The Quick Answer
6\u201310
weeks
Seattle DCI Standard Permit
8\u201314
weeks
King County DPER
+4\u20138
extra weeks
SMA / Shoreline review
These are permit review timelines only. Add 3\u20136 weeks for design and documentation preparation.
Complete Project Timeline
From "I want to build an ADU" to actual construction start, here's what a typical timeline looks like. For a detailed walkthrough of our approach, see our 3-step process.
Feasibility & Planning
1–2 weeks- Property feasibility check (zoning, setbacks, critical areas)
- Determine ADU type and approximate size
- Budget planning and financing decisions
- Select design/permitting team
Design & Documentation
3–6 weeks- Architectural design (floor plan, elevations, site plan)
- Structural engineering (if required)
- Energy code calculations
- Stormwater/drainage plan
- Site survey (if needed)
Permit Application & Review
6–14 weeks- Submit application with all required documents
- Seattle DCI: 6–10 weeks for Standard Building Permit review
- King County DPER: 8–14 weeks for unincorporated areas
- Address correction letters (if any)
- SMA zone (within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound): add 4–8 weeks for Shoreline Conditional Use Permit
- Permit issuance
Construction
4–8 months- Site preparation and foundation
- Framing, roofing, exterior
- Mechanical (plumbing, electrical, HVAC)
- Interior finishing
- Inspections throughout
- Final inspection and Certificate of Occupancy
Total typical timeline: 6\u20139 months from decision to move-in for a new detached ADU. Garage and basement conversions are typically faster: 3\u20136 months.
What Causes Delays
Incomplete application
Missing documents mean your application gets placed on hold. The clock restarts when you resubmit a complete package.
Critical areas discovery
If a critical areas assessment is required, hiring a qualified consultant and completing the study adds significant time.
Design revisions
If Seattle DCI or King County DPER requires design changes (setback issues, height concerns), the redesign and resubmission cycle adds time.
SMA / Shoreline review
Properties within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound require a Shoreline Conditional Use Permit on top of the Standard Building Permit.
Septic system review (rural King County)
King County Public Health septic review and any required system upgrades add weeks to the rural permit timeline.
Seasonal construction backlogs
Spring and summer are busiest. Seattle DCI and DPER plan reviews take longer when departments are processing more applications.
Want the fastest path to a permit? Our permit specialist knows exactly what Seattle DCI and King County DPER need and coordinates directly with each department. Your permit investment is applied as a build deposit when you sign the Letter of Intent.
How to Speed Things Up
Get a feasibility study first — know your constraints before spending money on design. Our free feasibility report checks your zoning, SMA proximity, and lot data automatically.
Use a designer with Seattle and King County ADU experience — they know what Seattle DCI and DPER reviewers expect. See our ADU design services for site-specific architecture.
Submit a complete application the first time — see our Seattle permit guide for the full document checklist.
Respond to correction letters within 48 hours.
Schedule pre-application conferences with Seattle DCI or King County DPER to surface issues early.
Confirm SMA zone status early — shoreline properties within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound need an additional permit and add 4–8 weeks.
Work with our permit specialist who has relationships with local planning staff. Learn about our Full Permit Package.
Last updated: June 2026. Timelines are estimates based on typical experiences and may vary.
Start Your ADU Timeline Today
Our free feasibility study is Step 1 — get your property checked in 60 seconds.
Book Free WalkthroughPrefer to call? (206) 222-2898
