The short answer
A detached ADU of 400 to 800 square feet typically costs $100,000 to $250,000. Garage conversions and attached units run $50,000 to $120,000. The national average build lands near $180,000.
Why the ROI is different
Most additions are valued at resale. An ADU is valued at resale and every month in between. It adds roughly 41% of its cost in appraised value, but the real engine is rent. At $1,500 to $2,500 a month in most US markets, a $120,000 unit can recoup its full cost in five to seven years on rental income alone. No other addition type does that.
Where ADUs make the most sense
High land-value metros, where building up or out beats moving. California, led by Los Angeles and San Diego, leads the country on ADU-friendly policy. Portland, Seattle, and San Jose are close behind. If you own a lot in one of these markets, an ADU is often the single highest-impact project available to you.
What separates a good ADU build
- Designed to local setback and ADU ordinance from the first sketch
- Separate metering and entrance so it actually rents cleanly
- Income-property finish standards, not builder grade
- Permitting handled by the builder, not pushed to you
See our ADU construction service or get a tailored estimate from the cost tool.