Garage conversion vs. new detached ADU in the Bay Area: which is right for you?
How does a garage conversion compare with a new ADU?
| Garage conversion | New detached ADU | |
|---|---|---|
| Structure | Reuses foundation, walls and roof | Built from the ground up |
| Size | Limited by the existing garage | At least 800 sq ft allowed statewide |
| Layout freedom | Works within the existing footprint | Designed around how you'll use it |
| Typical cost drivers | Slab moisture, insulation, new windows/door, utility runs | Foundation, framing, roof, utility trenching |
| Parking | Losing garage parking generally can't be used to block an ADU | Often no parking required near transit |
| Permit review | Ministerial; decision within 60 days of a complete application | |
When does a garage conversion make sense?
- The garage is structurally sound with a level, dry slab.
- You want a studio or one-bedroom and the footprint fits it.
- You'd rather not give up yard space.
When does a new detached ADU make more sense?
- You want two bedrooms or a specific layout the garage can't fit.
- The garage needs major structural repair anyway.
- Privacy between the main house and the unit is a priority.
What hidden issues should you check in a garage?
Garage slabs often lack a moisture barrier, walls may need insulation and drywall upgrades, and the floor may need to be raised or leveled. Sewer and water connections, electrical panel capacity and window egress for a bedroom all need to be checked before you commit. Our guide to hidden issues in older Bay Area homes covers what often turns up once walls open.
What fees apply, and how big should you build?
According to California's ADU rules, ADUs under 750 sq ft are exempt from impact fees, which is one reason many homeowners keep conversions and new builds just under that size. Plan-check and permit fees still apply. For a compact unit, our small kitchen layout guide helps you plan the kitchen, and our project gallery shows finished kitchens and baths we have built.
Full rules: ADU rules in California.
Frequently asked questions
Is a garage conversion cheaper than a new ADU?
Usually, because the foundation, walls and roof already exist. The savings shrink if the slab, framing or roof need major repair.
How big can a new detached ADU be in California?
State law guarantees at least 800 sq ft with 4-foot side and rear setbacks for most single-family lots. Local rules may allow more.
Do I lose required parking if I convert my garage to an ADU?
California law generally prevents cities from requiring replacement parking when a garage is converted into an ADU.
How long does ADU permit approval take?
Cities must approve or deny a complete ADU application within 60 days.
Planning a remodel in the Bay Area?
ProRock Remodeling is a licensed general contractor (CSLB #1154078) serving San Jose to South San Francisco, with $1M liability coverage and a 2-year workmanship warranty. Call (408) 991-4744 or request a free estimate.