ADU Costs in Seattle: What to Budget for Your Accessory Dwelling Unit
Accessory Dwelling Units have become one of the most popular residential projects in Seattle, driven by the 2020 ADU reforms that dramatically expanded where and how ADUs can be built. But budgeting an ADU is more complex than most homeowners realize. This guide breaks down realistic ADU costs in Seattle and explains what drives costs on your specific project.
ADU Types and Cost Ranges
Attached ADU (AADU)
Shares at least one wall with the primary home — typically a basement conversion, garage conversion, or ground-floor addition. AADUs are generally the least expensive ADU type because they use existing structure. Cost range: $80,000–$180,000 depending on the extent of renovation required.
Detached ADU (DADU)
A separate structure in the backyard — the classic backyard cottage. DADUs are new construction with full foundation, framing, mechanical, electrical, and plumbing. Cost range: $200,000–$400,000+ for a 400–1,000 sq ft unit in Seattle. Quality and site complexity are the main variables.
Garage Conversion ADU
Converting an existing detached garage to an ADU is often the most cost-effective path. The existing structure provides foundation and framing. Cost range: $100,000–$200,000, depending on garage size, condition, and extent of renovation.
ADU Cost Breakdown
| Cost Category | Typical Range (DADU) |
|---|---|
| Architecture and engineering | $18,000–$40,000 |
| Permit fees | $4,000–$10,000 |
| Site work (grading, utilities) | $15,000–$50,000 |
| Foundation | $20,000–$45,000 |
| Framing and structure | $30,000–$60,000 |
| Mechanical, electrical, plumbing | $35,000–$65,000 |
| Insulation and drywall | $12,000–$22,000 |
| Finishes (flooring, tile, cabinets) | $25,000–$60,000 |
| Windows and doors | $15,000–$30,000 |
| Roofing and exterior | $18,000–$35,000 |
ADU Return on Investment in Seattle
Seattle rental rates for a quality 600 sq ft ADU range from $1,800 to $2,800/month depending on neighborhood and amenities. A $250,000 DADU renting at $2,200/month generates $26,400/year gross rent — a 10.6% gross yield on construction cost. ADUs also typically increase property value by 1.5–2x their construction cost in desirable Seattle neighborhoods.
Architecture for ADUs
Piper Cole Architects designs ADUs as complete, well-considered homes. Our residential architecture services cover ADUs from feasibility and design through permit and construction support. We serve Seattle, Kirkland, Bellevue, and all Eastside communities. See our design process for how we manage ADU projects from first call to certificate of occupancy.
Related reading: our full ADU rental income & ROI breakdown.
Frequently Asked Questions
How much does an ADU cost in Seattle?
ADU costs in Seattle vary by type: a garage conversion typically runs $80,000 to $180,000, an attached ADU around $100,000 to $200,000, and a detached ADU (DADU) $200,000 to $400,000 or more, depending on size, site conditions, and finish level.
Is a detached ADU more expensive than an attached ADU?
Generally yes. An attached ADU shares walls and utilities with the main house, keeping costs around $100,000 to $200,000, while a standalone DADU needs its own foundation, roof, and utility connections, typically $200,000 to $400,000 or more.
What is the most affordable type of ADU to build in Seattle?
Converting an existing garage is usually the most affordable path, roughly $80,000 to $180,000, because the structure and foundation already exist. New detached units cost more because everything is built from the ground up.
Can an ADU pay for itself in Seattle?
Many do over time. As an example, a $250,000 DADU renting at about $2,200 per month generates roughly $26,400 in gross annual rent, plus added property value. Actual returns depend on your build cost and local rents.
What drives ADU costs up or down?
The biggest factors are the ADU type (garage conversion versus new detached), size, site conditions such as grading and utility runs, finish level, and permit and design fees. A feasibility review gives you an accurate number for your specific lot.
How much are ADU design and permit fees in Seattle?
Design and permitting are a meaningful share of the budget, commonly running in the range of $15,000 to $60,000 depending on scope and jurisdiction, on top of construction costs.
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