Free Tool
ADU ROI Calculator — Washington State
Washington State passed House Bill 1337 in 2023, requiring most cities to allow two ADUs per lot and removing owner-occupancy requirements by July 2025. Seattle is one of the highest-rent ADU markets in the country, with strong tech employment driving consistent rental demand.
Typical Project Cost
$180,000–$320,000
Typical Monthly Rent
$1,600–$2,800/mo
Cap Rate Range
7–11%
Break-Even Range
8–13 years
ADU Rental Income by City — Washington
Use these figures as a starting point when entering your expected monthly rent above.
| City | 1BR ADU Rent | 2BR ADU Rent | Avg Build Cost |
|---|---|---|---|
| Seattle | $1,800–$2,800/mo | $2,400–$3,400/mo | $200K–$320K |
| Bellevue | $1,900–$2,900/mo | $2,600–$3,600/mo | $210K–$330K |
| Tacoma | $1,200–$1,800/mo | $1,600–$2,300/mo | $160K–$250K |
| Spokane | $900–$1,400/mo | $1,200–$1,800/mo | $120K–$200K |
Washington ADU ROI — Frequently Asked Questions
What is a good ADU ROI in Washington State?
Seattle and Bellevue ADUs typically achieve 8–12% cap rates, making them among the strongest ADU markets nationally. Tacoma and other mid-tier markets yield 7–10%. ROI depends heavily on project cost — garage conversions and JADUs outperform new builds.
What did HB 1337 change for Washington ADU investors?
HB 1337 (effective July 2025) requires cities to allow two ADUs per single-family lot and eliminates owner-occupancy requirements statewide. This means investors can own and rent both the main house and ADU(s) without living on the property.
How much does it cost to build an ADU in Seattle?
Seattle ADU construction costs range from $150,000–$220,000 for garage conversions to $250,000–$380,000 for new detached builds. Seattle's permitting fees and design requirements add to costs, but high rents justify the investment in most locations.