Washington HB 1337 in 2026: Two ADUs on Every Urban Lot

Washington's House Bill 1337, passed in 2023, is one of the strongest state ADU laws in the country — and as of July 1, 2025, it is fully in force across the central Puget Sound region. The law requires cities and counties planning under the Growth Management Act to allow at least two accessory dwelling units on every lot in urban growth areas where single-family homes are allowed, bans owner-occupancy requirements, caps impact fees, limits parking mandates, and — almost uniquely among state ADU laws — protects the right to sell an ADU as a condominium. A year into full implementation, Seattle and Bellevue have both rewritten their codes, and the results tell you a lot about where backyard housing in Washington is headed.

What HB 1337 requires

The law's requirements took effect six months after each jurisdiction's periodic comprehensive plan update — July 1, 2025 for King County cities. Within urban growth areas, cities and counties must: • Allow two ADUs per lot in addition to the principal unit, in any configuration: one attached plus one detached, two attached, or two detached (in one or two structures). • Permit detached ADUs (DADUs) outright, on any lot that meets the minimum lot size for the principal unit. • Allow ADUs of at least 1,000 sq ft of gross floor area — cities cannot set a lower cap. • Allow DADU roof heights of at least 24 feet (unless the principal unit's height limit is lower). • Allow conversion of existing structures — including detached garages — into ADUs even if they violate current setback or lot-coverage rules. • Apply setback, yard-coverage, aesthetic, and design-review standards no more restrictive than those for the main house , and drop street-improvement requirements entirely.

Parking, impact fees, and condo sales

Three provisions do the heavy lifting on cost: • Parking: no off-street parking can be required within a half-mile walk of a major transit stop; at most one space per ADU on lots under 6,000 sq ft; at most two spaces on larger lots. (There is a narrow exception if a city proves a safety problem through a study certified by the Department of Commerce.) • Impact fees: capped at 50% of the fees that would apply to the principal unit — a meaningful saving, since impact fees on a new house can run into five figures. • Ownership and occupancy: cities may not require the owner to live on the property, and may not prohibit an ADU built as a condominium from being sold separately from the main house. That opens a genuine starter-home pathway: build a DADU, condominiumize, sell. Cities retain real authority too: they can still restrict short-term rentals of ADUs, apply health, safety, and environmental rules, prohibit ADUs on lots without sewer service, and limit ADUs in low-density zones overlapping wetlands, flood plains, or geologically hazardous areas. HB 1337 is a floor, not a free-for-all.

Seattle: consolidation and a height bump

Seattle was ahead of the state — its 2019 reform already allowed two ADUs per lot with no owner-occupancy and no parking requirement, and the city has been one of the nation's most productive ADU markets since. Still, HB 1337 (together with the HB 1110 middle-housing mandate) forced a June 30, 2025 code update via Ordinances 127211 and 127219. The city consolidated all ADU standards into a single code section, SMC 23.42.022, applying uniformly across residential zones. Key outcomes: • Two ADUs per lot plus the primary unit, everywhere residential development is allowed — as a detached unit, attached unit, or stacked apartment flat. • A uniform 1,000 sq ft ADU size limit across zones. • ADU height and setbacks no more restrictive than the main house — up to 32 feet in most Neighborhood Residential zones , a notable increase that makes two-story DADUs and rear-yard stacked flats far more practical. • An "attached" ADU now needs only a 3-foot connection to the main structure, and street-improvement requirements are gone. For current Seattle specifics — including how middle-housing rules can push many lots to three or more total units — see our Seattle ADU rules page.

Bellevue: a two-step reform

Bellevue's path is the more instructive one for suburban cities. In July 2023, ahead of the state deadline, the council adopted Ordinance 6746 , which overhauled attached-ADU rules: owner-occupancy was eliminated, the size cap rose from 800 sq ft to 1,200 sq ft (or 40% of the structure, whichever is greater) , extra design restrictions were dropped, and — ahead of the state on this point — the prohibition on condominium ownership was removed, so an ADU can be sold separately from the main home. Parking went tiered: none for units under 1,000 sq ft or near frequent transit. What Ordinance 6746 did not do was legalize detached ADUs — Bellevue was one of the last major Puget Sound cities where backyard cottages remained prohibited. That ended on June 24, 2025, when the council adopted Ordinances 6851 and 6852 to implement HB 1110 and HB 1337. Bellevue now must allow two ADUs per lot including DADUs, with DADU heights of at least the state's 24-foot floor, alongside broader middle-housing allowances of four to six units per lot near transit. For a city long defined by large-lot single-family zoning, allowing a 1,200 sq ft, 24-foot backyard cottage that can be sold as a condo is a genuine structural change. Current standards are on our Bellevue ADU rules page.

What about everyone else?

The same floor now applies across urban Washington — Tacoma, for example, allows two ADUs per lot at up to 1,000 sq ft with no parking or owner-occupancy requirements (see our Tacoma ADU guide). Implementation details still vary city to city: dimensional standards, tree and critical-area rules, sewer availability, and permit timelines are all local. And one honest caveat — statewide production data for the first year under full HB 1337 compliance is not yet published, so it is too early to say how much the 2025 code changes have moved permit volumes. The California experience suggests the effects compound over three to five years as designers, builders, and lenders adapt.

What it means for homeowners

If you own a single-family lot in a Washington urban growth area in 2026, your baseline rights are stronger than they have ever been: two units of up to 1,000 sq ft (1,200 in Bellevue), no requirement that you live on site, halved impact fees, little or no required parking, and the option to sell a DADU condo separately. The constraint that matters most now is usually your lot itself — sewer capacity, trees, slope, and budget. Start with our city-by-city guides for Seattle, Bellevue, and Tacoma to see exact setbacks, size limits, and permit steps for your city — then use our free ADU cost calculator to estimate construction costs and rental payback for your specific project.