Building an ADU in unincorporated Snohomish County

Which areas this page covers

This page covers unincorporated Snohomish County only. The county's ADU bulletins say they apply "only to property within unincorporated Snohomish County and does not apply to property within incorporated City limits." If your home is inside a city, ask that city's planning office about its ADU rules.

Permits for unincorporated areas come from Snohomish County Planning and Development Services (PDS). [1][2][4]

What you can build

Snohomish County's ADU rules are in Snohomish County Code (SCC) 30.28.010. The county's ADU bulletins, revised June 2025, summarize them.

Urban zones. When the main home is a single-family house or a duplex, you may build up to two ADUs: two attached, two detached, or one of each. That means a duplex lot can have up to four homes. When the main home is a single-family attached dwelling, you may build one ADU.

Rural, resource and other zones. You may build one ADU, attached or detached.

Limits on detached ADUs. The county's detached ADU bulletin says they need a legally established single-family home on the lot, and they are not allowed on:

• Lots below the minimum lot area for the zone
• R-5 lots under 5 acres
• RC lots under 100,000 square feet

A rural or resource lot with a duplex can't also have an ADU. In rural, resource and other zones, a detached ADU must use the same driveway as the house. A mobile home may be a detached ADU only on lots of 10 acres or more outside the urban growth area. [1][2][5]

Size, setbacks and design

An ADU "shall not exceed 1,200 square feet." Garages, porches, unheated storage areas and unfinished basements don't count toward that limit.

The county says urban lots "may be eligible for reduced setbacks and increased lot coverage when proposing an Accessory Dwelling Unit." The county says it recently dropped the rule that an ADU match the design of the main home, raised maximum lot coverage and relaxed ADU setbacks. Ask PDS which setbacks apply to your lot. [1][2][5]

Parking

The county's bulletins say you still need the two off-street spaces required for the house. For the ADU itself, no parking is required in urban zones. In rural, resource and other zones, you need one off-street space per ADU.

The state ADU law's parking limits, such as no parking requirement within a half-mile walk of a major transit stop, apply only inside urban growth areas, and the county's urban zones already require no ADU parking. See our Washington ADU law page.

A separate 2025 state law, RCW 36.01.397, says a county may not require "more than one parking space per single-family home" or "any minimum parking requirements" for "Residences under 1,200 square feet." It has exceptions, including where "the county's roads are not developed to the standards for streets and roads adopted by the cities within that county." Counties of 50,000 or more people must put it in place within 18 months of July 27, 2025. Snohomish County has 879,700 people, according to the state's April 1, 2026 estimate. The county's ADU bulletins date from June 2025, before this law, so ask PDS which parking rules apply to your lot. [1][2][6][7][8]

Water, sewer and septic

Every ADU must show physical and legal access to potable water of adequate flow. You must also show that the sewer or septic system can handle the extra demand.

If your lot is on public water and sewer, you submit availability letters. If it's on a well and septic system, you need construction clearance from the health department. The county strongly recommends contacting the Snohomish County Health Department about septic requirements before you hire designers or pay for plans. Call the Snohomish County Health Department at (425) 339-5250. [1][2]

Fees

PDS bases plan check and permit fees on the county's valuation of your structure. Assistance Bulletin #18 (revised March 13, 2026) values each floor of a new residence at $175.92 per square foot and a garage at $71.07 per square foot.

You pay the base application fee and plan check fee before your application goes to review, and the permit fee when the permit is issued. Other fees include:

• Base fee: $150 per application
• Site review fee: $320
• Technology surcharge: 3 percent of permit fees
• School, park and traffic impact fees, which vary by project and location

State law caps ADU impact fees at 50 percent of the fees for the main home. Health department review for septic systems costs extra. For budgeting, see our ADU cost guide. [5][6][9]

Pre-approved ADU plans

Snohomish County has a Pre-Approved ADU Plan Catalog of plans "reviewed and pre-approved by PDS." You choose a plan, pay any licensing fee to the designer, and use the plan to apply for a building permit. The catalog lists plans from about 500 to 1,050 square feet.

The program, set by PDS Rule 25-02 (adopted February 2, 2026), covers detached ADUs only. When you apply with a pre-approved plan, the county's checklist says "the plan check fee is reduced from 65% to 20%." The same checklist says the program "is also limited to builders and companies licensed as general contractors in the State of Washington," so ask PDS whether you can use a catalog plan as a homeowner. [3][10][11]

How this fits state law

Washington's ADU law (RCW 36.70A.681) requires at least two ADUs per lot in urban growth area zones that allow single-family homes. It also bars owner-occupancy rules and street improvement requirements, and it bars counties from prohibiting the sale of an ADU as a condominium just because it was built as an ADU. The county says it "allows ADUs in the urban unincorporated areas in line with HB 1337."

State law says counties must adopt these requirements "to take effect at the same time as the jurisdiction's next periodic comprehensive plan update." For Snohomish County, that update was due "on or before December 31, 2024." A July 2025 county presentation instead said the deadline for jurisdictions in Snohomish County "was July 1st, 2025." Both dates have passed. Where a county hasn't adopted the rules, state law says they "supersede, preempt, and invalidate any conflicting local development regulations." The state rules apply only inside urban growth areas, so the county's one-ADU limit in rural zones is a local rule. [5][6][12][13]

Permit steps in Snohomish County

1. Confirm your lot is unincorporated and check the zone

   Make sure your property is outside city limits, and find out whether it's in an urban or a rural zone. Contact PDS online through AskPermitTech, or call 425-388-3311. [1][4]

2. Check water and septic early

   On a well or septic system, contact the Snohomish County Health Department before you pay for plans. On public utilities, request water and sewer availability letters. [1][9]

3. Pick a pre-approved plan or have plans drawn

   Choose a detached ADU from the Pre-Approved ADU Plan Catalog, or prepare a site plan and structural plans that meet the county's checklists. [1][3]

4. Apply in the PDS Permit Portal

   Create an account and apply in the PDS Permit Portal. Choose Construction Permit, Residential Building Permit, then Accessory Dwelling Attached or Accessory Dwelling Detached, work type New. [1][2][14]

5. Pay fees and answer review comments

   PDS checks your application for completeness, then posts an invoice for the base and plan check fees, which you pay before review starts. You pay the permit fee when the permit is issued. [9][14]

6. Build and pass inspections

   Schedule inspections through the PDS Permit Portal as the work goes on. [14]

Pre-approved and sample plans

• Pre-Approved ADU Plan Catalog

  Detached ADU plans reviewed and pre-approved by PDS; pay the designer's licensing fee and use the plan to apply for a building permit. [3][10]

Who to call

• Snohomish County Planning and Development Services (PDS)

  ADU permits, zoning and building questions for unincorporated Snohomish County

  425-388-3311

  Admin West Building, 2nd Floor, 3000 Rockefeller Avenue, Everett, WA 98201

  [1][4]

• Snohomish County Health Department

  Septic and well approvals for ADUs

  (425) 339-5250

  [2][9]

Questions people ask