Build costs, cost per square foot, garage conversions and rents across 10 Washington cities, from each city’s own cost guide.
Building an ADU in Washington costs $130,000–$550,000 all-in across the 10 cities we cover (2026), or about $200–$650 per square foot. The middle of the 10 city ranges below sits around $437,500. Converting an existing garage runs $50,000–$225,000.
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| City | Typical build cost | Cost per sq ft | Garage conversion | 2-BR rent |
|---|---|---|---|---|
| Bellevue | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $2,900-$3,750/mo |
| Bothell | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $2,700-$3,150/mo |
| Kirkland | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $2,850-$3,350/mo |
| Redmond | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $3,100-$3,350/mo |
| Sammamish | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $3,400-$3,850/mo |
| Seattle | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $2,300-$3,000/mo |
| Spokane | $130,000-$260,000 | $200-$400 | $50,000-$120,000 | $1,300-$1,500/mo |
| Tacoma | $150,000-$350,000 | $300-$520 | $75,000-$180,000 | $1,780-$2,500/mo |
| Vancouver | $150,000-$350,000 | $200-$400 | $60,000-$150,000 | $1,750-$2,100/mo |
| Woodinville | $325,000-$550,000 | $400-$650 | $100,000-$225,000 | $2,800-$3,150/mo |
Each figure is the city’s own cost guide; click a city for what is included, permit fees and the sources behind it.
Across the 10 Washington cities we cover, building a new ADU runs $130,000 to $550,000 all-in, depending on the city, size and finish. The middle of the city ranges sits around $437,500. Spokane has the lowest published range ($130,000-$260,000) and Bellevue the highest ($325,000-$550,000).
$200 to $650 per square foot across the 10 Washington cities we cover. Smaller units cost more per square foot, because the kitchen and bathroom are the expensive rooms.
Usually. Converting an existing garage runs $50,000 to $225,000 in the Washington cities we cover, against $130,000 to $550,000 for a new detached unit, because the foundation, walls and roof already exist.
A two-bedroom unit rents for about $1,300 to $3,850 a month across these cities. Each city's cost page shows its own rent figure and source.
Yes. An ADU is a home, so it needs a building permit, and the zoning rules, fees and review times are set city by city. Our Washington ADU laws page compares them.
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