ADU Rules in Seattle, WA (2026)
ADU rules, permit requirements, size limits, and fees for Seattle, WA. Source-cited from official municipal code.
Yes, ADUs are allowed in Seattle, WA.
Seattle, WA permits accessory dwelling units (ADUs) on residential properties. ADUs — also called granny flats, in-law units, backyard cottages, or secondary suites — are secondary housing units on a single-family or multifamily lot. State and local law actively preempts local restrictions, making ADU approval a streamlined ministerial process. In Seattle, the Seattle Department of Construction and Inspections (SDCI) handles ADU permit applications. Below you will find the current size limits, setbacks, fees, and timeline for building an ADU in Seattle.
ADU Size Limits
ADU Size Limits in Seattle: Detached ADU: up to 1000 sq ft (max height: 24 feet). Attached ADU: 1000 sq ft. Washington does not use the California JADU (Junior ADU) concept. Seattle allows Detached ADU (DADU, sometimes called backyard cottage) and Attached ADU (AADU) up to 1,000 sqft each per Seattle Municipal Code 23.44.041. Both an attached and a detached ADU may be permitted on the same lot per HB 1337 and Seattle's 2019 ADU ordinance — up to 2 ADUs total.
| Type | Max Size | Max Height |
|---|---|---|
| Detached ADU | 1000 sq ft | 24 feet |
| Attached ADU | 1000 sq ft | Same as primary dwelling height limit per zone |
Permit Fees
ADU Permit Fees in Seattle: Plan check fee: $2,000–$5,000 depending on project value. Building permit fee: $1,500–$4,000. Impact fees: School impact fees apply: Seattle School District #1 charges approximately $3,700–$7,400 per new residential unit. School fees: Seattle School District #1 impact fees approximately $3,700 per attached ADU, $7,400 per detached ADU (check current rates with SDCI). Total estimated permit cost: $4,000–$12,000 in permit fees; school impact fees additional. Seattle charges school impact fees (SEPA mitigation) for new residential units including ADUs. Fee waivers may be available for affordable housing ADUs. Full fee schedule: https://www.seattle.gov/sdci/permits/fees
| Fee Type | Amount |
|---|---|
| Plan Check | $2,000–$5,000 depending on project value |
| Building Permit | $1,500–$4,000 |
| Impact Fees | School impact fees apply: Seattle School District #1 charges approximately $3,700–$7,400 per new residential unit |
| School Fees | Seattle School District #1 impact fees approximately $3,700 per attached ADU, $7,400 per detached ADU (check current rates with SDCI) |
| Total Estimate | $4,000–$12,000 in permit fees; school impact fees additional |
Permit Timeline
Seattle ADU Permit Timeline: Seattle SDCI (Department of Construction and Inspections) processes ADU permits. Standard DADU: 4-8 weeks plan review with SDCI standard queue. Pre-approved standard plans available for select DADU designs, reducing review time to 1-3 weeks. Seattle offers an Early Design Guidance process for complex projects. Construction typically 4-10 months. Apply through the Seattle Department of Construction and Inspections (SDCI) online permit portal. Phone: (206) 684-8850.
Setback Requirements
ADU Setback Requirements in Seattle: Front setback: Same as zone standard. Side setback: 5 feet. Rear setback: 5 feet. DADU setbacks: minimum 5 ft from side and rear property lines per SMC 23.44.041. Front setback follows zone standard. Corner lots: rear setback measured from lot line that abuts an alley or that would be the rear if no alley. DADUs in Seattle benefit from generous lot coverage allowances — lot coverage for DADUs is not counted against lot coverage maximum of primary dwelling.
Parking Requirements
Parking Requirements: No additional off-street parking required for ADUs within 0.25 miles of a major transit stop per Washington HB 1337. Seattle 2019 ADU ordinance eliminated all off-street parking requirements for ADUs citywide, regardless of transit proximity. Seattle is among the most transit-rich major US cities, reducing car dependency citywide.
Restrictions & Requirements
- ADU must comply with Seattle Municipal Code 23.44.041 objective standards
- Maximum 2 ADUs per lot (1 attached + 1 detached) per HB 1337 and SMC
- ADU cannot be sold separately from primary dwelling unless lot is subdivided
- Short-term rental of ADU requires Short-Term Rental Operator's License
- DADU must have permanent foundation — no recreational vehicles as permanent dwelling
- Alley access encouraged for DADU where alley exists
How Many ADUs Are Allowed?
Number of ADUs Allowed in Seattle: Single-family lots: 2 ADUs per lot (1 attached ADU + 1 detached ADU) per WA HB 1337 and SMC 23.44.041. Multifamily properties: 1 additional ADU per existing unit in LR1/LR2/LR3 zones; MR/HR zones follow floor area ratio standards. Seattle's 2019 ordinance allowed 2 ADUs before WA HB 1337 required it statewide. Both an attached ADU and a detached backyard cottage (DADU) may coexist on the same SF-zoned lot. LR zones allow additional units through lot coverage and FAR standards.
Single-Family Lots
2 ADUs per lot (1 attached ADU + 1 detached ADU) per WA HB 1337 and SMC 23.44.041
Multifamily Properties
1 additional ADU per existing unit in LR1/LR2/LR3 zones; MR/HR zones follow floor area ratio standards
State Law & Preemption
State Preemption: Washington House Bill 1337 (2023) requires all Washington cities and counties to allow at least 2 ADUs per single-family residential lot by-right. Removes owner-occupancy requirements. Reduces off-street parking requirements for ADUs within 0.25 miles of a major transit stop. Effective July 23, 2023. Seattle had already enacted a permissive ADU ordinance in 2019, making it among the first major US cities to allow 2 ADUs per lot before state law required it.
Historic District Considerations
Historic District Requirements: Seattle has 18 historic districts where additional design review may be required for ADU construction. Design review approval is required before obtaining a building permit for properties in historic overlay zones. Seattle has 18 Landmark Preservation Districts and numerous individually designated Seattle Landmarks as of 2026. Certificate of Approval required for exterior alterations to landmarks and for changes in landmark districts. Seattle Landmarks Preservation Board reviews significant changes. Designated landmarks database: https://www.seattle.gov/neighborhoods/historic-preservation/landmark-designation Look up historic district boundaries at: https://www.seattle.gov/neighborhoods/historic-preservation/landmark-designation/search
Frequently Asked Questions
Can I build an ADU in Seattle?
Yes. Seattle, WA allows accessory dwelling units. On a single-family lot you can build 2 ADUs per lot (1 attached ADU + 1 detached ADU) per WA HB 1337 and SMC 23.44.041.
How big can an ADU be in Seattle?
The maximum size for a detached ADU in Seattle is 1,000 square feet.
How long does it take to get an ADU permit in Seattle?
The typical ADU permit timeline in Seattle is Seattle SDCI (Department of Construction and Inspections) processes ADU permits. Standard DADU: 4-8 weeks plan review with SDCI standard queue. Pre-approved standard plans available for select DADU designs, reducing review time to 1-3 weeks. Seattle offers an Early Design Guidance process for complex projects. Construction typically 4-10 months.. Total estimated permit fees are $4,000–$12,000 in permit fees; school impact fees additional.
Do I need extra parking for an ADU in Seattle?
No, Seattle does not require additional parking for ADUs.
Do I have to live on-site to have an ADU in Seattle?
No, owner-occupancy is not required for ADUs in Seattle.
Source: Seattle Municipal Code 23.44.041 — Accessory Dwelling Units; Washington RCW 36.70A.681 (HB 1337 codification). Last verified April 5, 2026. View source