An ADU builder in San Mateo 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 Mateo'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 Mateo [4][5]
Based on our San Mateo 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) | $250,000-$450,000 |
| Cost per Sq Ft | $350-$550 |
| Garage Conversion | $120,000-$200,000 |
| Prefab / Modular ADU (installed) | $160,000-$320,000 |
| City Permit & Plan Review | Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees |
| Impact Fees | Waived for ADUs under 750 sq ft (state law); confirm larger-unit fees with the Building Division |
For a full line-item breakdown, see our San Mateo ADU cost guide. To confirm what you're allowed to build before you hire, review the San Mateo ADU rules (max detached size: Up to the FAR of the underlying zone and no larger than the primary unit; 800 sq ft is always allowed (ADU area up to 800 sq ft is FAR-exempt); owner-occupancy: Not required for ADUs. JADUs: deed restriction and owner-occupancy required (under AB 1154, from 2026 owner-occupancy applies only to JADUs sharing a bathroom with the main home — confirm current enforcement with the Planning Division)).
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How to Vet an ADU Builder in San Mateo
- 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 Mateo 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 (Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees), plan check (Standard building plan check; Code Consultation Counter open weekday mornings (8:30-11:30am) for pre-submittal questions), and impact fees (Waived for ADUs under 750 sq ft (state law); confirm larger-unit fees with the Building Division) are included.
- Permit handling: Confirm they manage San Mateo's permit process ("Ministerial — building permit and related permits only; no planning application or public notice for code-compliant projects", typically 60-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026)) 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 Mateo 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-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026)) 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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