As of November 2026, Maxable's 2026 Bay Area figures put garage conversions at about $120,000 to $150,000 and detached ADUs at about $250,000 to $500,000 or more. We found no dated, San Jose-only survey. A preapproved design can trim design fees and time; the site still drives construction cost.
Published ranges
| ADU type | Published range |
|---|---|
| Garage conversion | About $120,000 to $150,000 |
| Detached ADU | About $250,000 to $500,000 or more; most standard builds about $300,000 to $450,000 |
These are published ranges, not a quote for your lot.
What moves the number?
- Utility runs. Distance to the sewer and whether the electrical panel needs an upgrade.
- Foundation and soil. Slopes, expansive soil and tree roots.
- Size and finishes. Kitchens and bathrooms carry the most cost per square foot.
- Access. Narrow side yards slow every delivery.
- Fees. Plan check and permit fees, plus school or utility fees on larger units.
What lowers it?
- Keeping the unit at or under 750 square feet, which avoids impact fees under state law.
- A simple roofline and a plan that fits the setbacks without custom engineering.
- Converting existing space instead of building new, when the structure is sound.
Sources, checked November 2026: Maxable, "Building an ADU in the Bay Area"; California ADU fee provisions. Published ranges, not a quote.
Updated November 2026