It depends mostly on design time, SDCI review and corrections, and construction. Washington does not have California's 60-day ADU deadline. A pre-approved DADU plan, a complete first submittal and quick answers to corrections save the most time.
The phases
- Lot research. Zone, critical areas, trees, easements and utilities.
- Design. Custom plans or a pre-approved DADU plan.
- SDCI review. Submittal through the Seattle Services Portal, corrections and resubmittal.
- Construction. Foundation, framing, inspections, finishes and final.
What slows things down?
- Steep slopes or other critical areas.
- Significant trees near the building site.
- Sewer and drainage connections on difficult lots.
Sources, as of November 2026: City of Seattle SDCI and OPCD ADU pages. General information, not legal advice.
Updated November 2026