Seattle · ADU
How much does it cost to build a detached ADU in the Seattle area in 2026?
Published 2026-05-25 · Reviewed by Dror Gigi, Co-Founder
Short answer
A detached 600 to 800 sqft ADU (called a DADU in Seattle) costs $235,000 to $360,000 to build in the Seattle metro in 2026, including permits, design, and standard finishes. Bellevue and Eastside cities run roughly 10 to 15 percent higher due to stricter design review and Title 24-equivalent energy code enforcement.
Seattle calls a detached accessory dwelling a DADU and an attached one an AADU. The City of Seattle's 2019 ADU reform package eliminated parking minimums, removed owner-occupancy requirements, and allowed two ADUs per single-family lot (one attached, one detached). Eastside cities (Bellevue, Kirkland, Redmond) have similar provisions but tighter design review.
Cost breakdown for a 700 sqft DADU, Seattle metro 2026
| Component | Seattle | Bellevue / Eastside |
|---|---|---|
| Architectural design + structural | $14,000 | $18,000 |
| Permit + plan check + design review | $8,000 | $13,000 |
| Site prep + demolition | $9,500 | $11,500 |
| Foundation | $28,000 | $32,000 |
| Framing, sheathing, roof | $48,000 | $52,000 |
| Plumbing, electrical, HVAC rough | $38,000 | $42,000 |
| Energy code compliance (heat pump, envelope) | $14,000 | $16,000 |
| Drywall, interior finishes | $32,000 | $36,000 |
| Kitchen + bath | $26,000 | $30,000 |
| Flooring, paint, trim | $15,000 | $17,000 |
| Utility connections | $12,000 | $15,000 |
| Landscaping + final grade | $8,000 | $10,000 |
| Contractor overhead and profit | $40,000 | $48,000 |
| Total | $292,500 | $340,500 |
Why Bellevue and Eastside cost more
- Design review boards in Bellevue, Kirkland, and Mercer Island add 4 to 8 weeks and $4,000 to $8,000 to the permit phase
- Critical Areas Ordinance (steep slope, wetland buffer, tree retention) covers more Eastside lots and triggers geotechnical and arborist reports averaging $3,500 to $6,500 combined
- Stormwater detention requirements are stricter east of Lake Washington and typically add $4,000 to $9,000 for cisterns or drywell systems
Seattle-specific savings
- No off-street parking required for any ADU within Seattle city limits
- Pre-approved DADU plans available through the City of Seattle DADU Plan Library, cutting permit time to about 8 to 12 weeks
- No owner-occupancy requirement, meaning you can rent out both the main house and the DADU
Permit timeline
A custom DADU permit through Seattle SDCI runs 5 to 9 months. A pre-approved plan compresses that to 8 to 12 weeks. Bellevue and Eastside cities run 7 to 12 months for custom designs because of the additional design review step.