An ADU builder in San Jose is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through San Jose's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in California.
What ADU Builders Cost in San Jose [3][4]
Based on our San Jose cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $180,000-$400,000 |
| Cost per Sq Ft | $250-$450 |
| Garage Conversion | $100,000-$220,000 |
| Prefab / Modular ADU (installed) | $130,000-$300,000 |
| City Permit & Plan Review | $4,000-$14,000 |
| Impact Fees | Waived for units under 750 sq ft |
For a full line-item breakdown, see our San Jose ADU cost guide. To confirm what you're allowed to build before you hire, review the San Jose ADU rules (max detached size: 1,200 sq ft; owner-occupancy: Not required).
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How to Vet an ADU Builder in San Jose
- License: Confirm a current contractor's license in California and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near San Jose and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits ($4,000-$14,000), plan check (Standard review process), and impact fees (Waived for units under 750 sq ft) are included.
- Permit handling: Confirm they manage San Jose's permit process ("Ministerial review", typically 60 days) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in San Jose in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including permit review (60 days) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent California ADU client references?
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