San Carlos ADU Regulations & Zoning
When do San Carlos 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 Carlos allows detached ADUs up to 16-18 feet in height depending on transit proximity. Your ADU must maintain a minimum 6-foot separation from other structures on the lot.
Permitting typically takes 2 to 3 months. San Carlos is part of the San Mateo County ADU Resource Center, which provides free ADU feasibility consultations and troubleshooting support.
The good news? Most projects do not require a soils report, building survey, or stormwater mitigation, making the process smoother and more accessible.
What Are the Rules for ADUs in
San Carlos
?
Number of ADUs Allowed
On single-family residential lots in San Carlos, you can build one ADU (attached or detached) and one Junior ADU (JADU). There is no minimum lot size requirement for ADU eligibility. Your property must be zoned for residential use with an existing or proposed primary dwelling.
Multifamily properties in San Carlos can build up to two detached ADUs with 4-foot setbacks. Additionally, you can convert non-livable spaces (such as storage areas, garages, or basements) into ADUs—up to 25% of the existing unit count.
Yes, San Carlos allows Junior ADUs up to 500 square feet. A JADU must be located entirely within the existing footprint of the primary residence or attached garage. JADUs require a separate entrance and an efficient kitchen, and may share a bathroom with the main home. Owner-occupancy is required—you must live in either the primary dwelling or the JADU.
San Carlos does not have a separate guest quarters classification. Any secondary living space with kitchen facilities would be classified as an ADU and count toward your allowable units.
ADU Size Limits
Detached ADUs in San Carlos can be up to 1,200 square feet. ADUs up to 800 square feet with 4-foot setbacks and 16-foot height limits qualify for streamlined ministerial approval and cannot be denied due to lot coverage, FAR, or open space requirements.
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 square feet for one-bedroom units or 1,000 square feet for two or more bedrooms.
Yes. Under California state law, ADUs up to 800 square feet with 4-foot setbacks and 16-foot height cannot be denied due to lot coverage, FAR, or open space requirements. This means you can build an 800 square foot ADU even if your lot is otherwise 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, with up to 18 feet allowed near transit or to match the primary dwelling's roof pitch. 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 within the 18-foot height limit in some zones or near transit. Most detached ADUs are single-story. Consult with the city planning department for specific guidance on your property.
Detached ADUs require minimum 4-foot setbacks from side and rear property lines, plus at least 6 feet from existing structures. Front setbacks follow the underlying zoning district requirements. Conversions of legally established structures are exempt from setback requirements.
San Carlos requires detached ADUs to be at least 6 feet from the primary residence and any other existing structures on the lot for fire safety and maintenance access.
Permitting & Timeline
San Carlos processes ADU permits within 60 days as required by state law. Overall, expect 2-3 months from complete application to permit issuance. The San Mateo County ADU Resource Center offers free consultations and troubleshooting support to help expedite the process.
Permit costs in San Carlos typically range from $5,000 to $15,000 depending on project scope and valuation. No impact fees are charged for ADUs under 750 square feet.
San Carlos does not routinely require soils reports or building surveys for standard ADU projects. If your property is on septic (rare in San Carlos), San Mateo County Environmental Health will need to review capacity.
Parking Requirements
No parking is required for ADUs within half a mile of public transit—which covers much of San Carlos given its Caltrain station and SamTrans bus service. Where parking is required, it cannot exceed one space per ADU. Conversion ADUs and JADUs never require parking.
No. When converting a garage or carport to an ADU, replacement parking is not required under California state law. This makes garage conversions a cost-effective option.
If parking is required, it can be located in setback areas, existing driveways (tandem parking), or elsewhere on the property per city standards.
ADU Costs & Rental Income
The total cost to build an ADU in San Carlos typically ranges from $250,000 to $450,000 depending on size, design, and site conditions. This includes soft costs (design, permits, engineering) and hard costs (construction, materials, labor). ADUs under 750 square feet qualify for reduced or waived impact fees.
San Carlos benefits from its central Peninsula location and excellent schools. Based on current market data, you can expect the following monthly rents for ADUs: 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, finishes, and proximity to downtown or Caltrain.
Several factors impact your total ADU cost: lot conditions (slope, access, soil quality), whether you're building detached or converting existing space, the level of finishes, and utility upgrade requirements. San Carlos's established neighborhoods may require design sensitivity. Using pre-reviewed plans and keeping your ADU under 750 square feet can significantly reduce costs.
.svg)
.svg)
.svg)