Estimate construction costs, permit timelines, and rental yield for an ADU on your Peninsula property. Covers San Mateo and Santa Clara County cities.
A detached ADU in San Mateo County costs $350–$500 per square foot for hard construction, plus $12,000–$30,000 in permit fees depending on city. A typical 800-sqft detached ADU in San Mateo or Burlingame runs $300,000–$430,000 all-in. Atherton, Hillsborough, and Woodside tend to be at the top of the range due to stringent plan check requirements and a high-cost contractor market. Garage conversions are materially less: $180–$280 per sqft in San Mateo County, making them the most cost-effective way to add rental income to an existing property.
An 800-sqft ADU on the Peninsula rents for approximately $3,400–$4,200 per month in 2026. At a $350,000 all-in cost, that represents a 12–14% gross yield before vacancy, management, and maintenance. After expenses (30–35% of gross), net yields are typically 8–10%. ADUs have appreciated alongside the underlying property, so total return including appreciation is generally higher. The Peninsula rental market is among the tightest in the country, driven by tech employment and limited rental housing supply — vacancy rates for well-maintained ADUs are consistently below 2%.
Permit timelines vary widely. Ministerial (by-right) ADU permits in most cities run 3–6 months under SB 9 and SB 68 streamlining requirements. Detached ADUs in design-review-required cities — Atherton, Hillsborough, and Woodside — can run 6–12 months due to architectural review processes. San Mateo, Redwood City, and Burlingame have streamlined to 3–5 months for standard designs. Pre-approved ADU plans offered by Redwood City and other cities can reduce review to 1–2 months and lower design costs by $5,000–$15,000. Mountain View and Sunnyvale have among the fastest ministerial review timelines on the Peninsula.
In California, Prop 13 limits reassessment to completed new construction. Adding an ADU will result in the ADU improvement value being added to your Prop 13 base; the existing home value is not reassessed. A $300,000 ADU addition might add $3,000–$3,600 per year to your property tax bill at a 1.1–1.2% effective rate in San Mateo County. The existing home value remains at its historical Prop 13 base, so long-term owners with low assessed values will see a modest, proportional increase rather than a full reassessment.
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