Bay Area ADU cost by type
| ADU type | Typical Bay Area range | What it is |
|---|---|---|
| Detached ADU | $280K – $600K+ | A new standalone backyard unit with its own foundation, roof and utilities. |
| Attached ADU | $160K – $350K | A new unit built onto the existing home, sharing at least one wall. |
| Garage-conversion ADU | $200K – $320K | An existing garage converted into a legal living unit. |
| Junior ADU (JADU) | $65K – $130K | A smaller unit created within the existing home’s footprint. |
Note: Ranges are typical Bay Area estimates; your price depends on your site and scope.
What drives the cost of an ADU
Size and layout
Larger units and more bedrooms and bathrooms mean more materials and labor.
Site work and access
Slopes, tight backyards, demolition and grading all affect the budget.
Utilities and sewer lateral
New or upgraded water, sewer, gas and electrical service can be a major line item.
Permits and city fees
Plan-check, permit and impact fees vary from city to city.
Foundation and structure
Soil conditions and engineering requirements shape the foundation design.
Design, engineering and finishes
Architectural plans, structural engineering and your choice of finishes and fixtures.
What’s included in an ON Design ADU project
- Consultation and site review – we look at your lot, goals and budget.
- Design – layout and plans for your ADU.
- Permits – we prepare and submit plans and work with your city through approval.
- Build – construction from foundation to final finishes as a licensed general contractor (CSLB #1080660).
- Final inspection and handover – your ADU, ready to live in or rent.
See a Bay Area ADU we built
Walkthrough of our Redwood City ADU build, from backyard to finished home.
ADU cost FAQ
How much does an ADU cost in the Bay Area?
Typical Bay Area estimates are about $280K–$600K+ for a detached ADU, $160K–$350K for an attached ADU, $200K–$320K for a garage-conversion ADU and $65K–$130K for a JADU. Your price depends on your site and scope.What is the least expensive type of ADU?
A junior ADU (JADU), created inside the existing home, is typically the lowest-cost option at about $65K–$130K, because it reuses existing structure and utilities.Why is a detached ADU more expensive?
A detached ADU is a new standalone building, so it needs its own foundation, framing, roof and utility connections, and often more site work.Do permits and city fees add to the cost?
Yes. Permit, plan-check and city fees vary by city and project size, and they are part of the total budget alongside design and engineering.How do I get an accurate price for my ADU?
Book a free consultation with ON Design & Remodeling. We review your lot, utilities, goals and finishes to give you a project-specific estimate.Cities we serve
ON Design & Remodeling designs and builds ADUs across the Bay Area, including San Jose, Palo Alto, Redwood City, San Mateo, Sunnyvale, Mountain View, Fremont, Santa Clara, Cupertino, Los Altos, Menlo Park and nearby cities.
Planning an ADU?
Get a clear, project-specific estimate for your property.
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