ADU Design in Seattle & Kirkland, WA — Piper Cole Architects

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