ADU vs. garage conversion: which fits your San Jose home?

There are three ways to add a legal living unit in San Jose: build a detached ADU in the yard, convert the garage, or carve a JADU out of the house you already have. They solve the same problem at very different prices, and the right one usually gets obvious once you look honestly at your lot, your budget, and what the space needs to do.

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. What actually separates them is money and square footage, so start there.

What each one costs in San Jose

Cost depends on the size, finishes, site conditions and utility work. A garage conversion and a JADU reuse structure you already have, while a detached ADU is a new building. For published cost ranges, see garage conversion costs and our ADU cost guide, then get a written estimate for your lot.

San Jose and California rules worth knowing

• Size limits. State guidance puts the ADU size limit at generally up to 1,200 sq ft, or 50% of the living area for an attached unit. 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, The City says to contact the school district directly about school fees for larger units. Both garage conversions and JADUs almost always stay under that line, and many smaller detached units do too.
• Parking. Under state ADU law, parking for an ADU is capped at one space and cannot be required within a half mile walk of public transit, and a garage converted to an ADU does not have to have its parking replaced.
• Setbacks. Under state ADU law, a city cannot require more than a four-foot setback from side and rear lot lines for an ADU.

All three need permits, and in our experience the paperwork decides the schedule as much as the construction does. 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.

Already have an unpermitted conversion?

If the garage was converted into a living unit before January 1, 2020, the question is not which to build but how to legalize what you have. State law limits how a city can deny that permit, and San Jose runs a program for it. See legalizing an unpermitted unit as an ADU and California's ADU amnesty.

Common questions

Related reading: whether an ADU fits on your San Jose lot and how to evaluate ADU rental income.

Sources

• City of San José: ADU FAQs
• City of San José: ADU Fees
• California HCD: Junior ADU FAQs