San Mateo ADU Regulations & Zoning
When do San Mateo ADU regulations apply?
Planning an Accessory Dwelling Unit (ADU) starts with knowing what's possible on your property. Here's a quick snapshot of the key build guidelines and permitting details to help you move forward with clarity.
You can build an ADU up to 1,200 square feet, with 4-foot setbacks from the side and rear property lines. San Mateo allows taller ADUs near transit, with heights up to 18-20 feet depending on location and design. Your ADU must maintain adequate separation from other structures.
Permitting typically takes 2 to 3 months. San Mateo is part of the San Mateo County ADU Resource Center, which provides free consultations, troubleshooting support, and cost/rent calculators to help homeowners plan their projects.
What Are the Rules for ADUs in
San Mateo
?
Number of ADUs Allowed
On single-family residential lots in San Mateo, you can build one ADU (attached or detached) and one Junior ADU (JADU). There is no minimum lot size requirement for ADU eligibility.
Multifamily properties can build up to two detached ADUs and convert non-livable spaces (garages, basements, storage) into ADUs—up to 25% of the existing unit count.
Yes, San Mateo allows Junior ADUs up to 500 square feet. A JADU must be within the existing single-family residence or attached garage. JADUs require a separate entrance and efficiency kitchen. Owner-occupancy is required.
San Mateo does not have a separate guest quarters classification. Any secondary living space with kitchen facilities would be classified as an ADU.
ADU Size Limits
Detached ADUs can be up to 1,200 square feet in San Mateo. ADUs up to 800 square feet with 4-foot setbacks and 16-foot height qualify for streamlined approval regardless of lot coverage or FAR.
Attached ADUs are limited to 1,200 square feet or 50% of the primary dwelling's floor area, depending on configuration. State law guarantees attached ADUs of at least 850-1,000 square feet.
Yes. Under California state law, ADUs up to 800 square feet are exempt from lot coverage, FAR, and open space requirements. This ensures you can build a qualifying ADU even on lots at maximum coverage.
About MolaADU
This page offers a clear summary of what’s allowed—how many ADUs you can build, size limits, setbacks, and other key requirements. But we go beyond the basics. Our insights are grounded in real-world experience, not just recycled bullet points.
Whether you're planning a rental unit, a guest house, or a space for multigenerational living, we’re here to help you move forward with clarity and confidence.
Height & Setbacks
Detached ADUs are typically limited to 16 feet in height. Near transit, heights up to 18-20 feet may be allowed. Attached ADUs can match the primary dwelling's height up to 25 feet. Conversion ADUs have no height limit since they use existing structures.
Limited two-story configurations may be possible depending on zoning, transit proximity, and design. Most detached ADUs are single-story. Consult with the city planning department for specific guidance.
Detached ADUs require minimum 4-foot setbacks from side and rear property lines, plus at least 6 feet from existing structures. Conversions of existing legal structures are exempt from setback requirements.
Detached ADUs must typically maintain 6 feet of separation from the primary residence and other structures for fire safety and maintenance access.
Permitting & Timeline
San Mateo processes ADU permits within 60 days as required by state law. Overall, expect 2-3 months from application to permit issuance. The San Mateo County ADU Resource Center offers free 20-minute troubleshooting calls for homeowners in the permitting process.
Permit costs vary based on project scope, typically ranging from $5,000 to $15,000. ADUs under 750 square feet qualify for reduced or waived impact fees. Use the county's ADU Calculator to estimate costs and potential rental income.
San Mateo does not routinely require soils reports or building surveys for standard ADU projects. Properties in special zones or with unusual conditions may require additional documentation.
Parking Requirements
No parking is required for ADUs within half a mile of public transit, which covers much of San Mateo given Caltrain and SamTrans service. Where parking is required, it cannot exceed one space per ADU. Conversion ADUs and JADUs never require parking.
No. When converting a garage to an ADU, replacement parking is not required under California state law.
If parking is required, it can be located in setback areas, driveways (tandem), or elsewhere on the property per city standards.
ADU Costs & Rental Income
The total cost to build an ADU in San Mateo typically ranges from $250,000 to $450,000 depending on size, design, and site conditions. Permit fees run $5,000-$15,000. The county resource center offers free tools to estimate costs.
San Mateo benefits from Peninsula location and excellent transit access. Based on current market data: Bedrooms Average Rent Studio $2,100 - 2,400 1 Bedroom $2,500 - 2,800 2 Bedroom $3,100 - 3,500 3 Bedroom $3,800 - 4,200 Actual rents vary by neighborhood and proximity to downtown or Caltrain
Several factors impact cost: lot conditions, design complexity, utility upgrades, and finishes. Conversion projects typically cost less than new construction. Using pre-approved or standard plans can reduce design costs.
.svg)
.svg)
.svg)