Permitting is where most ADU projects stall. The process involves zoning verification, site plan preparation, Standard Building Permit applications, and coordination with Seattle DCI or King County DPER. The good news: with HB 1337 and Seattle's 2019 ADU ordinance, the path is more streamlined than ever. Start with a free feasibility study to confirm your property qualifies before investing in design.
Whether you DIY the permitting or use our Full Permit Package, these guides will help you understand what's involved.
The ADU Permit Process at a Glance
Feasibility
Zoning check, lot analysis, setback review
Design
Site plan, floor plan, architectural drawings
Application
Submit to planning dept with all required documents
Review
Plan review, corrections, approvals
Permit
Building permit issued — construction begins
Detailed Guides
Seattle Permitting
Step-by-step guide to ADU permits inside Seattle city limits — Standard Building Permit process, Seattle DCI, and 6–10 week timeline.
Read guideKing County Permitting
Permit process for unincorporated King County — DPER requirements, rural vs UGA, septic and well considerations.
Read guidePermit Timeline
Realistic timelines from application to approval — what takes 4 weeks, what takes 12, and how to avoid common delays.
Read guidePermit Costs & Fees
Complete breakdown of permit fees, impact fees, utility connection charges, and professional service costs.
Read guide5 Common Permit Mistakes (and How to Avoid Them)
Mistake: Skipping the pre-application meeting
Fix: A quick meeting with planning staff can catch zoning issues before you invest in architectural drawings.
Mistake: Incomplete application submissions
Fix: Missing documents (site plan, drainage plan, energy calculations) cause the most delays. Use our checklist to submit complete the first time.
Mistake: Ignoring critical areas
Fix: Wetlands, streams, and steep slopes can void your entire plan. Check GIS maps early — our feasibility study does this automatically.
Mistake: Not checking utility capacity
Fix: Sewer, water, and electrical service must support an additional dwelling. Verify with utility providers before committing to a design.
Mistake: Designing before understanding setbacks
Fix: Setback requirements determine where and how large your ADU can be. Get a site survey and zoning analysis first.
Our Full Permit Package
Don't want to navigate permitting yourself? Our permit specialist has processed 50+ ADU permits in King County and knows exactly what Seattle DCI and DPER need. The package covers:
Want to understand the full investment? See our complete ADU cost breakdown covering construction, permits, and soft costs for Seattle and King County.
Last updated: March 2026.
