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 [12][13][14]
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) | $300,000-$480,000 |
| Cost per Sq Ft | $450-$600 |
| Garage Conversion | $180,000-$280,000 |
| Prefab / Modular ADU (installed) | $160,000-$350,000 |
| City Permit & Plan Review | Flat ADU building permit fees, FY2026-27 Comprehensive Fee Schedule p. 2.5: $2,831.57 new-construction ADU; $2,031.12 ADU remodel/conversion; $1,455.43 eligible manufactured, prefab or pre-approved-plan ADU; $1,454.02 JADU. A detached ADU also pays a wastewater capacity charge of $5.87 per sq ft (Public Works schedule p. 9.5), about $4,700 for 800 sq ft; conversions are exempt (27.19.050(d)). School district developer fees may apply |
| Impact Fees | None for ADUs of 750 sq ft or less; a larger ADU pays impact fees in proportion to the primary home's floor area (SMMC 27.19.050(b)). Utility connection and capacity charges for a new detached unit are proportional to its burden, and the ADU is not treated as a new residential use unless built with a new primary home |
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: No fixed cap: up to the FAR of the underlying zone and no larger than the primary home. The first 800 sq ft of ADU living area is exempt from FAR, and an 800 sq ft state-exempt ADU is always allowed even when the lot's floor area is used up or the main house is under 800 sq ft (SMMC 27.19.060(b)); owner-occupancy: Not required for ADUs. Required for a JADU only when it shares a bathroom with the main home; a JADU with its own bathroom has no owner-occupancy requirement (27.19.080(n)). Every JADU needs a deed restriction recorded before permit issuance (27.19.090)).
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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 ADU building permit fees, FY2026-27 Comprehensive Fee Schedule p. 2.5: $2,831.57 new-construction ADU; $2,031.12 ADU remodel/conversion; $1,455.43 eligible manufactured, prefab or pre-approved-plan ADU; $1,454.02 JADU. A detached ADU also pays a wastewater capacity charge of $5.87 per sq ft (Public Works schedule p. 9.5), about $4,700 for 800 sq ft; conversions are exempt (27.19.050(d)). School district developer fees may apply), plan check (Electronic plan check through the CSS portal (PDF sheets, 11x17 minimum). Fire sprinklers are required only if the main house is sprinklered or site conditions (hydrant flow, driveway length) demand them; solar panels are required on a new detached, non-manufactured ADU and may be placed on the main house instead; a smoke alarm interconnected with the main house and a CO detector are required in a JADU), and impact fees (None for ADUs of 750 sq ft or less; a larger ADU pays impact fees in proportion to the primary home's floor area (SMMC 27.19.050(b)). Utility connection and capacity charges for a new detached unit are proportional to its burden, and the ADU is not treated as a new residential use unless built with a new primary home) are included.
- Permit handling: Confirm they manage San Mateo's permit process ("Ministerial building permit only; no planning application or public notice for a code-compliant unit (27.19.040). In order: 1) confirm zoning, FEMA flood zone and heritage trees; 2) optional free Code Consultation Counter visit, weekdays 8:30-11:30am; 3) register on the Customer Self-Service (CSS) portal and upload a PDF plan set per the Residential Plan Submittal Guide (site plan, floor/roof/elevation plans, Title 24 energy forms, CALGreen checklist); 4) building, fire (San Mateo Consolidated), public works and utility review; 5) separate address assignment and fee payment; 6) permit issued, then inspections. A unit that misses an objective standard (height, setback, daylight plane, deck size) can instead seek discretionary Zoning Administrator approval under 27.19.045 with public notice", typically 60 days from a complete application for the city to approve or deny, or the ADU is deemed approved (state law, incl. SB 543 from Jan 2026). The city publishes no shorter target for pre-approved plans; expect at least one correction cycle) 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 days from a complete application for the city to approve or deny, or the ADU is deemed approved (state law, incl. SB 543 from Jan 2026). The city publishes no shorter target for pre-approved plans; expect at least one correction cycle) 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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