Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your San Mateo ADU cost →Building an ADU in San Mateo costs between $300,000-$480,000 for new construction, with per-square-foot costs averaging $450-$600. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $180,000-$280,000, while prefab/modular ADUs fall in between at $160,000-$350,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 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 |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving San Mateo, where we have them — free, no obligation.
Get San Mateo builder quotes →Permit fees in San Mateo are 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. The review timeline is 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, with 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. Impact fees are 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. Pre-approved plans: Yes. The city's ADU Plans Gallery (cityofsanmateo.aduaccelerator.org) listed 17 pre-approved designs in Sept 2026, from 340 to 1,110 sq ft: 2 studios, 4 one-bedroom, 10 two-bedroom and 1 three-bedroom; 16 prefab and 1 site-built. Using one unaltered costs the reduced $1,455.43 permit fee, but the lot must be residential, outside a high-hazard FEMA flood zone and WUI area, on a slope under about 15%, and free of easements and open code cases (Pre-Approved ADU Plans Guide).
| 1-Bedroom Rent | $3,100-$3,400 (Zumper Sept 2026 $3,185; RentCafe Aug 2026 $3,406 apartment averages; ADUs typically rent slightly below)/month |
| 2-Bedroom Rent | $3,900-$4,400 (Zumper Sept 2026 $3,900; RentCafe Aug 2026 $4,448)/month |
| Annual Rental Income (est.) | $39,000/year |
| Payback Period (est.) | 10.0 years |
| Short-Term Rental | No. An ADU or JADU may not be rented for less than 30 consecutive days, and neither may be sold separately from the main home (SMMC 27.19.030) |
| Overall ROI Estimate | 7-9% gross rental yield on a $350,000-$480,000 detached build at $3,100-$3,400/mo (estimate before taxes, vacancy and financing) |
A typical ADU in San Mateo generates approximately $39,000 per year in rental income based on average 1-bedroom rents. At an average construction cost of $390,000, the estimated payback period is 10.0 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in San Mateo and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in San Mateo: $180,000-$280,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $160,000-$350,000.
3. Pre-approved plans — available in San Mateo, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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