Can I Build ADU in Seattle, WA? (2026)
Find out if you can build a adu in Seattle, WA. Rules, permit requirements, costs, and next steps.
Yes, ADUs are allowed in Seattle, WA. California state law ensures by-right approval.
An Accessory Dwelling Unit (ADU) is a secondary housing unit on a residential lot — also called a granny flat, in-law suite, or backyard cottage. In Seattle, WA, ADUs are permitted by right under state law, making them one of the most accessible ways to add housing on your property. ADUs can be detached, attached to the main home, or created by converting an existing garage or accessory structure.
ADU Rules in Seattle
ADU Rules in Seattle: Detached ADU max size: 1000 sq ft. Attached ADU max size: 1000. Setbacks: 5 ft side, 5 ft rear. Owner-occupancy: Washington HB 1337 (2023) prohibits local owner-occupancy requirements for ADUs statewide. Seattle eliminated its owner-occupancy requirement in 2019 as part of its ADU ordinance update, predating state law. Neither the primary dwelling nor the ADU is required to be owner-occupied. ADU may be rented short-term with a Short-Term Rental Operator's License.. State preemption applies: WA law requires Seattle to approve compliant ADUs by right.
Costs & Fees
ADU permits in Seattle: Total estimated permit cost: $4,000–$12,000 in permit fees; school impact fees additional. Contact the Seattle Department of Construction and Inspections (SDCI) for current fee schedule.
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.
Next Steps
Next steps for building an ADU in Seattle: 1. Review full ADU rules including size limits, setbacks, and parking. 2. Apply for an ADU permit through the Seattle Department of Construction and Inspections (SDCI) at https://cosaccela.seattle.gov/portal/welcome.aspx. 3. Hire a licensed contractor familiar with WA ADU requirements.
Seattle Department of Construction and Inspections (SDCI)
Phone: (206) 684-8850
Website: https://www.seattle.gov/sdci
Online Permits: https://cosaccela.seattle.gov/portal/welcome.aspx
Frequently Asked Questions
Can I build an ADU (backyard cottage) in Seattle?
Yes. Seattle is one of the most ADU-friendly cities in the US. You can build both a Detached ADU (backyard cottage/DADU, up to 1,000 sqft) and an Attached ADU (up to 1,000 sqft) on the same lot — 2 ADUs total per Washington HB 1337. No owner-occupancy requirement. No additional parking required. Building permit from Seattle SDCI required. Pre-approved standard DADU plans are available to speed up the permit process.
What changed in Seattle's ADU rules because of Washington HB 1337?
Washington HB 1337 (effective July 2023) codified what Seattle had already done in 2019: allows 2 ADUs per single-family lot, removes owner-occupancy requirements, and eliminates parking mandates near transit. Seattle was a national leader in ADU reform before HB 1337. The state law now ensures that other Washington cities must follow Seattle's example. Seattle's existing ADU ordinance continues to govern locally.
Do I need a permit for a home business in Seattle?
No city-issued permit is required to operate a qualifying home occupation in Seattle. You may work from home as long as you have no client visits, no non-resident employees working on-site, no external evidence of the business, and your business use is incidental to the residential use. You will need a Seattle business license and a Washington State business license regardless. Cottage food operations (baked goods, jams, etc.) require Washington Dept of Agriculture registration.
Can I do short-term rentals on Airbnb in Seattle?
Yes, with a Short-Term Rental Operator's License (approximately $75/year). If you live on-site (operator-occupied), you may rent your home for an unlimited number of nights annually. Airbnb automatically collects and remits Seattle and Washington lodging taxes (combined approximately 15.6%). Display your license number on your listing. Operating without a license carries fines of $500+ per violation.
What are the setbacks in Seattle's SF 5000 zone?
In Seattle's SF 5000 zone (most common single-family zone), the front setback is 20 feet, side setbacks are 5 feet on each side, and the rear setback is typically 25 feet (20% of lot depth, minimum 20 ft, maximum 25 ft). Maximum lot coverage is 35% (plus additional allowance for DADUs). Maximum building height is 30 feet. Verify your specific parcel using SDCI's parcel data tool — shoreline, environmental overlay, or historic designation may add requirements.
Is solar required on new homes in Seattle?
No. Washington does not have a statewide new-construction solar mandate. Seattle encourages solar through streamlined permitting and incentive programs, but it is not required. Seattle City Light offers net metering and the federal Investment Tax Credit (ITC) reduces solar installation cost by 30%. Standard residential solar permits are processed quickly through SDCI.
How do I look up the zoning for my Seattle property?
Use SDCI's Parcel Information tool at seattle.gov/sdci/permits/do-i-need-a-permit/parcel-information. Enter your address to find your zoning designation, ADU eligibility, lot size, and any overlays (environmental critical areas, shoreline, landmark, flood zone). You can also view Seattle's interactive zoning map at https://www.seattle.gov/opcd/ongoing-initiatives/seattle-2035.
Source: Seattle Department of Construction and Inspections (SDCI) — Building Regulations. Last verified April 5, 2026. View source