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 $250,000-$450,000 for new construction, with per-square-foot costs averaging $350-$550. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $120,000-$200,000, while prefab/modular ADUs fall in between at $160,000-$320,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 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 |
| 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. Get up to 3 free quotes from ADU builders serving San Mateo — no obligation.
Get San Mateo builder quotes →Permit fees in San Mateo are Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees. The review timeline is 60-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026), with plan check: Standard building plan check; Code Consultation Counter open weekday mornings (8:30-11:30am) for pre-submittal questions. Impact fees are Waived for ADUs under 750 sq ft (state law); confirm larger-unit fees with the Building Division. Pre-approved plans: Yes — the city offers a Pre-Approved ADU Plans program.
| 1-Bedroom Rent | $2,700-$3,100/month |
| 2-Bedroom Rent | $3,400-$3,900/month |
| Annual Rental Income (est.) | $34,800/year |
| Payback Period (est.) | 10.1 years |
| Short-Term Rental | No — short-term rental of ADUs/JADUs prohibited per SMMC 27.19.030 |
| Overall ROI Estimate | 5-8% annual return (Peninsula rents are among the strongest in the state) |
A typical ADU in San Mateo generates approximately $34,800 per year in rental income based on average 1-bedroom rents. At an average construction cost of $350,000, the estimated payback period is 10.1 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: $120,000-$200,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $160,000-$320,000.
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.
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.
Compare prices from local ADU contractors in your area. Free, no-obligation estimates.
Find ADU Contractors →