ADU Pass
Snohomish County
ADU Pass helps homeowners in Snohomish County, Washington navigate the permit paperwork for building an accessory dwelling unit. We cover 16 cities and 32 ZIP codes in this county.
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County ADU details
County ADU ordinance
Snohomish County, WA (841,000 residents north of Seattle) administers a county code for unincorporated territory under the Washington Growth Management Act (RCW Ch. 36.70A) and the 2023 statewide ADU preemption (HB 1337, codified at RCW 36.70A.681 et seq.). HB 1337 requires Washington counties planning under GMA to permit two ADUs (one attached, one detached, or two attached) on each lot in residential zones, with no owner-occupancy requirement, no minimum lot-size impositions exceeding the underlying district, no off-street parking required for ADUs within 1/2 mile of major transit, and ministerial review for code-compliant applications.
Code citations:
State-floor overlay: Washington HB 1337 preemption is comprehensive; the county ordinance must conform.
Adopting body: Snohomish County Council
County permitting (unincorporated parcels)
Snohomish County issues building permits for parcels in unincorporated territory through its development services / planning department, with separate review tracks for zoning conformance, building-code compliance, on-site sewage where applicable, floodplain compliance, and addressing. Inside incorporated municipalities, city departments handle their own permits; the county's authority is geographically limited to unincorporated territory. An ADU permit application is typically processed as a residential building permit with a zoning verification step against the county's ordinance for the parcel's zoning district.
Process overview: Typical workflow: (1) jurisdictional verification (parcel confirmed in unincorporated Snohomish County, not inside city limits or extra-territorial jurisdiction); (2) zoning verification against the county ordinance; (3) building-code plan review against the adopted state building code; (4) site-plan, septic (where applicable), and floodplain review; (5) issuance, construction with inspections, and certificate of occupancy.
Impact fees: Snohomish County permit fees are itemized at intake. Counties in Washington commonly do not levy municipal-style impact fees on residential additions in unincorporated areas; verify current fee schedule at the development-services counter.
County assessor
The Snohomish County property assessor / equalization office maintains parcel-level assessment records for all real property in Snohomish County. ADU additions are typically captured as improvements to the host parcel via shared permit data with the building department. Washington property-assessment rules govern annual revaluation cycles, homestead or principal-residence caps where applicable, and the procedures for protesting an appraisal.
Assessment policy: Improvement value for an ADU is added to the parcel record on the next regular revaluation cycle. Homestead / principal-residence caps where applicable shield the existing structure from rapid valuation increases but do not exempt new improvement value.
County overlays (4)
Snohomish County administers flood-hazard, and (where mapped) coastal, wildland-fire, historic, and airport overlays that shape ADU project feasibility. The most consistent overlay across the county is FEMA NFIP floodplain regulation; other overlays apply to specific geographies inside the county.
- FEMA NFIP Special Flood Hazard Areas in Snohomish County — A new ADU in a mapped SFHA must be elevated to or above the Base Flood Elevation; cost impact on the project is often material.
- Coastal / hurricane wind exposure — Confirm design wind speed and exposure category at the building department.
- Wildland-Urban Interface (WUI) hazard areas
- Historic districts and individually-listed historic resources
Washington state — ADU law and programs
State ADU law
Washington preempts local ADU regulation in urban growth areas of cities and counties subject to the Growth Management Act through Engrossed Second Substitute House Bill 1337 (2023), codified principally at RCW 36.70A.680-.681. Cities and counties planning under the GMA must allow at least two ADUs per lot in urban growth areas, must not require owner-occupancy, must not impose minimum-lot-size or setback rules stricter than those for the primary residence, and must allow ADUs to be sold separately as condominiums. Related House Bill 1110 (2023), codified at RCW 36.70A.635, requires 'middle housing' (duplexes, triplexes, fourplexes) in most residential zones of large cities.
- RCW 36.70A.680 — Accessory dwelling units — Requirements for cities and counties planning under this chapter — Sets the minimum entitlement floor for ADUs in GMA-planning jurisdictions: at least two ADUs per lot in urban growth areas, no owner-occupancy requirement, no stricter setback or lot-coverage rules than for the primary residence, no parking requirements within a half-mile of major transit stops, and no prohibition on selling an ADU separately as a condominium.
- RCW 36.70A.681 — Accessory dwelling units — Design, size, and aesthetic standards — Caps what local governments may require in ADU design and size standards. Maximum ADU size must be at least 1,000 square feet in most zones. Street-facing facades and aesthetic review may not be used to categorically prohibit ADUs.
- RCW 36.70A.696 — Accessory dwelling units — Definitions — Statutory definitions of 'accessory dwelling unit,' 'attached accessory dwelling unit,' 'detached accessory dwelling unit,' 'owner,' and 'urban growth area' used by RCW 36.70A.680-.681.
- RCW 36.70A.635 — Middle housing — Cities planning under this chapter — HB 1110 (2023). Requires cities over specified population thresholds to allow duplexes, triplexes, fourplexes, townhouses, cottage housing, and stacked flats in most residential zones. Interacts with ADU law because ADUs and middle housing both contribute to the state's response to the GMA's housing element.
State HOA preemption
Washington enacted HOA preemption of ADU bans in Substitute House Bill 1337 (2023), which amended the Washington Uniform Common Interest Ownership Act and the Washington Horizontal Property Regimes Act to prohibit common-interest communities from banning ADUs on lots in GMA-planning jurisdictions. A homeowners' association may impose reasonable aesthetic or architectural standards, but may not categorically prohibit ADUs.
- RCW 64.38.057 — Homeowners' associations — Accessory dwelling units — Prohibits a homeowners' association governing a plat within a GMA-planning city or county from recording or enforcing a provision that prohibits the construction, use, or rental of an accessory dwelling unit on a lot. Reasonable design guidelines consistent with RCW 36.70A.681 remain enforceable.
State financing programs
The Washington State Housing Finance Commission (WSHFC) administers state-level housing finance programs. Washington does not currently operate an ADU-specific statewide grant or forgivable-loan program comparable to California's CalHFA ADU Grant, but several state programs can be used for ADU construction when program criteria are met.
State housing programs
Washington supports ADU implementation through Department of Commerce technical assistance and grant programs rather than through a single statewide pre-approved-plan catalog. The state provides model ADU code under the GMA, distributes GMA update grants to help jurisdictions comply with RCW 36.70A.680-.681, and has produced ADU guidance documents for local governments.
Federal (United States) — ADU-relevant rules and programs
Federal ADU law
The United States has no federal statute that directly regulates accessory dwelling unit entitlement or design. Land-use authority over ADUs resides with states and local governments under the traditional police power. Federal engagement is limited to financing (Fannie/Freddie/FHA/VA/USDA), flood insurance (FEMA/NFIP), and discretionary housing programs (HUD), which are recorded in sibling sections of this file.
Federal financing programs
Federal housing-finance agencies and GSEs set nationwide underwriting rules that govern whether an ADU can be financed, appraised, and counted toward mortgage qualifying income. The relevant actors are Fannie Mae, Freddie Mac, FHA (HUD), VA, and USDA Rural Development.
Federal tax credits
There is no ADU-specific federal tax credit. ADUs may incidentally qualify for existing federal energy-efficiency and clean-energy tax credits when the ADU construction includes qualifying measures.
Federal housing programs
HUD administers several discretionary programs that can fund ADU-related activity at the grantee's election, but none is an ADU-specific program.