As of November 2026, a 2026 Bay Area guide from Maxable puts garage conversions at about $120,000 to $150,000 and detached ADUs at about $250,000 to $500,000 or more. We did not find a dated, San Francisco-only survey. Older buildings, tight access and structural work under a house often push city projects toward the high end.
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.
- Seismic and structural work. Ground-floor units under a house often need new foundations or framing.
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 Government Code ADU fee provisions, as amended by SB 543. Published ranges, not a quote.
Updated November 2026