Washington’s ADU Laws: What Changed and What It Means for You

Over the last few years Washington has rewritten the rules on accessory dwelling units — and it's a big reason searches for ADUs across the state have surged. The short version: it's now easier, in more places, to build a second home on a lot you already own. Here's the plain-English version for homeowners.

Why Washington changed its ADU laws

Washington is short on housing, and ADUs are one of the fastest, lowest-impact ways to add homes without changing neighborhood character. To encourage them, the state passed a 2023 ADU law (House Bill 1337) that requires many cities and counties to roll back the rules that used to make backyard cottages hard to build.

What the law changed

Under Washington's ADU reforms, cities planning under the state Growth Management Act generally have to allow:

• More units per lot — up to two ADUs on most single-family lots (for example an attached ADU plus a detached DADU). In Seattle those two now sit inside a wider density allowance — see what you can build on a Seattle lot.
• No owner-occupancy requirement — you no longer have to live on-site to have an ADU, which makes rentals far more practical.
• Reduced parking mandates — within half a mile of a major transit stop, cities cannot require any added off-street parking for an ADU. Seattle goes further and requires none anywhere.
• Reasonable size allowances — cities must permit ADUs up to at least around 1,000 square feet.
• Capped fees — limits on the impact fees local governments can charge for an ADU.

The intent across all of it is the same: fewer arbitrary barriers between a homeowner and a permitted backyard home.

What it means for Seattle-area homeowners

Seattle was already ahead of the curve — the city loosened its ADU rules back in 2019 — so many Seattle lots have been DADU-friendly for a while. What the state law does is push that same direction out across King, Snohomish and the rest of Washington, so homeowners in more cities now have a realistic path to build. If you've wondered whether your lot qualifies, the answer is “more likely than it used to be.” See the specifics in our ADU & DADU FAQ and DADU permit guide.

One provision of the state law gets very little attention: RCW 36.70A.681(1)(k) bars a city from prohibiting the sale of a condominium unit separately from the principal unit just because it was built as an ADU. Seattle mirrored it in 2025. What that does and does not make possible is in can you sell a DADU separately in Washington.

2026 update: HB 1345 and rural detached ADUs

Everything above applies inside urban growth areas. In 2026 the Legislature filled the gap for rural land: House Bill 1345 (effective June 11, 2026) lets a county that plans under the Growth Management Act choose to allow one detached ADU per parcel outside its UGAs, capped at 1,296 sq ft, sited within 150 feet of the house, sharing its driveway, with metered water and documented septic capacity — paired with mandatory enforcement against unpermitted units. It is permissive, so your county has to adopt it before it applies to your parcel. The full breakdown, including what it means in Snohomish and King County, is in Washington's new rural DADU law.

Does this apply to my city?

Here's the important caveat: the state sets the floor, but each city and county implements the rules locally, on its own timeline, with its own design standards, setbacks and critical-area rules. So the exact standards for your property depend on where you are. That's exactly what a feasibility study settles — we confirm what your specific lot allows before you spend real money.

Thinking about building under the new rules?

Easier rules don't change the fundamentals of a good build — you still want honest pricing and a builder who knows local permitting. We've completed 100+ ADUs, DADUs and homes across the Greater Seattle area. Outside Seattle city limits the details shift — see ADU rules in Snohomish and King County. Inside Seattle city limits, the implementing ordinance sets the numbers you'll design around — see how big a DADU can be in Seattle. Estimate your project with the ADU cost calculator, learn how to choose an ADU contractor, or get in touch and we'll tell you what your lot can do under today's rules.

State law is the floor, not the whole answer — each city builds its own rules on top of it, and on the Eastside the three big ones diverge a long way. For what Bellevue, Kirkland and Redmond actually allow in 2026, see ADU rules on the Eastside. Outside an urban growth area the state mandate does not apply at all, which matters on most septic parcels — building an ADU on a septic system covers that.