A well-designed detached garage serves immediate needs while preserving the option for future ADU conversion. Here is what Seattle homeowners need to know about detached garage design in 2026.
Size Limits for Detached Garages in Seattle
Detached garages in Seattle residential zones are limited by lot coverage rules. The garage, primary home, and any other structures cannot exceed the allowed lot coverage percentage (typically 35% in SF5000 zones). Height is typically limited to 15 feet or the height of the primary structure, whichever is less.
Permit Requirements
All detached garages require a building permit regardless of size. Garages are utility occupancy (Group U) but still require a site plan, floor plan, elevation drawings, structural design, and energy code compliance if heated.
Planning a project?
Working out what your lot will allow, and what it will cost?
Send David the address and a few lines about what you want. The first feasibility check is free and usually takes about a day: what the site and the code are likely to allow, and what to check first.
Architect since 1996, member of the AIA and NCARB. Three Masonry Excellence Awards for commercial projects; the Carr residence was featured in Above The Lake and Lux magazines.
Designing for Future ADU Conversion
The most valuable garages are designed from the start to allow future ADU conversion with minimal additional work:
- Full ceiling height (10-12 ft) to allow insulation and drywall without losing headroom
- Electrical panel sized for additional circuits (kitchen, bathroom, HVAC)
- Rough-in plumbing for future bathroom and kitchen (capped at slab)
- Windows positioned for natural light and privacy as a residence
- Structural framing for a potential future loft/second floor
Cost in Seattle (2026)
- Basic 2-car garage (600 sq ft): $90,000-$150,000
- Garage with ADU-ready design: $120,000-$190,000
- Architect fees: $12,000-$22,000
Piper Cole Architects designs detached garages and garage ADUs throughout Seattle. free feasibility check, call 425-753-6452.
