ADU Pass
Seattle
King County portion
Also in: No County
ADU Pass helps homeowners in Seattle, King County, Washington navigate the permit paperwork for building an accessory dwelling unit. This area covers 29 ZIP codes.
Map
ADU details
ADU legality: allowed
Washington preempts most local ADU restrictions. Seattle permits ADUs by right in single-family zones per its zoning ordinance.
Cost scenarios
| Scenario | Sq ft | Permit | Build | Total |
|---|---|---|---|---|
| minimum | 150 | $2,900 | $82,200 | $85,100 |
| 600 | 600 | $2,900 | $328,800 | $331,700 |
| midpoint | 675 | $2,900 | $369,900 | $372,800 |
| maximum | 1,200 | $2,900 | $657,600 | $660,500 |
Fee breakdown (as of 2026-04)
Permitting process
- Site eligibility and ADUniverse feasibility check (~4d)
Use ADUniverse (aduniverse-seattlecitygis.hub.arcgis.com) to confirm SMC 23.42.022 development standards on the parcel, identify Neighborhood Residential subzone (NR1/NR2/NR3) under SMC 23.44, and check overlay districts (SDCI mapping for shorelines, ECA, landmark district). - Pre-Application Site Visit (optional, recommended for DADU) (~14d)
Request an SDCI Pre-Application Site Visit (PASV) for early staff feedback on DADU siting, tree-protection ordinance compliance, and shoreline/ECA constraints before formal intake. - Submit construction addition/alteration permit via Seattle Services Portal (~1d)
Apply at cosaccela.seattle.gov (Seattle Services Portal) using the Construction Addition/Alteration permit type. Include site plan, floor plans, elevations, structural calcs, WSEC 2021 energy compliance package, and SMC 23.42.022 compliance worksheet. Pre-approved DADU packets streamline intake. - Intake screening and routing (~7d)
SDCI intake reviewer screens for completeness; routes to land use, building, drainage, and Seattle Fire concurrent review queues. Pre-approved DADU plans bypass design portion of land-use review. - Concurrent corrections cycle (zoning, building, drainage, fire, SPU) (~35d)
Plan reviewers issue correction notices through the portal. Typical 1-2 cycles for custom DADU; pre-approved track usually 0-1. Seattle Fire reviews access roads and sprinkler exemption per WSAFC. SPU reviews stormwater per Stormwater Code. - Permit issuance and fee payment (~3d)
Final fees due include SDCI permit/plan-review fees, King County Sewage Capacity Charge, and impact fees if any. Permit issued through portal; valid 18 months with extension. - Construction inspections via Seattle Services Portal
Foundation, framing, mechanical/plumbing/electrical rough, insulation, drywall, final. Seattle City Light coordinates separate electrical service permit. Inspections requested through portal or 24-hour phone line. - Certificate of Occupancy and DADU recording (~5d)
Final inspection triggers CO. ADU eligible for occupancy and rental; STR registration handled separately under SMC 6.600.
Viability (permitted uses)
- Long-term rental: yes Long-term rental of ADU explicitly permitted; Washington owner-occupancy preemption (where applicable) makes ADU eligible for full landlord-tenant treatment.
- Short-term rental: with-restrictions STR rules vary by city. Seattle regulates STRs separately from ADU permitting; check local STR ordinance and HOA covenants.
- Office rental: with-restrictions Detached office rental requires home occupation permit or rezoning.
- Home office: yes Home occupation permitted with restrictions on signage and customer traffic.
- Studio / workshop: yes Personal artist studio is a permitted accessory use.
- Agriculture: with-restrictions Limited urban agriculture permitted in residential zones; livestock varies by district.
- Relative support: yes Family-occupancy ADU explicitly permitted in single-family zones.
Incentives
Pre-approved plans Pre-approved plans
Contacts
Staff: SDCI Applicant Services Center (Permit intake and general questions), Seattle City Light - New Service (Electrical service for ADUs), Seattle Public Utilities - Side Sewer Permits (Side sewer / drainage review)
Utilities
- Water: Seattle Water Utility · 21d connect · $4,500
- Sewer: Seattle Sewer / Wastewater · 21d connect · $5,500
- Electric: Seattle Electric Utility · 14d connect · $1,800
- Gas: Seattle Gas Utility · 21d connect · $1,500
Property values & taxes
Construction timeline
Realistic total: best 7mo · typical 11mo · worst 17mo
Financing
State ADU loans:
Insurance impact
HOA prevalence & 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.
Regulatory overlays (2)
- historic-district
Seattle historic districts trigger Architectural Review Board / Historic Preservation Commission review for ADUs in historic boundaries. - airport-noise-zone
Airport noise contours affect some parcels and may require sound-attenuation construction.
Technical envelope (climate & building code)
Climate & energy code
Building code
Amendments:
- Amendment
- Amendment
Legal history (timeline)
Current ordinance: Seattle Municipal Code Chapter 23.42.022 (commonly applied ADU standards) and 23.44 (Neighborhood Residential development standards), adopted 2019-07-01, last amended 2025-07-07
- 2010-11-22 — Seattle Backyard Cottage Ordinance citywide expansion (Ord. 123473) (city-ordinance)
Expanded backyard cottage (DADU) eligibility from Southeast Seattle pilot to all single-family zones citywide.
Effect: First citywide DADU framework; retained owner-occupancy and parking minimums later removed in 2019. - 2019-07-01 — Seattle ADU Reform Ordinance 125854 (Ord. 125854 / CB 119544) (city-ordinance)
Comprehensive AADU/DADU reform allowing two ADUs per single-family lot, removing owner-occupancy, eliminating off-street parking minimum, and raising height/FAR.
Effect: Permits more than doubled within 24 months; established the modern SMC 23.42.022 commonly applied standards for ADUs across all residential zones. - 2023-05-08 — Washington EHB 1337 (Chapter 332, Laws of 2023) (state-law)
Statewide ADU enabling act for cities and counties planning under GMA, requiring two ADUs per lot, no owner-occupancy, no off-street parking within 1/2 mile of major transit, and 1,000 sqft minimum size allowance.
Effect: Seattle's ordinance already exceeded the state floor; minor compliance edits adopted to align height-measurement and lot-coverage exceptions. - 2024-09-10 — ADU Code Updates Director's Report (D4 SEPA Draft) (city-ordinance)
SDCI Director's Report initiating ADU height and bulk amendment package to harmonize with One Seattle Plan zoning.
Effect: Refined DADU height permitted in low-rise zones and eased lot-coverage limits where green-roof or solar incorporated. - 2025-07-07 — One Seattle Plan permanent state zoning compliance ordinance (city-ordinance)
One Seattle Plan land-use update reconciling SMC 23.42 / 23.44 ADU rules with HB 1110 (middle housing) and EHB 1337 (ADUs) statewide mandates.
Effect: Codified four-plex / six-plex middle housing alongside two-ADU-per-lot allowance; preserved Neighborhood Residential ADU pathway.
King County — county ADU rules and overlays
County permitting (unincorporated parcels)
King County handles unincorporated permitting under its planning/development department.
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.
ZIP Codes
- 98101
- 98102
- 98103
- 98104
- 98105
- 98106
- 98107
- 98108
- 98109
- 98112
- 98115
- 98116
- 98117
- 98118
- 98119
- 98121
- 98122
- 98126
- 98133
- 98134
- 98136
- 98144
- 98146
- 98154
- 98164
- 98174
- 98177
- 98178
- 98199
Post Office
- 1329 N 47th St, 98103
- 2010 15th Ave W, 98119
- 212 Broadway E, 98102
- 2420 4th Ave S, 98134
- 2460 4th Ave S, 98134
- 2721 SW Trenton St, 98126
- 301 Union St, 98101
- 3019 NE 127th St, 98125
- 3613 S Juneau St, 98118
- 3795 S Alaska St, 98118
- 415 1st Ave N, 98109
- 4244 University Way NE, 98105
- 4412 California Ave SW, 98116
- 4501 9th Ave NW, 98107
- 620 S Orcas St, 98108
- 7633 S 126th St, 98178
- 7724 35th Ave NE, 98115
- 929 N 145th St, 98133