City · Washington

ADU rules in Seattle

4 sources

These are the primary sources for accessory dwelling unit regulations in Seattle: the city's own zoning code, planning department pages, and permit guides. Start here before talking to anyone: the rules below are what your permit reviewer will apply.

Rules at a glance

Detached (backyard) ADU

Allowed

ADUs are allowed as a housing use in all zones where housing uses are allowed, and may be attached, detached, or stacked. Up to two ADUs per lot.

SMC 23.42.022.A, .C, .D

Attached or internal ADU

Allowed

Attached ADUs (AADUs) allowed in all zones where housing uses are allowed; an attached ADU may exceed the 1,000 sq ft limit if the portion of the structure it occupies existed as of July 23, 2023.

SMC 23.42.022.A, .D, .H.4

Maximum size

1,000 sq ft gross floor area (ADU with up to two bedrooms); 1,200 sq ft (three or more bedrooms); up to 1,500 sq ft on qualifying LR-zone lots in a frequent transit service area. Excluded from the limit: up to 250 sq ft of attached garage, all underground stories or portions, and up to 35 sq ft of long-term bicycle parking.

The 1,500 sq ft tier also requires that the lot has not been purchased for more than $1,000 in the past 20 years (aimed at public or land-trust lots). Attached ADUs in structure portions existing as of July 23, 2023 may exceed 1,000 sq ft.

SMC 23.42.022.G, .H.4

Owner occupancy

Not required

SMC 23.42.022 (the complete ADU section, read in full) contains no owner-occupancy requirement, and a full-text search of the current code found no ADU owner-occupancy provision. Seattle repealed its requirement in 2019; SDCI even offers a 'Release of Owner Occupancy Covenant for ADUs' process for covenants recorded under the old rules.

SMC 23.42.022 (absence of requirement)

Extra parking

None. 'No off-street motor vehicle parking is required for an accessory dwelling unit.'

Citywide exemption, not transit-conditioned.

SMC 23.42.022.I

Approval process

Permitted use

ADUs are allowed outright as a housing use in all zones where housing uses are allowed; residential uses are permitted outright (P) in the Neighborhood Residential zone use table. Standard construction (addition/alteration) permit through SDCI; pre-approved DADU designs can shorten permit review.

SMC 23.42.022.A; SMC 23.44.020 Table A

Rental restrictions

No minimum lease term in the ADU section. Short-term rental use is expressly allowed in an ADU, but the operator must hold a City business license tax certificate and a short-term rental operator's license.

STR operator licensing and unit limits are administered under separate city licensing law (Title 6); the land use code provision read here is SMC 23.42.060.

SMC 23.42.060.B, .C

The fine print: 6 caveats that can change the picture

Maximum two ADUs per lot, in any combination of attached, detached, or stacked (SMC 23.42.022.C, .D).
ADUs count toward the zone's density limit along with principal units (SMC 23.42.022.J, 23.44.060.D.5); under the 2025 Neighborhood Residential rewrite, most NR lots are capped around 4 dwelling units (up to 6 near major transit or with affordable units per 23.44.060.C), so the two-ADU allowance operates inside those totals.
In the Shoreline District, ADUs must also comply with the Shoreline Master Program (Ch. 23.60A), which prevails over 23.42.022.
Conversions of accessory structures existing before July 23, 2023 into DADUs are permitted notwithstanding lot coverage, yard, and setback provisions, with Type I waivers available (SMC 23.42.022.H.3).
The 1,500 sq ft size tier has an unusual gate: LR zone, frequent transit service area, and the lot not purchased for more than $1,000 in the past 20 years (SMC 23.42.022.G.1.c).
The former ADU section SMC 23.44.041 was repealed; Chapter 23.44 is now the Neighborhood Residential chapter (Ord. 127376, 2025) and all ADU standards live in SMC 23.42.022. Old deep links to 23.44.041 are dead.

Summarized from Seattle's ordinance text, read July 2026; every row links the code section it came from. Rules often vary by zone and lot, so treat this as orientation, not a permit determination, and confirm with the planning department before you commit to a design.

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ADU regulations

Short-term rental rules

Planning to rent your ADU on Airbnb or VRBO? Seattle regulates short-term rentals separately; check these before counting on nightly rental income.

library.municode.comOfficial sourceVerified July 2026Municipal Code §23.42.060https://library.municode.com/wa/seattle/codes/municipal_code?nodeId=TIT23LAUSCO_SUBTITLE_IIILAUSRE_CH23.42GEUSPR_23.42.060SHRMRE

Washington statewide law also applies

Washington sets baseline ADU rights at the state level. Where local rules are stricter than state law allows, state law usually wins, so both are worth reading.

See Washington statewide rules

Spotted something outdated?

Zoning codes change. If a link is dead or Seattle has updated its ordinance, report it and we'll verify against the official source.

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