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: varies — see official source). 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 · Verified June 1, 2026 · Source | See official source |
| Attached ADU | 1000 sq ft · Verified June 1, 2026 · Source | See official source |
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: Varies.
| 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 | See official source |
Permit Timeline
Seattle ADU Permit Timeline: Not codified 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: follows zone standard. Side setback: 5 feet. Rear setback: 5 feet.
Parking Requirements
Parking Requirements: No off-street motor vehicle parking is required
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
Statewide Preemption
ADU Preemption
Washington state law preempts local ADU rules — local governments cannot impose stricter standards.
View Washington preemption detailsState 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/programs-and-services/historic-preservation Look up historic district boundaries at: https://www.seattle.gov/neighborhoods/programs-and-services/historic-preservation
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 Not codified. Total estimated permit fees are Total ADU project cost not codified; all-in construction and fee estimate not established — see SDCI fee schedule for permit costs.
Do I need extra parking for an ADU in Seattle?
Seattle parking rules for ADUs: No off-street motor vehicle parking is required
Do I have to live on-site to have an ADU in Seattle?
Owner-occupancy requirement in Seattle: No owner-occupancy requirement. The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot
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