If you're thinking about adding living space in San Jose, you've probably run into three terms: detached ADU, garage conversion, and JADU. They solve similar problems but cost very different amounts, so the right pick depends on your lot, your budget, and what you want the space to do.
- Detached ADU: the most space and value, roughly $250k-$450k+ (about $350-$500/sq ft).
- Garage conversion: the budget pick, roughly $85k-$200k — typically 20-40% cheaper because the shell already exists.
- JADU: the cheapest start, roughly $60k-$150k, carved from your home's existing footprint (max 500 sq ft).
- Keeping a unit under 750 sq ft waives school and parkland impact fees, saving roughly $5,000-$15,000 or more.
The three options, in plain terms
A detached ADU is a brand-new, standalone unit built in your yard, with its own foundation, walls, and roof. A garage conversion turns your existing garage into a legal dwelling, reusing the shell that's already there. A JADU (junior ADU) is a small unit carved out of the existing footprint of your house, capped at 500 square feet, usually a bedroom plus a small kitchen and a private entrance.
All three are legal in San Jose, and the city has worked to streamline approvals. The big differences come down to cost and how much space you end up with.
What each one costs in San Jose
These are current local ranges, not promises. Your number depends on finishes, site conditions, and utility work.
- Detached ADU: roughly $250,000-$450,000 and up, or about $350-$500 per square foot for a typical 600-800 sq ft unit.
- Attached ADU addition: roughly $150,000-$300,000.
- Garage conversion: roughly $85,000-$200,000, often $250-$380 per square foot, because the foundation, walls, and roof already exist.
- JADU: roughly $60,000-$150,000, since it stays inside your home's existing walls.
Reusing an existing structure typically cuts 20-40% off the cost of building a comparable detached unit. For a deeper breakdown, see our ADU cost guide.
San Jose and California rules worth knowing
- Size limits. Detached ADUs can go up to 1,000 sq ft on lots under 9,000 sq ft, and up to 1,200 sq ft on larger lots. JADUs are capped at 500 sq ft and must stay within the existing home.
- The 750 sq ft fee waiver. In San Jose, ADUs under 750 square feet are fully exempt from parkland and school impact fees, which can save you roughly $5,000 to $15,000 or more. Cross 750 sq ft and the city charges per-square-foot school fees (often $3,000 to $6,000 or more). Both garage conversions and JADUs almost always stay under that line, and many smaller detached units do too.
- Parking. San Jose generally requires one off-street space for an ADU, but that requirement is waived if your property is within a half-mile walk of a transit stop. Garage conversions are also exempt from replacing the parking the garage removed.
- Setbacks. Detached ADUs typically need a four-foot setback from rear and side property lines.
Permits are required for all three, and the process matters as much as the build. Our permits overview walks through what San Jose expects.
When a garage conversion makes the most sense
Go this route if your garage is in decent shape, you want to keep costs down, and you don't mind giving up the parking or storage. It's the fastest, cheapest way to add a real one-bedroom or studio, and it usually lands under 750 sq ft, so you capture the fee waiver. The trade-off: you're limited to the garage's existing dimensions, and older slabs or low rooflines sometimes need work that eats into the savings. See our garage conversion page for what's involved.
When a detached ADU is worth the extra cost
A detached unit costs the most but gives you the most: a true private home in the backyard, more square footage, and the strongest rental income and resale value. It makes sense when you have the yard space, the budget, and a longer-term goal, like housing family or building steady rental income. If financing is the hurdle, our financing options can help you compare.
When a JADU is the smart starter move
A JADU is the lowest-cost path to a legal, income-producing or family-friendly space. It's ideal when you have an extra bedroom or unused interior area and want a separate suite without a full addition. The limits: 500 sq ft and an efficiency kitchen.
A simple way to decide
Tightest budget and an existing garage? Convert it. Want the most space and value, with room in the yard? Build detached. Have unused space inside and want to start small? A JADU. The best answer usually comes from walking your property with a builder who knows San Jose's rules.