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 governs the approval process. In Seattle, the local building department 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: Varies. Building permit fee: Varies. Impact fees: None. School fees: None or N/A. 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: Permit timeline has not been confirmed — contact the department for current processing times. Apply through the local building department online permit portal. Phone: (206) 684-8600.
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: No lot may have more than two accessory dwelling units.. Multifamily properties: not confirmed — see official source.
- Single-Family Lots
- No lot may have more than two accessory dwelling units. · Verified July 14, 2026 · Source
- 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 detailsFrequently Asked Questions
Can I build an ADU in Seattle?
Yes. Seattle, WA allows accessory dwelling units. On a single-family lot you can build No lot may have more than two accessory dwelling units..
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