Everything You Need to Know About Building an ADU in San Mateo County

(NOTE: Always consult with your local planning department or a qualified professional before making any decisions. This article is for informational purposes only and does not constitute legal, financial, or zoning advice.)

Adding an Accessory Dwelling Unit (ADU) to your property in San Mateo County can be a smart way to increase your home’s value, generate rental income, or create extra space for multigenerational living. Whether you’re considering a detached guesthouse, converting your garage, or building a rental unit in your backyard, it’s essential to understand the regulations, costs, and permitting process specific to San Mateo County.

Here’s everything you need to know before you start your ADU project.

What Is an ADU?

An Accessory Dwelling Unit (ADU) is a self-contained residential unit located on the same lot as a primary residence. It includes its own kitchen, bathroom, and entrance. There are three main types of ADUs:

• Detached ADU – a standalone unit, often located in the backyard.

• Attached ADU – an extension built onto the existing home.

• Converted ADU – an interior space repurposed (e.g., a garage or basement).

You may also consider a Junior ADU (JADU), which is a smaller, up-to-500 sq ft unit created within the main home. JADUs must include an efficiency kitchen and may share a bathroom with the main house.

Zoning and Size Regulations in San Mateo County

Building an ADU in San Mateo County is subject to specific zoning and development standards. Here are some of the key rules:

• Lot Eligibility: Most single-family and multi-family lots are eligible for one ADU. Some may also qualify for a JADU.

• Maximum ADU Size:

  • Detached: up to 1,200 sq ft

  • Attached: up to 50% of the primary residence

  • JADUs: up to 500 sq ft

• Setbacks: Minimum 4 feet from side and rear lot lines

• Height Limits: Typically up to 16 feet for detached units; higher may be allowed depending on zoning

• Parking Requirements: One off-street parking space may be required unless your property is:

  • Within half a mile of public transit

  • In a historic district

  • Involved in a garage conversion (no replacement parking required)

The Permitting Process: Step-by-Step

1. Feasibility Review
   Contact your local Planning Department or use the San Mateo County ADU Resource Center to assess what's allowed on your lot.

2. Design Your ADU
   Work with an architect or designer familiar with San Mateo’s ADU regulations. You’ll need site plans, floor plans, elevation drawings, and Title 24 energy compliance documentation.

3. Submit Permit Applications
   File your plans with your city or the county’s Planning and Building Department. Most jurisdictions offer streamlined ADU review processes.

4. Receive Approvals
   Your application will go through zoning, building, and potentially environmental review. Typical timelines range from 2 to 6 months.

5. Begin Construction
   Once your permits are approved, you can begin construction. Licensed contractors must be used, and inspections will occur throughout the build.

ADU Construction Costs & Financing

The cost of building an ADU in San Mateo County varies based on the size, materials, and whether it's detached or a conversion. On average:

• Garage Conversions: $80,000–$150,000

• Detached ADUs: $250,000–$400,000+

• Attached Additions: $200,000–$350,000

You’ll also need to factor in:

• Design and engineering fees

• Permit fees and utility connections

• Landscaping and site prep

Financing Options:

• Home equity loans or HELOCs

• Construction or renovation loans

• Cash-out refinancing

• Local and state incentives (check for San Mateo County programs)

Legal and Ownership Considerations

• Rental Terms: ADUs must be rented for terms of 30 days or longer. Short-term rentals (like Airbnb) are prohibited.

• Separate Sale: ADUs cannot be sold separately from the main home.

• Owner Occupancy: As of recent California law updates, owner occupancy is no longer required for ADUs built before 2025 (subject to change, so check the latest local rules).

Pro Tips for Homeowners

• Work with an ADU-experienced architect or design-build firm to avoid costly mistakes.

• Get familiar with your city-specific rules—each municipality in San Mateo County may have slight variations.

• Consider your long-term goals: rental income, family needs, or aging-in-place.

Thinking About Building an ADU?

If you're a homeowner in San Mateo County and are curious about the value, rental potential, or permitting process for adding an ADU, I’d love to help.

As a local real estate expert with a background in science and analytics, I help homeowners and investors navigate the complexities of the market with clarity and confidence.

📞 Call or text me at 415-828-0047
📧 Email: nona.ehyaei@compass.com