Washington’s New Rural DADU Law (HB 1345): Building Outside the Urban Growth Area

Washington's 2023 ADU reform (HB 1337) was deliberately limited to land inside urban growth areas. If your parcel is rural — and a great deal of Snohomish County and eastern King County is — the state mandate simply did not reach you, and whether you could build a backyard home depended entirely on your county's code and, in Snohomish County's case, on a Growth Management Hearings Board decision. That changed on June 11, 2026, when House Bill 1345 took effect. Here is what the law actually says, what it does not do, and what it means if you own acreage around Monroe, Snohomish, Duvall or anywhere else outside a city's growth boundary.

The one-paragraph version

HB 1345 (Chapter 231, Laws of 2026) lets a county that plans under the Growth Management Act choose to allow one detached accessory dwelling unit per parcel outside its urban growth areas, provided the county adopts a specific set of development limits, a specific set of code enforcement measures, and tracks and reports the units it permits. It passed the House 86–5 and the Senate 33–15 and was signed on March 27, 2026. The key word is choose: it is permissive, not a mandate. A county may adopt it, adopt something stricter, or do nothing.

The development limits a rural DADU must meet

If a county opts in, any detached ADU it permits outside a UGA must be subject to regulations that include all of the following:

• One unit, total. No parcel may have more than one ADU, attached or detached. Inside a UGA the floor is two; out here it is one.
• A 1,296 sq ft ceiling. Gross floor area may not exceed 1,296 square feet, excluding garages, porches and unfinished basements — and may not exceed what the county would have allowed as an addition to the house to create an attached ADU.
• Within 150 feet of the house. The DADU must be sited within 150 feet of the principal unit, and must share its driveway or other access. This is the rule that decides most rural site plans.
• Water, proven and metered. The unit must meet the State Building Code's water-supply requirements and any county or Ecology groundwater mitigation rules; the combined withdrawal for the house, the DADU and other domestic use may not exceed the state's domestic-use limits for a permit-exempt well; and each dwelling's water use must be metered.
• Septic capacity, documented. The applicant must show the sewage or septic system can handle the added demand. On most rural parcels this, not zoning, is the real gate — see building an ADU on a septic system.
• Not on small nonconforming lots. No detached ADU on nonconforming lots under one acre, and a parcel may not be subdivided to get around the rules.
• The old house can become the ADU. If an existing single-family home meets these limits, a new principal house may be built and the original becomes the DADU — a useful path on older farm parcels.

The enforcement side nobody reads

The bill pairs the new permission with teeth, and if you already have an unpermitted cabin, converted barn or “guest house” on rural land this part applies to you. An opting-in county must offer a voluntary compliance process for owners of unpermitted detached ADUs, with a permit penalty of at least double the normal fee. Owners who do not come forward and are found with an unpermitted unit face a civil infraction of at least $1,000, must remove the unit or bring it up to code (at a penalty of at least triple the fee if it stays), and are barred from any new ADU permits for at least three years. Counties must also report completed rural DADU permits to the Department of Commerce every year and fold the numbers into their growth targets — rural counties may allocate no more than 10 percent of rural population growth to detached ADUs, other counties no more than 7 percent. That cap is the quiet mechanism that keeps this from becoming open rural subdivision, and it is also why county adoption may come with its own limits on top.

What it means in Snohomish County

Our home county is the obvious test case. Snohomish County rewrote its ADU code in 2021, capped units at 1,200 sq ft and required a rural DADU to sit near the main house and share its driveway — rules that already rhyme with HB 1345. Since then the rural side of that code has been contested and revised more than once, including a Hearings Board decision that pulled detached units back on smaller rural lots. Whether and how the County adopts the HB 1345 framework is a County Council decision, and the County's current rules are what govern until it does. Do not assume the state numbers apply to your parcel yet. Check the current code with Planning and Development Services, or send us the address and we will. The broader picture of how county rules differ from Seattle's is in ADU rules in Snohomish and King County.

What it means in King County

Most of King County's population is inside a UGA, where HB 1337 already applies and this bill changes nothing. The rural east — the Snoqualmie Valley, the foothills, the parcels between the cities — is where HB 1345 could matter, and again only if the County chooses to use it. The same advice holds: the UGA line, not the mailing address, is what decides which set of rules you are under, and a feasibility study starts by finding that line.

Does any of this change how we would build it?

No. A rural DADU is a small house on a well-and-septic site, and the sequence is the one we have always used on acreage: confirm water and septic capacity first, because they decide the bedroom count and sometimes the whole project; then site the unit where the 150-foot rule, the drainfield reserve area and the access all agree; then design to the size cap. The feasibility study is where those questions get answered, and on rural land it is the single best money in the budget. If you are weighing a backyard home on acreage in Monroe, Snohomish or anywhere in the two counties, get in touch and we will tell you what the parcel can do under the rules that apply to it today. For the state law this one builds on, see Washington's ADU laws explained.