ADU construction costs in San Mateo County in 2026 vary significantly by project type — and the difference between a garage conversion and a new detached ADU is not just a matter of price. It is a difference in scope, timeline, and what your site can support. Here are the real numbers, broken down by ADU type, with the cost drivers that most online calculators do not account for.
In 2023, San Mateo County permitted over 600 ADUs across its jurisdictions. With permitted ADUs in the county generating $2,500–$3,500 per month in rental income, the return on investment calculation is genuinely different from any other renovation category — making accurate cost information more important, not less.
ADU Cost Ranges in San Mateo County by Type
Junior ADU (JADU) — $50,000 to $120,000: Converting an existing interior space within the main dwelling — a bedroom or bonus room — to a semi-independent unit with its own entrance and kitchen. The most affordable ADU type but limited to 500 square feet under California law and must share some utilities with the main home.
Garage Conversion ADU — $180,000 to $280,000: Converting an existing detached garage to a fully habitable ADU. Includes Title 24 energy compliance, electrical panel upgrade, plumbing rough-in, HVAC installation, and all finished interior work. Cost varies based on garage size, utility connection distance, and existing site conditions.
Attached ADU (Addition to Main House) — $220,000 to $350,000: Adding a self-contained unit attached to the main residence. Requires structural engineering, shared wall insulation to full residential code, and separate utility metering in most San Mateo County jurisdictions.
New Detached ADU — $300,000 to $450,000+: New construction from foundation up on an existing lot. Most flexible in terms of design and size but requires the longest permitting timeline and the highest upfront cost. Foundation type and site access are the primary cost variables.
What Drives ADU Costs Up on the Peninsula
Three site-specific factors add cost to Peninsula ADU projects above base estimates. First, utility trenching: connecting water, sewer, and electrical from the main structure to a detached unit requires trenching across the property. Cost varies from $8,000–$25,000 depending on distance and site conditions — this is the most variable line item on any detached ADU project.
Second, soil conditions: hillside areas in San Carlos and Bay-adjacent parcels often require a geotechnical report before a foundation can be designed or permitted. A geotech report adds $3,000–$6,000 and 4–6 weeks to the pre-construction timeline. Third, the age of existing structures: most Peninsula garages were built alongside 1940s–1980s homes and were never designed to meet habitable space standards — bringing them to California Building Code requires more work than the exterior condition suggests.
Learn more about the full construction and permit process on our ADU construction service page or see completed projects in our home addition portfolio.
ADU Return on Investment in San Mateo County
A permitted ADU generating $3,000 per month in rental income produces $36,000 per year in gross income. At a $250,000 construction cost, the gross payback period is approximately 7 years — before accounting for property value appreciation. Real estate appraisals typically value a permitted ADU at approximately 100 times the monthly rent, meaning a unit renting for $3,000 per month adds approximately $300,000 in appraised value to the property.
An unpermitted ADU receives none of this appraised value. Lenders and buyers discount or exclude unpermitted structures entirely. The permit is not overhead — it is what creates the financial value of the project.
Frequently Asked Questions
How much does a garage conversion ADU cost in San Mateo County in 2026?
A garage conversion ADU in San Mateo County runs $180,000–$280,000 fully permitted and finished. Cost varies based on garage size, distance from utility connections, and site conditions. This range includes Title 24 compliance, electrical panel upgrade, plumbing rough-in, HVAC, and all interior finishes.
Is a garage conversion cheaper than building a new detached ADU?
Yes, typically $100,000–$180,000 cheaper. A garage conversion repurposes an existing structure, eliminating foundation work and reducing the structural scope significantly. A new detached ADU offers more design flexibility and size options but starts at $300,000–$450,000+ on the Peninsula.
What is a Junior ADU and how much does it cost in San Mateo County?
A Junior ADU converts an existing interior space within the main home — typically a bedroom or bonus room — into a semi-independent unit with its own entrance and kitchen. JADUs are limited to 500 square feet under California law. Cost in San Mateo County runs $50,000–$120,000 depending on the extent of kitchen and bathroom work required.
What is the ROI on an ADU in San Mateo County?
A permitted ADU renting for $3,000 per month generates $36,000 per year in gross income. At a $250,000 construction cost, the gross payback period is approximately 7 years. Appraised value added is typically 100 times monthly rent — a $3,000 per month unit adds approximately $300,000 in appraised property value.
What is the biggest hidden cost in an ADU project in San Mateo County?
Utility trenching is the most variable and most frequently underestimated cost on Peninsula ADU projects — ranging from $8,000–$25,000 depending on the distance between the main structure and the ADU and site conditions. This cost cannot be estimated accurately without a site visit and is absent from most online ADU cost calculators.
Get an Accurate ADU Cost Estimate
ADU costs on the Peninsula depend entirely on your site. Brian visits your property, assesses utility connection distances, soil conditions, and zoning, and gives you a realistic cost range before any design fees are incurred.
Request a Free ADU Consultation or call Brian at 650-966-4190.