Quick Answer: In San Jose, a prefab (modular or panelized) ADU typically costs $180,000-$350,000 and takes 3-6 months from permit approval to move-in — it’s faster because the unit is manufactured in a factory while your site is prepared. A site-built (stick-built) ADU costs $210,000-$420,000 and takes 6-12 months from permit approval — but it can be fully customized to your lot, existing home aesthetic, and personal design preferences. Prefab pros: faster, more predictable cost, factory quality control. Prefab cons: limited design flexibility, manufacturer lead times vary, crane required to set the unit (adds $3,000-$6,000), may not match the existing home’s architectural style. Site-built pros: fully custom, matches any home style, can use your lot’s odd dimensions. Site-built cons: more exposed to subcontractor scheduling, more variables in cost. For most San Jose homeowners, site-built is preferred for detached ADUs over 600 sq ft due to design flexibility; prefab makes more sense for garage conversions or JADUs where the structure already exists.
Prefab ADU vs. Site-Built ADU — San Jose 2025 Comparison
Helpful Resources:
| Factor | Prefab/Modular ADU | Site-Built ADU |
|---|---|---|
| Total cost (600 sq ft) | $180,000 – $280,000 | $210,000 – $360,000 |
| Time from permit to move-in | 3–6 months | 6–12 months |
| Design flexibility | Limited — manufacturer catalog | Fully custom to your lot |
| Architectural match to existing home | Moderate — generic designs | Excellent — custom exterior |
| Quality control | Factory-controlled; very consistent | Depends on subcontractor quality |
| Crane required for installation | Yes — $3,000 – $6,000 extra | No |
| Best for | Speed, predictable budget | Custom design, large ADUs |
Quartz Construction builds custom site-built ADUs throughout San Jose. We also advise clients evaluating prefab vs. site-built options. Learn about ADU construction. CSLB #1003436 | Call (650) 749-7436.
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