ADUs in Seattle, WA
A detached ADU in Seattle costs $325,000 to $525,000 all-in, a basement ADU $160,000 to $320,000 and a garage conversion $130,000 to $230,000. Seattle has the region's most established ADU market: rules that allow two ADUs on most single-family lots, a city ADU portal, and pre-approved backyard cottage plans.
Seattle ADU costs
| ADU | Typical Seattle range | Seattle factors |
|---|---|---|
| Detached backyard cottage, 600 – 1,000 sq ft | $325,000 – $525,000 | Alley access helps; side sewer and trees |
| Basement ADU | $160,000 – $320,000 | Headroom, egress, fire separation, dig-out if needed |
| Garage conversion | $130,000 – $230,000 | Detached alley garages convert well |
| Pre-approved plan cottage | Saves design time | Site work and utilities still vary |
Seattle ADU rules
Seattle allows two ADUs on most lots in its neighborhood residential zones, detached or attached in any combination, with no owner-occupancy requirement. Detached units can be up to 1,000 square feet, with height and placement limits that depend on lot size and alley access. The city's ADU portal explains the rules and links to pre-approved backyard cottage plans, which skip much of design review when they fit a lot.
What Seattle lots bring to an ADU
Alleys. Lots with alley access make backyard cottages easier: construction access, parking and a street-facing presence for the unit.
Side sewers. A new unit usually connects to the house's side sewer or needs a new one. Older side sewers may be clay pipe in poor condition, and SDCI permits side-sewer work.
Trees. Seattle's tree protection rules apply to large trees, and a big tree in the back yard can shape or prevent a cottage.
Slopes. Lots in environmentally critical areas may need geotechnical review for the unit's foundation.
Basements. Many Seattle basements need dug-out floors, egress windows and fire separation before they can become legal units.
Seattle ADU permits
ADUs go through SDCI with full building plans, energy compliance, utility plans and often side-sewer and drainage permits. Pre-approved plans shorten review; custom designs follow the standard path.
Seattle ADU questions
Can I build two ADUs on my Seattle lot?
On most single-family lots, yes, such as a basement unit and a backyard cottage.
Do I need parking for my ADU?
Seattle generally does not require parking for ADUs.
Can I use a pre-approved cottage plan?
If it fits your lot's setbacks, trees and utilities. Site work still needs design.
How long does a Seattle ADU take?
Twelve to eighteen months from design to move-in for a backyard cottage is typical.