An Accessory Dwelling Unit (ADU) is one of the smartest investments a Seattle-area homeowner can make. Whether you want rental income, multi-generational living, or a private home office, Piper Cole Architects designs ADUs that fit your property, meet Seattle and Kirkland permit requirements, and complement your existing home.
What Is an ADU?
An ADU is a second, self-contained dwelling unit on the same lot as a primary residence. In Washington State and the Seattle metro area, there are two main types:
- Detached ADU (DADU / Backyard Cottage) — A freestanding structure separate from the main home. Up to 1,000 sq ft in Seattle.
- Attached ADU — An addition to or internal conversion of the existing home — basement unit, garage conversion, or added suite.
ADU Design Services
- Site and zoning feasibility analysis
- Schematic design and floor plan development
- Full permitted construction documents
- City of Seattle, Kirkland, Bellevue, and Redmond permit submissions
- Construction administration and contractor coordination
ADU Costs in the Seattle Area (2026)
- Garage conversion ADU: $90,000 to $180,000
- Basement ADU: $80,000 to $160,000
- Detached backyard cottage: $250,000 to $450,000+
- Architect design fees: $15,000 to $35,000 depending on scope
ADU Permitting in Seattle, Kirkland, and the Eastside
ADU permitting requires a Site Plan Review and Building Permit with full architectural drawings. Key requirements include setbacks (typically 5 ft rear, 5 ft side), lot coverage limits, height limits (18 to 24 ft depending on zone), and utility connections. Piper Cole Architects navigates all Eastside jurisdictions, having completed ADU projects across the entire Seattle metro area.
Free ADU Consultation
Ready to explore what is possible on your lot? Contact Piper Cole Architects for a free initial consultation. We will review your property, walk through your goals, and give you an honest assessment of what your ADU project will require.
Call: 425-753-6452 | Email: pipercolearchitects@gmail.com | Kirkland, WA
Frequently Asked Questions
How much does it cost to build an ADU in Seattle?
In the Seattle area in 2026, a garage conversion ADU runs $90,000 to $180,000, a basement ADU $80,000 to $160,000, and a detached backyard cottage $250,000 to $450,000 or more. Architect design fees typically add $15,000 to $35,000 depending on scope.
Do I need a permit for an ADU in Seattle or Kirkland?
Yes. Every ADU requires a building permit with architectural drawings, structural drawings, and energy calculations. Piper Cole Architects prepares complete permit sets and manages the review process with the City of Seattle, Kirkland, Bellevue, and other Eastside jurisdictions.
How big can a backyard cottage (DADU) be in Seattle?
Seattle allows a detached ADU up to 1,000 sq ft, up to 24 feet tall with a pitched roof. There is no owner-occupancy requirement and both the main home and the DADU can be rented.
How long does it take to design and permit an ADU?
Design typically takes 6 to 12 weeks and Seattle permit review runs 8 to 14 weeks, so most ADU projects reach permit-in-hand in roughly 6 to 10 months depending on site complexity and correction cycles.
Ready to Start Your Project?
Free initial consultation — no obligation. Licensed architect. 800+ projects. Kirkland, WA — serving the entire Seattle metro area since 2000.
Or call us directly: 425-753-6452
Ready to talk about your project?
Piper Cole Architects has designed 800+ Eastside projects since 2000. Get a free, no-pressure consultation with David Meade, AIA, NCARB.
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