ADU or garage conversion: California rules, permits and cost

For Silicon Valley homes · Updated October 2026 · Sources below

California law makes it much easier to add an ADU (a second home on your lot) or turn a garage into one. Cities must approve a complete application within 60 days, can't require replacement parking when a garage is converted, and can't charge impact fees on ADUs under 750 square feet. You still need building permits and a licensed contractor.

Facebook Text WhatsApp

The short version

Cities must approve or deny a complete ADU application within 60 days
No replacement parking when a garage becomes an ADU
No impact fees on ADUs under 750 square feet
Side and rear setbacks for new ADUs can't be more than 4 feet
A junior ADU (inside the house) can be up to 500 square feet

Need a general contractor?

Tell us about the job in 2 minutes - free. We send it to one local pro at a time, never a crowd.

Get matched

What it costs

Garage conversion to living space$5,986 - $27,348
Garage to apartment or guest suite$20,000 - $50,000

National figures for garage conversions - a full ADU with its own kitchen and bathroom in the Bay Area usually costs far more, and new detached ADUs more still. Plans, utilities, fees and the sewer connection drive the price, so get detailed bids.

Typical ranges from published cost guides - mostly national figures. Bay Area prices usually run higher. Your actual price depends on your home and the job, and a pro gives you the real number. Source: HomeAdvisor cost guide (national) (Jun 2026). Have a quote? Check it with the Quote Explainer

Which kind of ADU fits?

Garage conversion: often the lowest cost, since the walls, roof and slab are there. No replacement parking, and no new setbacks if you stay in the footprint.
Junior ADU: up to 500 square feet inside the house (an attached garage counts) and can share a bathroom with the main house - but the owner must live in the house or the JADU if it does.
New detached ADU: the most room, the most cost. State law requires cities to allow at least 16 feet of height on most lots (18 feet near major transit).
Attached ADU: cities must allow 25 feet or the main home's height limit, whichever is lower.

In Silicon Valley

California HCD: cities must allow at least an 800-square-foot ADU with 4-foot side and rear setbacks, and can't set a maximum size below 850 square feet (1,000 square feet for more than one bedroom).
California HCD: a single-family lot can have an ADU in existing space, a junior ADU and a new detached ADU together. Owner occupancy can't be required for a regular ADU unless it's sold separately.
California HCD: fire sprinklers are only required in an ADU if the main house has them. Some ADUs and all JADUs must be rented for more than 30 days at a time.
San José: the city runs an ADU permit service for garage and basement conversions and new ADUs. Applications go through SJPermits.org and plans upload through SJePlans. The city warns that many projects get delayed waiting on the water company's hydrant flow letter - ask for it early.

Questions homeowners ask

Do I need a permit to convert my garage into an ADU in San Jose?

Yes. A garage conversion needs building, electrical, plumbing and mechanical permits, applied for through San José's ADU service. Ask your contractor or designer to request the water company's hydrant flow letter early, since the city says it's a common cause of delays.

Do I have to replace the parking when I convert my garage?

No. California law doesn't let cities require replacement parking when an ADU replaces a garage, carport or other off-street parking, or when a junior ADU replaces a garage.

How long does the city have to approve an ADU?

Under California law, the city must approve or deny a complete ADU application within 60 days when there's already a home on the lot. A complete, well-prepared plan set helps avoid delays.

How big can an ADU be in California?

Cities can't set a maximum below 850 square feet, or 1,000 square feet for an ADU with more than one bedroom, and must allow at least 800 square feet with 4-foot side and rear setbacks.

Will I pay impact fees on my ADU?

Not if it's under 750 square feet, and never for a junior ADU. Larger ADUs can be charged fees in proportion to the size of the main home. Permit and utility connection fees are separate, so ask the city for a fee estimate.

How much does a garage conversion cost?

National cost guides put a garage-to-living-space conversion at about $6,000 to $27,300, and a garage apartment at about $20,000 to $50,000. A full ADU with a kitchen and bathroom in the Bay Area usually costs much more. A contractor quotes it after seeing the garage and the sewer and electrical service.

What license should my ADU contractor have?

Usually a B General Building license from the CSLB, since an ADU involves several trades. Any job of $1,000 or more needs a license. The down payment can't be more than 10% or $1,000, whichever is less.

Do you need a permit?

Yes - ADUs and garage conversions need permits and plans.

General guidance based on California's building code - your city can have stricter rules, and zoning, historic and HOA rules can apply too. Check with your city's building department before you start.

Check the rule for your city

Questions to ask the pro

Have you built ADUs or garage conversions in my city, and can I see them?
Who draws the plans, who submits them, and who meets the inspector?
Will my electrical panel and sewer line handle the ADU, or do they need upgrades?
Which city fees and utility connection costs are in your bid, and which are on me?
What's the payment schedule, tied to which finished stages?

Before you sign, check the contractor's license at cslb.ca.gov. How to hire a contractor in California

Go deeperConverting a garage into an ADU in San Jose: what to know

Know a neighbor dealing with this?

Send them this page - it's free, and every answer has its sources.

Facebook Text WhatsApp Email