Quick Answer: Building an ADU in San Jose in 2025 costs approximately $350-$600 per square foot for a detached ADU (new construction), $250-$450 per square foot for an attached ADU (addition to the primary dwelling), or $150-$300 per square foot for a garage conversion to ADU. The wide range is driven by: design quality (pre-approved plans vs. custom architect-designed), foundation type (slab vs. raised), whether plumbing must be trenched through the slab, HVAC type (mini-split vs. ducted), and finish quality (standard vs. luxury finishes). A 600 sq ft detached ADU in San Jose at mid-range costs $210,000-$360,000 total including permits, design, and all construction. Don’t forget to add $15,000-$40,000 for permit fees, utility connections (PG&E service upgrade, water/sewer lateral if separated), and design/engineering fees.
ADU Cost Per Square Foot by Type — San Jose 2025
Helpful Resources:
| ADU Type | Cost Per Sq Ft | 300 Sq Ft ADU | 600 Sq Ft ADU | 1,000 Sq Ft ADU |
|---|---|---|---|---|
| Detached ADU (new construction) | $350 – $600 | $105K – $180K | $210K – $360K | $350K – $600K |
| Attached ADU (addition) | $250 – $450 | $75K – $135K | $150K – $270K | $250K – $450K |
| Garage conversion to ADU | $150 – $300 | — | $90K – $180K (2-car) | — |
| JADU (interior conversion) | $100 – $200 | $30K – $60K | $50K – $100K | — |
Quartz Construction provides detailed ADU cost estimates for San Jose properties — including permit fees, utility connections, and all construction costs. Learn about ADU construction. CSLB #1003436 | Call (650) 749-7436.
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