Seattle's ADU (Accessory Dwelling Unit) and DADU (Detached Accessory Dwelling Unit) program is one of the most permissive in the country — a result of zoning reforms passed in 2019 and expanded through subsequent legislative updates. As of 2023, Seattle allows up to two ADUs per lot, eliminates owner-occupancy requirements, and streamlines the review process. But navigating SDCI's permit system remains complex. This guide walks you through every step, from pre-application to final inspection.
What Changed: Seattle's 2019–2023 ADU Reforms
Seattle's 2019 ADU reforms were among the most significant zoning changes in the city's history. The key changes relevant to homeowners are: allowance of both an attached ADU (AADU) and a detached ADU (DADU) on the same lot; elimination of owner-occupancy requirements; removal of parking minimums for ADU projects; and expanded maximum square footage for DADUs to 1,000 square feet (or the size of the primary dwelling, whichever is smaller).
Step 1: Pre-Application Research
Before submitting any permit application to SDCI, complete this pre-application checklist to avoid surprises:
- 1Check your lot's zoning designation using Seattle's online Parcel Viewer. Most single-family and some multifamily zones allow ADUs. Urban Village overlays may have additional requirements.
- 2Review your lot's development standards: minimum lot size (typically 4,000 sq ft for a DADU), required setbacks, lot coverage limits (typically 35–45%), and height restrictions for your zone.
- 3Check for easements, critical areas (steep slopes, wetlands, riparian corridors), or historic designation that may restrict development. Capitol Hill and First Hill historic districts have additional review requirements.
- 4Review any existing covenants or CC&Rs if your property is in a planned development. Some older King County subdivisions recorded pre-1970 covenants restricting ADUs.
Step 2: Design Development and Site Plan
Your permit application will require a complete set of construction documents. For a DADU, SDCI typically requires a site plan showing lot boundaries, existing structures, proposed DADU footprint, and all setbacks; floor plans with dimensions; elevations showing all four sides; structural calculations for foundations and framing; and energy code compliance documentation (Seattle follows the Washington State Energy Code). Hiring a licensed architect or designer for the permit drawings is strongly recommended — incomplete applications are the single most common cause of permit delays.
Step 3: Permit Submission and Review
Seattle permits for ADUs are submitted online through SDCI's Seattle Services Portal. As of 2024, SDCI has made significant progress reducing review times, but expect the following timelines:
- Simple DADU (under 750 sq ft, no critical areas): 4–8 weeks for initial review
- Standard DADU (750–1,000 sq ft): 8–14 weeks
- Complex DADU (critical areas, design review): 16–24+ weeks
- Attached ADU (AADU): Typically 4–10 weeks, depending on scope of primary home modifications
Step 4: The Correction Process
First-time permits almost always receive a 'Correction Notice' — a list of items the permit reviewer wants clarified or modified. This is normal, not a rejection. Respond to corrections promptly and completely. Each round of corrections adds 3–6 weeks to the review timeline. Work with your contractor or designer to prepare correction responses accurately to minimize rounds.
Step 5: Construction and Inspections
Once your permit is approved and issued, construction can begin. SDCI requires inspections at specific milestones: foundation (before concrete pour), framing (before insulation), rough plumbing and electrical (before drywall), insulation, and final. The contractor is responsible for scheduling inspections at least 24–48 hours in advance through SDCI's scheduling system. Missing an inspection or failing to pass one can halt construction — and re-inspections cost additional fees.
What Does an ADU Cost in Seattle? (2025)
A detached ADU in Seattle typically costs $250,000–$450,000 fully permitted and constructed, depending on size, site complexity, and finish level. The wide range reflects the significant variation in lot conditions across Seattle neighborhoods — a DADU on a flat Ravenna lot is dramatically simpler than one on a steep Capitol Hill lot with drainage requirements. An attached ADU (garage conversion or basement ADU) typically runs $80,000–$180,000 depending on the existing condition of the space.
Working with an Experienced ADU Contractor
The ADU permit process rewards contractors who know SDCI's system. Repair Point LLC has experience managing ADU permits for Seattle homeowners across multiple neighborhoods and lot configurations. We coordinate with designers, manage permit submissions, respond to corrections, and schedule all required inspections — so you don't have to navigate the process alone.