Answers › How much does a 400-square-foot ADU cost?
How much does a 400-square-foot ADU cost?
A 400-square-foot ADU in Los Angeles or the San Francisco Bay Area typically costs roughly $150,000 to $300,000 in 2026, which works out to about $375 to $750 per square foot depending on whether it is a garage conversion or a new detached build, how far the utilities have to run, and the finish level. Small units cost more per square foot than large ones because the kitchen, bathroom and utility connections are fixed costs no matter the size.
| Typical total (LA and Bay Area, 2026) | Roughly $150,000 – $300,000 |
|---|---|
| Per square foot | About $375 – $750 |
| Garage conversion of this size | Roughly $120,000 – $220,000 |
| Design, engineering and permits | Commonly $15,000 – $35,000 |
Why small ADUs are expensive per foot
Every ADU, regardless of size, needs a kitchen, a bathroom, a water heater, heating and cooling, an electrical subpanel, a sewer connection and a front door. Those items cost about the same in a 400 square foot unit as in an 800 square foot unit, so the smaller unit carries them over fewer square feet. The result is a per-foot cost that looks high next to a house addition even when the total is reasonable.
What a 400 square foot unit buys
A studio or compact one-bedroom with a full bathroom, a kitchenette or small kitchen, a living and sleeping area, and a closet. Well-designed 400 square foot units use built-ins, a wall bed or sleeping alcove, and tall storage to live larger than the number suggests. At this size the layout decisions matter more than the finishes.
Budget ranges by type
Garage conversion to a 400 square foot unit: roughly $120,000 to $220,000 depending on the slab, fire separation and utilities. New detached 400 square foot unit: roughly $150,000 to $300,000 depending on foundation, site work and utility distance. Add design, engineering and permit fees, which commonly total $15,000 to $35,000 across the project. Oria Builders (CSLB #1029255) builds compact ADUs in both Los Angeles and the Bay Area; local fees and requirements should be confirmed with your city before budgeting.
Related Questions
Is 400 square feet big enough for a one-bedroom?
Yes, as a compact one-bedroom or a generous studio. A separate bedroom at this size usually means a small living area, so many owners choose a studio with a sleeping alcove.
Can I build an 800 square foot ADU instead?
California law requires cities to allow an ADU of at least 800 square feet on most lots, subject to setbacks and height. The cost per foot drops as the unit grows.
Do I need a separate electrical meter for an ADU?
Not usually; a subpanel off the main service is common. A separate meter is an option if you want the tenant billed directly, and it adds cost.
Sources
Related
- ADU Construction
- Project: Full Remodeling and garage conversion to an ADU in Los Angeles – Precision Punch List Completion
- Case study: Modern ADU Creation in Santa Monica’s Ocean Park Neighborhood
- How much does an ADU cost in Santa Monica?
- Can I convert my garage into an ADU in Los Angeles?
- How long does an ADU take to build in Los Angeles?
Answered by Gilad Segev — Founder & Licensed General Contractor, Oria Builders (CSLB #1029255). Last updated October 2, 2026.