Snohomish County, WA ADU (accessory dwelling unit) Rules and Regulations (2026)

Snohomish County’s ADU rules are short and specific, and they cover unincorporated land only. The code section, SCC 30.28.010, was last amended by Ordinance 25-014, effective March 30, 2025. Planning and Development Services backs it with two assistance bulletins, #12 for detached ADUs and #13 for attached ADUs, both revised June 2025.

Both bulletins state that they apply only to unincorporated Snohomish County, not to property inside city limits. For the statewide rules behind the county code, see our Washington ADU laws guide.

Who reviews your lot

A city mailing address does not settle jurisdiction; confirm city limits for the parcel.

Rules that apply everywhere in the county

SCC 30.28.010(1) sets the baseline for every unincorporated ADU:

• Same lot as a legally established principal unit.
• Water. Physical and legal availability of water, with documentation that it is potable and of adequate flow.
• Sewage. Documentation that the existing or proposed sewer or septic system can handle the added demand.
• Size. Floor area can’t exceed 1,200 square feet, not counting garages, porches, unheated storage, and unfinished basements.
• Parking under chapter 30.26 SCC. The bulletins translate that: no added parking in urban zones; one off-street space per ADU in rural, resource, and other zones, on top of the two spaces for the house.

Urban zones

In the urban zones, ADUs are a permitted use on lots with a single-family, single-family attached, or duplex dwelling.

How many:

• Single-family home: up to two ADUs — one attached and one detached, two attached, or two detached in one or two structures.
• Single-family attached dwelling: one ADU, attached or detached. The code also lets detached ADUs on two adjacent lots be combined into one structure under specific lot-line conditions.
• Duplex: up to two ADUs in any attached/detached mix.

Bulk standards are relaxed for lots with ADUs:

• Maximum lot coverage rises to 55 percent.
• Setbacks from private roads drop to 5 feet for new principal units, ADUs, and covered parking.
• Setbacks from public roads for ADUs drop to 5 feet from the right-of-way for roads 60 feet wide or more and for narrower roads inside a recorded subdivision, short subdivision, or binding site plan. Other narrow public roads follow a separate centerline rule.
• An existing structure that doesn’t meet current setback or lot-coverage standards can be converted into an ADU, with any conditions the director needs for health and safety.

Fire access. Any driveway or shared driveway serving three or more dwelling units must meet the county’s fire-lane requirements. A house plus two ADUs is three units, so check the fire-lane standard before you site the second unit.

Rural, resource, and other zones

Outside the urban zones, the rules tighten:

• One ADU, attached or detached, on a lot with a legally established single-family dwelling.
• Detached ADUs are prohibited on lots that don’t meet the zone’s minimum lot area, on R-5 lots under five acres, and on RC lots under 100,000 square feet.
• Shared driveway. The ADU must use the same driveway as the house.
• Duplex lots aren’t eligible for an ADU in rural or resource zones.
• Mobile homes. A mobile home subordinate to the house may serve as a detached ADU only on lots of 10 acres or more outside the Urban Growth Area. That rule is about mobile homes; a modular home built to the residential code is reviewed as a dwelling.

Water, septic, and the Health Department

The bulletins require one of two things with the application:

• Water and sewer availability letters for projects on public water and sewer, or
• Construction clearance from the Snohomish County Health Department for projects on septic and a well.

The county “strongly” recommends contacting the Health Department about septic requirements before hiring experts or paying for plans. The septic answer can decide the bedroom count, the location, and whether a detached unit works at all.

Critical areas, shoreline designations, steep slopes, and wetland buffers can also shrink the buildable area. State law lets counties restrict ADUs in residential zones of one dwelling unit per acre or less that fall inside wetlands, fish and wildlife habitat, floodplains, or geologically hazardous areas, and to apply the same health and environmental rules as for a principal unit (RCW 36.70A.680).

Applying

Applications go through the county’s online permit portal. For a detached unit, the bulletin’s path is Construction Permit → Residential Building Permit → Accessory Dwelling Detached → New. For an attached unit, choose Accessory Dwelling Attached. Expect to provide:

• Residential building permit fees — part at application, the rest at issuance.
• A site plan meeting the county’s residential site plan checklist.
• Construction plans meeting the structural submittal checklist.
• The water and sewage verification above.

Owner occupancy, rentals, and state law

SCC 30.28.010 doesn’t include an owner-occupancy requirement, and inside the Urban Growth Area state law bars one (RCW 36.70A.681). The same statute caps ADU impact fees at half of the principal unit’s on qualifying urban lots and bars a county from prohibiting the sale of a former ADU as a condominium unit solely because it was built as an ADU. Counties may still restrict short-term rentals of ADUs. Rental plans need their own check.

How a factory-built ADU fits

Wolf homes are modular, built to Washington’s residential code — not HUD (the federal standard for manufactured homes) manufactured homes and not mobile homes. Washington L&I reviews the factory plans and oversees factory inspections, and each home carries its L&I insignia before shipping. The county still reviews zoning, the foundation, utility connections, and site inspections.

The 1,200-square-foot cap leaves room for a range of floor plans. Compare our models against it and against the setbacks on your lot. Snohomish County sites are beyond the standard 20-mile delivery in Wolf’s published installation scope, so transport and setting are quoted after the site review; see cost and the Washington ADU cost guide.

What to have ready

1. Address or parcel number, and whether it is inside a city.
2. The zone and lot area. On a rural lot, whether the lot meets the detached-ADU minimum.
3. Water and sewer letters, or your septic permit, as-built, and well records.
4. The driveway layout, especially if a second ADU would make three units.

Send us the address and we’ll confirm the rule set, the Health Department path, and the model fit before you spend on design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.