What HB 1337 actually changed about building on your lot

In 2023 the Washington legislature passed House Bill 1337, now codified at RCW 36.70A.681. It is the reason a second home on a single-family lot went from “ask your city and hope” to “your city has to allow it, within limits.” Cities and counties in King, Snohomish, Pierce and Kitsap had until June 30, 2025 to bring their codes into line; the ones that missed the deadline have the conflicting local rule preempted by the statute.

Here is what the law requires, in plain terms, and then the part that matters more: what it leaves to your city.

Where it applies

Inside an urban growth area, in zones that allow single-family homes. It does not apply to lots with critical areas or their buffers (wetlands, streams, steep slopes, landslide hazard areas), and it does not apply outside the UGA. A related 2026 bill, HB 1345, lets counties choose to allow one detached unit per lot outside the UGA, but does not require it.

The floor the state sets

Every city and county covered by the law must, at minimum:

• Allow at least two accessory dwelling units on every lot — one attached and one detached, two attached, or two detached.
• Allow an ADU on any lot that meets the minimum lot size for the main house. No separate, larger minimum for lots with ADUs.
• Not cap ADU size below 1,000 square feet. Cities may allow more; several do.
• Not cap ADU height below 24 feet, unless the main house’s limit is lower.
• Not impose setbacks, lot coverage, tree retention, entry-location or design-review rules stricter than those for the main house.
• Allow existing structures, including detached garages, to convert — even if they violate current setback or coverage rules. A garage two feet off the property line can often become a dwelling where a new building could not.
• Require no parking within half a mile of a major transit stop, and no more than one space per unit on lots under 6,000 square feet or two on larger lots elsewhere.
• Not require the owner to live on the property. Owner-occupancy requirements are gone.
• Not prohibit selling the unit as a condominium solely because it was built as an ADU.
• Cap ADU impact fees at half of what the main house would pay, and not require public street improvements as a condition of the permit.

What your city still decides

The statute is a floor, not a plan. Everything above the floor is local, and it varies more than most people expect between neighboring jurisdictions.

The size cap. Seattle allows 1,000 square feet, or 1,200 with three or more bedrooms. Everett defines an ADU as a unit under 1,000 square feet and treats a larger one as simply another dwelling — and in its NR, UR and MU zones there is no cap on the number of dwellings on a lot, only on how many get the reduced fees. Unincorporated Snohomish County allows 1,200 square feet, excluding garage, porches and unheated storage.

The height limit. Seattle’s neighborhood residential zones allow 32 feet. Snohomish County’s R-9,600 and R-8,400 zones allow 30. Everett applies the zone’s height.

Setbacks and coverage. Seattle requires 5 feet from lot lines and 0 at an alley, with 50 percent coverage. Snohomish County’s common zones set front 10, side 5, rear 5, and raise coverage from 35 to 55 percent when an ADU is added — on a tight lot, that single number is often the difference between yes and no. Everett applies the main house’s setbacks.

Fees. Seattle exempts ADUs from impact fees entirely. Everett waives transportation impact fees on the first ADU and halves them on the second, and halves park fees on the first two. Snohomish County applies the state’s 50 percent cap.

Pre-approved plans. Seattle’s ADUniverse program has ten plans that permit in two to six weeks. Everett unveiled three designs (820 to 1,000 square feet) in June 2026 with a three-month permit target. Snohomish County’s program was in development as of July 2026, and Edmonds launched one on July 28, 2026. A plan “pre-approved” in one city is not pre-approved in the next.

Short-term rentals. RCW 36.70A.680 preserves a city’s authority to restrict short-term rental use, to apply its building and environmental codes, and to prohibit ADUs on unsewered lots. Seattle requires a short-term rental license, at $75 per unit per year.

The separate-sale part, because people ask

Under the statute, a city cannot prohibit the sale of a condominium unit solely on the grounds that it was originally built as an accessory dwelling unit. That makes the second home a separate address, not just a spare room. It has been done: Lumina Unit C, a 502-square-foot one-bedroom in Bothell, was permitted, built by [Builder of record — pending] and sold as its own condominium unit. Whether it works on your lot depends on your city, your lender and your title company, and it takes a declaration, a survey and legal work, so it is a thing to decide early rather than late.

What this means for a homeowner

The state removed the three objections cities used to raise: you can have two, you don’t have to live there, and you can sell it. What it did not do is make every lot buildable. Your buildable area, your access, your trees, your sewer and your city’s actual numbers still decide the project, and those are the things the report checks. Read the buildability checklist for the eleven items, then send us the address.