How much does an ADU cost in San Jose?
Researched 2026 price ranges for detached ADUs, garage conversions, and junior ADUs in San Jose — with sources — plus what drives the number and how families pay for it.
An accessory dwelling unit is essentially a small, complete home — its own kitchen, bath, and utility connections — so it's priced more like new construction than a remodel. The ranges below are 2026 planning figures for the San Jose area, compiled from the City of San José and local ADU cost data (sources at the bottom). Your actual cost depends on size, ADU type, and your lot.
Cost per square foot
In the San Jose area, ADUs generally run about $300–$600 per square foot. Smaller units cost more per square foot than larger ones, because the kitchen and bathroom — the most expensive rooms — make up a bigger share of a small building.
Detached ADU
A new, free-standing backyard ADU is the most involved option, since it needs its own foundation, roof, and utility runs. In San Jose these typically run from about $200,000 to $450,000+, with larger or high-end units reaching $600,000. As a rough guide by size:
- 550–750 sq ft (one-bedroom): about $200,000–$245,000
- 800–1,000 sq ft: about $260,000–$300,000
- 1,050–1,200 sq ft (near the state size limit): about $310,000–$330,000+
Garage conversion
Converting an existing garage is usually the most affordable ADU because the foundation, walls, and roof are already there. In San Jose, a fully permitted garage conversion commonly runs about $80,000–$225,000, depending on the garage's condition and how much structural and utility work is needed. See our garage conversion guide for details.
Junior ADU (JADU)
A JADU is built within the home's existing footprint and is capped at 500 sq ft by state law. Because it reuses the home's plumbing, electrical, and HVAC, it's typically the lowest-cost path to a unit — roughly $30,000–$80,000 in San Jose, depending on the starting condition and whether you add a private bathroom.
Permits, fees, and what drives cost
The biggest cost factors are size, ADU type, and your lot — utility connections, foundation, and site access. A key San Jose savings lever: impact fees are waived for ADUs under 750 square feet, so building slightly smaller can save thousands. San Jose permits, Title 24 energy requirements, and SB 407 plumbing rules apply to most ADUs; see our permit guide.
What an ADU returns
An ADU is one of the few projects that can pay for itself. In San Jose, a backyard or garage ADU commonly rents for about $3,000–$4,000 per month, many owners recover their investment in roughly 5–8 years, and an ADU can add an estimated 20–30% to property value. Homeowners often fund the build with a HELOC, cash-out refinance, or renovation loan — see our financing options.
Sources
Figures are 2026 planning ranges for the San Jose area and vary by project, lot, and finish level — they are not quotes. Compiled from the sources below; the only way to know your number is a free, lot-specific estimate.
- City of San José — Accessory Dwelling Units — permits, requirements, and the impact-fee waiver for ADUs under 750 sq ft.
- How Much Does an ADU Cost in San Jose? (2026) — size-based price ranges.
- Maxable — Cost to Build a Granny Flat: San Jose — per-square-foot and total cost data.
- Cost to Build an ADU in San Jose (2026 Pricing Guide) — detached, conversion, and JADU ranges.