owner must live in either the main house or the ADU — are prohibited. Fifth, jurisdictions may not require off-street parking for an ADU located within a half-mile walking distance of a major transit stop, defined under RCW 36.70A.030 to include bus rapid transit, light rail stations, ferry terminals, and certain frequent bus routes. Sixth, impact fees on ADUs are capped at 50% of the impact fee that would apply to the primary dwelling, and ADUs under 750 square feet are exempt from impact fees entirely under the related RCW 82.02.090.

The seventh and eighth provisions address conversions. ADUs created from existing detached structures, like a garage or shed, must be permitted even if the existing structure does not meet current setback or lot-coverage requirements — a major protection for the typical "convert the detached garage" project. Jurisdictions may not require any setback, lot-coverage, or floor-area-ratio modification for an ADU built within the footprint of a pre-existing legal nonconforming structure. Ninth, jurisdictions must allow ADUs to be sold separately from the primary dwelling through a condominium-style ownership structure, although individual fee-simple sale of a backyard cottage on the same lot remains restricted in most cities. Tenth, the statute caps the maximum side and rear setback a city may impose on a new-construction DADU at five feet, and the maximum height at twenty-four feet, with limited exceptions for steep slopes.

The combined effect is a regulatory floor — cities can still impose more permissive rules, but they cannot go below the statutory minimums. A city that wants to allow three ADUs per lot can do so. A city that wants to set the height limit at twenty-eight feet can do so. But a city cannot require owner-occupancy, cannot cap an ADU at 800 square feet, and cannot demand a parking space on a lot inside the transit half-mile radius. The bill applies to all GMA-planning cities and counties, which covers approximately 95% of Washington's population.

The "two ADUs per lot" rule

The most-discussed provision of HB 1337 is the two-ADU mandate. Before 2023, most Washington cities allowed at most one ADU per single-family lot, and many smaller cities did not allow ADUs at all. The two-ADU rule changes the basic unit of single-family zoning. A typical Seattle, Bellevue, or Tacoma single-family lot now permits three dwelling units: the original primary residence plus two accessory units. Those two units can be arranged in any combination — one AADU (basement apartment, attached cottage) and one DADU (backyard cottage), or two DADUs, or two AADUs split between basement and an attic conversion.

The practical effect on housing supply is substantial. King County's department of local services estimated in 2024 that the two-ADU rule would permit, in theory, roughly 350,000 additional dwelling units across the county's single-family lots if every owner exercised the right. Actual uptake will be far lower — most homeowners will never build an ADU — but the rule reframes the long-term capacity of single-family zoning. Combined with the middle-housing provisions of HB 1110 (discussed below), Washington has essentially ended single-family-exclusive zoning as a legal category in its larger cities.

HB 1110 and the middle-housing context

HB 1337 did not pass in isolation. The same 2023 legislative session enacted House Bill 1110 (Chapter 333, Laws of 2023), codified at RCW 36.70A.635, which requires cities over 25,000 population to permit four-unit and six-unit residential buildings on lots formerly zoned for single-family only. Cities over 75,000 — Seattle, Spokane, Tacoma, Vancouver, Bellevue, Kent, Everett, Renton, Federal Way, Spokane Valley, Yakima, Bellingham, Kirkland — must allow at least four units per lot, and six units within a quarter-mile of major transit. Smaller cities between 25,000 and 75,000 must allow at least two units per lot, with four units near transit.

The two laws interact directly. A Seattle owner who builds a four-plex under HB 1110 cannot then add two ADUs under HB 1337 on top of that; the ADU statute applies to lots where a single-family dwelling is the primary use. But a homeowner who keeps the existing single-family house can pair an AADU and a DADU for a total of three units on a lot that, twenty years ago, would have been capped at one. The choice between "four-plex under HB 1110" and "house plus two ADUs under HB 1337" is now the central design question for redevelopment on Washington urban single-family lots.

Seattle: the pre-HB 1337 model

Seattle is the city HB 1337 was modeled on. Seattle Municipal Code 23.42.054 — the city's ADU regulation — was rewritten in 2019 under Council Bill 119544 after a six-year legal battle that ended with the Washington Supreme Court's decision in Knapp v. City of Seattle. The 2019 ordinance allowed two ADUs per single-family lot, eliminated owner-occupancy requirements, dropped off-street parking mandates citywide, and increased the maximum DADU size from 800 to 1,000 square feet. Seattle's ADU permitting volume jumped from roughly 300 per year before the reform to more than 900 per year by 2022.

The 2019 ordinance also created a generous tree-preservation framework around DADUs — the city offers a "tree credit" allowing DADU footprint expansion if mature trees are preserved on site — and a pre-approved DADU plan-set program through the Seattle Department of Construction and Inspections (SDCI). The DADU plan-set program offers ten architect-designed cottages with permitting times of six to eight weeks and design fees waived. By 2024, more than 20% of Seattle's permitted DADUs used a pre-approved plan.

After HB 1337 took effect, Seattle's existing code was already more permissive than the statutory floor in most respects, so the city's response was largely confirmatory. A 2024 ordinance brought SMC 23.42.054 into alignment with the state impact-fee cap, clarified that the state's half-mile-transit parking exemption applied citywide (Seattle had already eliminated ADU parking, but the codification matters for legal challenges), and extended the two-ADU right to additional zoning categories that had previously been excluded.

Bellevue: the post-HB 1337 update

Bellevue's ADU code lived at Land Use Code 20.20.120 and was significantly more restrictive than Seattle's before HB 1337. Bellevue had required owner-occupancy, capped ADUs at 800 square feet, required one off-street parking space per ADU, and prohibited DADUs in some single-family zoning districts entirely. The city's 2024 comprehensive plan update, adopted in October 2024 by Council Ordinance 6810, rewrote the ADU regulations to comply with state law. The rewrite eliminated owner-occupancy, raised the size cap to 1,000 square feet (with up to 1,200 for DADUs that meet design standards), dropped parking within a half-mile of the Spring District, Bel-Red corridor, and Bellevue Transit Center light rail stops, and allowed two ADUs per lot in all single-family residential zones.

Bellevue's compliance included a substantive permitting reform. The city introduced an ADU pre-approval program in early 2025, modeled on Seattle's, with eight approved DADU designs ranging from 500 to 1,000 square feet. Permitting through the pre-approval program runs six to ten weeks versus the standard four-to-six-month timeline for custom plans.

Tacoma: TMC Title 13 rewrite

Tacoma's ADU rules sit in Tacoma Municipal Code Title 13, the city's land use regulatory code, primarily at TMC 13.06.150 (accessory dwelling unit standards). Tacoma had a relatively permissive ADU code before HB 1337 — the city allowed one ADU per single-family lot at up to 1,000 square feet, no owner-occupancy, and only nominal parking requirements — but the two-ADU mandate and the transit parking exemption required code changes. Council Substitute Ordinance 28858, adopted in November 2023, brought TMC 13.06.150 into compliance, allowing two ADUs per single-family lot, eliminating parking within a half-mile of the Tacoma Link light rail stations, and clarifying that DADUs built within the footprint of an existing detached garage are exempt from current setback rules.

Tacoma's ordinance also implemented the impact-fee cap, setting ADU impact fees at exactly 50% of the corresponding single-family-dwelling fee, with the under-750-square-foot exemption from RCW 82.02.090 fully implemented. The city runs a streamlined permitting track for ADUs under 1,000 square feet with administrative review only — no neighborhood notification, no public hearing.

Spokane: Title 17 zoning

Spokane's ADU regulations are at Spokane Municipal Code Chapter 17C.300, within Title 17 (Land Use). Before HB 1337, Spokane allowed one ADU per single-family lot up to 800 square feet with an owner-occupancy requirement and one off-street parking space. The city's 2024 code update, Ordinance C-36459, adopted in June 2024, eliminated owner-occupancy, raised the cap to 1,000 square feet, allowed two ADUs per lot, and removed parking mandates within a half-mile of the planned Central City Line bus rapid transit stations (the BRT route opened in 2023).

Spokane is also home to one of Washington's most active ADU pre-approval programs, run through the city's Building Services Department. The "Spokane ADU Toolkit" includes seven pre-approved designs from local architects and offers a flat permit fee structure that runs roughly half the cost of custom permitting. Spokane County's unincorporated areas inside the UGA were updated separately under Spokane County Code Title 14 with similar provisions.

Vancouver: Title 20 zoning

Vancouver, Washington — not to be confused with its larger Canadian neighbor across the Columbia River — regulates ADUs through Vancouver Municipal Code Title 20, the city's zoning code, at VMC 20.840 (Accessory Dwelling Units). The pre-HB 1337 code allowed one ADU per single-family lot up to 800 square feet, with parking and owner-occupancy requirements. Council Ordinance M-4378, adopted September 2024, rewrote VMC 20.840 to allow two ADUs per lot, eliminate owner-occupancy, raise the cap to 1,000 square feet, and remove parking requirements within a half-mile of C-TRAN's Vine bus rapid transit route along Fourth Plain Boulevard.

Vancouver also lifted its previous prohibition on ADUs in some "Residential Low" zoning districts and added a specific allowance for DADUs built within existing detached garages, even where those garages encroach into current setbacks — a direct codification of the HB 1337 garage-conversion protection.

Bellingham: small-city framework

Bellingham, with a population around 95,000, regulates ADUs through Bellingham Municipal Code Title 20 (Land Use) at BMC 20.10.036 and 20.13.020. Bellingham's pre-2023 code was already moderately permissive — one ADU per lot up to 1,000 square feet, no owner-occupancy — but the city had aggressive design and parking requirements. The 2024 update, Ordinance 2024-10-035, brought the code into HB 1337 compliance by allowing two ADUs per lot, removing parking within a half-mile of the WTA GO Line bus rapid transit corridor, and confirming the impact-fee cap.

Bellingham's ADU code also includes a distinctive "Cottage Housing" provision (BMC 20.13.040) that allows clusters of small ADU-style units on larger lots, which has been used in several infill developments near Western Washington University.

Olympia: capital city

Olympia regulates ADUs through Olympia Municipal Code Title 18 (Unified Development Code), at OMC 18.04.060.K and 18.06.080. The city's pre-2023 ADU code allowed one ADU per single-family lot at up to 800 square feet, with parking and design standards. Council Ordinance 7378, adopted June 2024, raised the cap to 1,000 square feet, allowed two ADUs per lot, eliminated owner-occupancy, and dropped parking within a half-mile of Intercity Transit's planned BRT and frequent-route corridors along Capitol Way, Martin Way, and Pacific Avenue.

Olympia also introduced a small-DADU exemption from design review for units under 600 square feet — administrative permitting only — which has streamlined the small-cottage path significantly.

Federal Way, Renton, Kent: South King County compliance

The South King County cities — Federal Way, Renton, and Kent — all updated their ADU codes in 2024 in response to HB 1337. Federal Way's update sits in Federal Way Revised Code Title 19 at FWRC 19.195 and 19.265, allowing two ADUs per lot, raising the cap to 1,000 square feet, eliminating owner-occupancy, and dropping parking within a half-mile of the Sound Transit Federal Way Link Extension stations scheduled to open in 2026.

Renton regulates ADUs through Renton Municipal Code Title IV at RMC 4-2-110.A and 4-9-100. The city's 2024 update, Council Ordinance 6126, brought the code into HB 1337 alignment with two ADUs per lot, the 1,000-square-foot floor, no owner-occupancy, and a transit parking exemption tied to the South Renton Transit Center and the Sound Transit Stride S1 bus rapid transit corridor.

Kent's ADU rules are at Kent City Code Chapter 15.08.350. Kent's 2024 update, Ordinance 4513, eliminated the city's previous one-ADU-per-lot cap, raised the size limit to 1,000 square feet, dropped owner-occupancy, and exempted ADUs from parking within a half-mile of the Kent Sounder Station and the planned Stride S1 corridor along I-405.

Counties under HB 1337

HB 1337 applies to GMA-planning counties as well as cities, but only inside designated urban growth areas. King, Snohomish, Pierce, Kitsap, Thurston, Clark, Spokane, and Whatcom counties have all updated their unincorporated UGA zoning codes between 2023 and 2025 to comply. The most-relevant county codes are King County Code Title 21A.08.030, Snohomish County Code Title 30, Pierce County Code Title 18A, and Clark County Code Title 40.220.030. Outside UGAs — meaning on truly rural lots zoned RA-5, RA-10, F (Forestry), and similar — the two-ADU rule does not apply, and counties retain broad authority to limit accessory uses.

Practical effect: if you own a lot inside a city or within the unincorporated UGA donut around a city, HB 1337 applies. If you own a rural-zoned parcel outside the UGA, your county zoning controls, and most rural Washington zoning still allows only one ADU at most, subject to local discretionary review.

Common pitfalls

Five recurring issues catch Washington ADU builders. First, septic capacity on lots outside city sewer service: a DADU on a septic lot requires an upgraded design and Department of Health approval, often costing $20,000 to $40,000. Second, side-setback nonconforming garages: HB 1337 protects the setback nonconformity for the existing footprint, but a vertical expansion (adding a second story) does not inherit that protection. Third, design review in historic districts: HB 1337 does not preempt local historic preservation overlays, so a DADU in a Seattle landmark district or a Tacoma historic neighborhood still requires Landmarks Preservation Board review. Fourth, separate utility connections: Washington has no statutory bar on cities requiring separate water meters for ADUs, and several utility districts charge full new-connection fees. Verify with both the city planning department and the water/sewer utility. Fifth, building code separation: an AADU sharing a wall with the primary dwelling requires a one-hour fire-rated assembly under the Washington State Residential Code, and existing walls often fail without retrofit.

Financing

Washington benefits from both the 2023 FHA update — which allows borrowers to count up to 75% of projected ADU rental income toward mortgage qualification — and a state-specific program. The Washington State Housing Finance Commission launched the Covenant Homeownership Program in 2024 and a related ADU loan pilot in 2025, offering low-interest construction loans of up to $200,000 for ADU construction by low- and moderate-income homeowners, with deferred-payment second-mortgage structures designed to minimize monthly cash-flow impact.

Fannie Mae's HomeStyle Renovation loan and Freddie Mac's CHOICERenovation loan both finance ADU construction as part of a purchase or refinance. Seattle, Bellevue, and Tacoma all maintain lists of local credit unions and community banks offering ADU-specific construction-to-permanent loan products, which simplify financing compared with the typical separate-construction-loan-then-refinance pathway.

HOA preemption status

This is the most-asked question and the answer is more nuanced than the headline coverage of HB 1337 suggests. The statute does not contain an explicit override of homeowner association CC&Rs the way California Civil Code §4751 does. RCW 36.70A.681 governs municipal regulation, not private covenants. As of 2026, a Washington HOA covenant that prohibits accessory dwelling units is generally enforceable against the underlying property even though the city would permit the ADU. Several proposals to add an HOA preemption clause have been introduced in subsequent legislative sessions, most recently SB 5258 in 2025, but none has been enacted at the time of writing.

If you are buying in a planned community in the Seattle, Bellevue, or Vancouver suburbs and counting on the right to build an ADU, read the CC&Rs before closing. Older subdivisions in unincorporated King and Snohomish counties often have decades-old covenants that prohibit any secondary residence.

Tribal land considerations

Tribal land within Washington reservation boundaries is generally outside the GMA framework and therefore outside HB 1337. ADU rules on tribal trust land are set by the tribe under its own land-use code and by the Bureau of Indian Affairs through allotment-leasing rules. The Tulalip, Muckleshoot, Puyallup, Suquamish, Yakama, Spokane, and Colville tribes each have separate planning and permitting authorities, and several have adopted ADU-permissive codes mirroring HB 1337 in substance. If you are building on tribal land — including fee land within reservation boundaries in many cases — start with the tribal planning department, not the surrounding county.

How to check your city

State law tells you the floor. City code tells you whether your specific lot can support the ADU you want to build, with what design standards, what fees, and what permitting timeline. Before you hire an architect, answer six questions: Is my lot inside an urban growth area? What does my city code say about maximum height, design review, and historic overlay? Are impact fees waived under the 750-square-foot threshold? Is my lot within a half-mile of a major transit stop, exempting me from parking? Do CC&Rs apply to my lot? And what does the building permit pre-application process look like in my city? CityRuleLookup maintains an accessory structures page for every Washington city we cover, summarizing ADU allowance, size caps, setbacks, parking, and permit contacts. Start there, confirm with the city code, and if you are in an HOA, talk to a Washington real-estate attorney familiar with both HB 1337 and the underlying covenants before you commit to a design.