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Millbrae ADU Zoning And Building Standards

Millbrae has specific zoning and building standards for ADUs to ensure they integrate well into the community. These standards cover aspects such as building height, size, and placement on the property. Adhering to these regulations is essential to obtain the necessary permits and avoid potential issues down the line. These guidelines are designed to maintain the character of the neighborhood while accommodating the growing need for additional housing options. By adhering to these standards, you ensure that your ADU project is not only legally compliant but also enhances the overall appeal of your property and the community.

At DevArt8 Construction, we are well-versed in Millbrae’s zoning and building standards for ADUs. We provide expert advice and handle the permitting process, ensuring your ADU meets all local requirements. Our team’s experience and knowledge will help streamline your project, saving you time and stress. We offer comprehensive services from initial consultation to final construction, ensuring that every aspect of your ADU project is meticulously planned and executed.

Millbrae ADU Zoning and Building Standards

Property Requirements

When planning an ADU in Millbrae, it’s essential to understand the property requirements. These include lot size, setbacks, and the number of ADUs allowed per property. Compliance with these requirements ensures your project can proceed without legal complications. Understanding the specific requirements for your property type is crucial in the initial planning stages. Whether you have a single-family, duplex, or multi-family property, knowing these requirements helps in creating a feasible and compliant ADU design.

Building Size

The size of your ADU is determined by various factors, including the size of your lot and the zoning regulations. Millbrae allows different maximum sizes for ADUs depending on whether they are attached or detached and the zoning of the lot. Ensuring your ADU meets these size regulations is crucial for obtaining the necessary permits and ensuring the structure fits well within the existing property layout. Properly planning the size and layout of your ADU can maximize functionality and comfort.

  • Single-Family Zoned Lots:

In single-family zoned lots, ADUs are subject to specific size and placement regulations. Typically, the maximum size for a detached ADU on a single-family lot is 1,200 square feet, while attached ADUs can be up to 50% of the existing main residence’s square footage, with a cap at 1,200 square feet. These regulations help ensure that the ADU complements the primary residence and fits harmoniously within the neighborhood.

  • Duplex Family Zoned Lots:

For duplex family zoned lots, you can have both attached and detached ADUs, but the total square footage must not exceed the limits set by local ordinances. Detached ADUs can be up to 1,200 square feet, and attached ADUs can be up to 50% of the main structure’s size. These guidelines help maintain the balance between added living space and the overall property layout.

  • Multi-Family Zoned Lots :

Multi-family zoned lots have more flexibility regarding ADU placement and size. These lots can accommodate multiple ADUs, both attached and detached, depending on the available space and zoning regulations. The flexibility in multi-family zoning allows for creative solutions to maximize living space while adhering to local standards.

  • Attached ADUs:

Attached ADUs in multi-family zones can be up to 50% of the main structure’s size, capped at 1,200 square feet. These units can utilize existing space efficiently, providing additional housing options within the current property framework.

  • Detached ADUs:

Detached ADUs on multi-family lots can also reach up to 1,200 square feet, provided they meet the setback and lot coverage requirements. Detached units offer independent living spaces, ideal for rental opportunities or extended family accommodations.


Millbrae requires that either the primary residence or the ADU be owner-occupied. This rule ensures that the property is well-maintained and integrates smoothly into the residential neighborhood. Owner-occupancy requirements help maintain the quality and care of the property, fostering a sense of community and responsibility among residents.

Number Of Units Allowed

The number of ADUs allowed per property varies based on the zoning. Single-family lots typically allow one ADU, while duplex and multi-family lots may accommodate multiple ADUs. Understanding these limits is crucial for planning your project. Proper planning ensures that your ADU complies with local regulations and makes the most efficient use of available space.

Millbrae Building Height

Building height restrictions for ADUs in Millbrae are designed to ensure that new structures fit seamlessly into the existing neighborhood. These height limits vary depending on the type of property and the ADU configuration.

  • Single-family: For single-family properties, the maximum height for a detached ADU is typically 16 feet, while attached ADUs must not exceed the height of the main residence. These height restrictions ensure that the ADU remains a secondary structure, maintaining the visual hierarchy of the property.
  • Duplex family: On duplex family lots, detached ADUs can be up to 16 feet high, while attached ADUs must conform to the height of the main structure. This helps maintain a consistent aesthetic and prevents overshadowing neighboring properties.
  • Multi-family: In multi-family zones, detached ADUs are also limited to 16 feet, but attached ADUs should align with the existing building’s height. This ensures that new ADUs blend seamlessly with the surrounding buildings and do not disrupt the neighborhood’s overall appearance.

Millbrae Building Size

The size of an ADU in Millbrae is influenced by the property’s zoning and existing structures. It’s important to plan your ADU to comply with these regulations to ensure a smooth approval process.

  • Single-Family Freedom: Single-family properties can have detached ADUs up to 1,200 square feet or attached ADUs that do not exceed 50% of the main residence’s size, with a maximum of 1,200 square feet. This allows homeowners to add significant living space without compromising the property’s primary use.
  • Duplex house: Duplex properties can support a combination of ADUs, with detached units up to 1,200 square feet and attached units up to 50% of the primary structure’s size. This provides flexibility in adding living spaces while adhering to zoning regulations.
  • Multi-Family Mix: Multi-family lots can include multiple ADUs, each up to 1,200 square feet, provided they comply with setback and lot coverage rules. This flexibility allows for creative use of space and maximizes the property’s potential.

Fire Safety Regulations

Fire safety is a critical consideration in ADU construction. Millbrae requires ADUs to adhere to specific fire safety regulations, including smoke detectors, fire alarms, and proper egress windows. Compliance with these standards ensures the safety of the occupants. Proper planning and installation of fire safety measures can prevent accidents and ensure that your ADU is a secure and safe living space.


Parking requirements for ADUs in Millbrae may vary based on the property’s zoning and the ADU’s size. Generally, new ADUs must provide one parking space, but there are exceptions, such as proximity to public transit. Adequate parking ensures convenience for residents and prevents congestion in the neighborhood.

Utility Connections

Connecting utilities like water, electricity, and sewage to your ADU requires careful planning. Millbrae has specific guidelines for utility connections to ensure they are safe and efficient. Consulting with experts like DevArt8 Construction can simplify this process. Proper utility connections ensure that your ADU is fully functional and meets all health and safety standards.


Setbacks refer to the minimum distance an ADU must be from the property lines. In Millbrae, typical setback requirements include a minimum of four feet from the side and rear property lines for detached ADUs. These regulations help maintain privacy and neighborhood aesthetics. Adhering to setback requirements ensures that your ADU is appropriately spaced and does not encroach on neighboring properties.

Zoning Regulations

Zoning regulations for ADUs in Millbrae dictate where and how ADUs can be built. These rules vary depending on whether the property is zoned for single-family, duplex, or multi-family use.

Millbrae ADU Zoning Regulations

Single Family

Single-family zones allow one ADU per lot, with specific requirements for size, height, and setbacks to ensure compatibility with the neighborhood. These regulations ensure that the ADU is a secondary structure that complements the primary residence.

Duplex Family

Duplex family zones can accommodate both attached and detached ADUs, with regulations designed to maintain the residential character of the area. This provides flexibility while ensuring that the ADUs do not overwhelm the primary structures.


Multi-family zones offer more flexibility, allowing multiple ADUs per lot, provided they meet the space and setback requirements. This flexibility supports the creation of additional housing units within the existing property framework.

RF Zone Minimum Lot Area In Millbrae

In Millbrae’s RF Zone, the minimum lot sizes ensure ample space for your ADU and lifestyle. These range from RF-20 for single-family homes to RF-40 for sprawling estate properties.

  • RF-20: Minimum lot size of 20,000 square feet, suitable for spacious single-family homes. This size allows for substantial living spaces and outdoor areas.
  • RF-30: Minimum lot size of 30,000 square feet, offering more space for larger residences. This provides additional room for landscaping and amenities.
  • RF-40: Minimum lot size of 40,000 square feet, ideal for expansive estate properties. This allows for extensive living areas and luxurious outdoor spaces.

Room Specifications

Understanding room specifications is essential for designing a functional and comfortable ADU. Each room type has specific requirements to meet building codes and ensure livability.

Millbrae ADU Room Specifications

Room TypeSpecifications
Bathroom1. Minimum size: 30 square feet

2. Ventilation: required

3. Waterproof flooring: required

4. Shower or tub: mandatory

5. Sink and toilet: required

Kitchen1. Minimum size: 50 square feet

2. Ventilation: required

3. Stove and oven: mandatory

4. Sink: required

5. Refrigerator: required

Sleeping Areas1. Minimum size: 70 square feet

2. Egress window: required

3. Closets: recommended

4. Smoke detectors: required

5. Outlets: required

Storage1. Minimum size: 20 square feet

2. Shelving: required

3. Ventilation: optional

4. Lighting: required

5. Accessibility: required

Millbrae Requirements

Millbrae has specific requirements for different types of ADUs to ensure they meet the city’s standards and contribute positively to the community.

  • Detached ADU Millbrae: Detached ADUs must meet specific size, setback, and height requirements. They offer the flexibility of a separate living space while adhering to local regulations. Detached units provide privacy and independence, ideal for rental properties or family members.
  • Attached ADU Millbrae: Attached ADUs must integrate with the existing structure and are subject to size limitations relative to the main residence. They provide additional space without the need for a separate building. Attached units are perfect for extending living areas while maintaining a single cohesive structure.
  • JADU Millbrae: Junior ADUs (JADUs) are smaller units created from existing space within the main residence. They are limited to 500 square feet and require an efficiency kitchen. JADUs are a cost-effective way to add living space, ideal for rental opportunities or guest accommodations.

Rental Requirements

Renting out your ADU involves meeting specific requirements to ensure compliance and attract quality tenants.

  • Proof of income: Tenants must provide proof of stable income to ensure they can afford the rent. This typically includes recent pay stubs or tax returns.
  • Credit score: A good credit score is typically required to assess the tenant’s financial responsibility. This helps ensure that tenants can reliably pay rent on time.
  • Rental history: Previous rental history helps verify the tenant’s reliability and behavior. Landlords often contact previous landlords for references.
  • Application fee: An application fee covers the cost of processing rental applications and background checks. This fee is non-refundable and helps cover administrative costs.
  • Security deposit: A security deposit protects against potential damages and unpaid rent. This deposit is typically equal to one month’s rent and is refundable upon lease termination, provided there are no damages.
  • Proof of renter’s insurance: Tenants must have renter’s insurance to cover personal property and liability. This insurance protects both the tenant and landlord in case of unforeseen events.
  • Pet policy: Specific rules regarding pets ensure they are managed appropriately and do not cause damage. Some landlords may charge a pet deposit or pet rent.
  • Smoking policy: Clear rules about smoking help maintain a healthy living environment. Non-smoking policies are common to prevent damage and ensure safety.

Development Standards For Millbrae ADU

Understanding the development standards for different types of ADUs is crucial for compliance and planning.

ADU TypesDevelopment Standards Specifications
Single-family1. Maximum size: 1,200 square feet

2. Height limit: 16 feet

3. Setbacks: 4 feet side and rear

4. Parking: 1 space

5. Owner-occupancy: required

Duplex1. Maximum size: 1,200 square feet

2. Height limit: 16 feet

3. Setbacks: 4 feet side and rear

4. Parking: 1 space

5. Owner-occupancy: required

Multi-family1. Maximum size: 1,200 square feet

2. Height limit: 16 feet

3. Setbacks: 4 feet side and rear

4. Parking: 1 space per unit

5. Owner-occupancy: required

Junior ADUs (JADUs)1. Maximum size: 500 square feet

2. Height limit: within existing structure

3. Setbacks: N/A

4. Parking: N/A

5. Owner-occupancy: required

Millbrae Property Designations For ADU

Specific property designations in Millbrae can impact the feasibility of building an ADU.

  • Flood zones: Properties in flood zones may require additional engineering and safety measures to ensure compliance with floodplain regulations. These measures might include elevated foundations or floodproofing techniques.
  • Geohazard zones: Properties in geohazard zones might need geotechnical assessments and specific construction techniques to address potential risks like landslides or earthquakes. These precautions ensure the safety and stability of the structure.
  • Easements: Easements on a property can limit where an ADU can be built, as they often restrict construction in certain areas. Understanding any easements on your property is crucial for proper planning and compliance.

ADU Permit Requirements And Fees

Obtaining the necessary permits is a critical step in the ADU construction process. Millbrae requires several permits, each with associated fees.

Millbrae ADU Permit Requirements and Fees

  • Building Permit: A building permit ensures your ADU complies with structural and safety standards. This permit is mandatory for all ADU projects. It covers inspections and approvals for the overall construction.
  • Electrical Permit: An electrical permit is required for any electrical work involved in the ADU, ensuring all installations meet safety codes. This includes wiring, outlets, and fixtures.
  • Plumbing Permit: Plumbing permits are necessary for any plumbing work, including water supply and sewage systems, to ensure proper functionality and safety. This covers piping, fixtures, and drainage systems.
  • Other Permits: Depending on the project, additional permits such as mechanical or grading permits may be required to cover specific aspects of the construction. These permits ensure that all elements of the ADU meet local standards.

Design Your Dream ADU Today!

Creating an ADU in Millbrae is a rewarding endeavor that enhances your property and provides valuable living space. At DevArt8 Construction, we are dedicated to helping you design and build the ADU of your dreams. From initial planning to final construction, our team is here to guide you through every step, ensuring your ADU meets all requirements and exceeds your expectations. Our comprehensive services include site evaluation, design consultation, permit acquisition, and high-quality construction. Contact us today to start your ADU project and turn your vision into reality. With DevArt8 Construction, you can transform your property with an ADU that offers both functionality and aesthetic appeal.


Yes, but specific setbacks and design considerations apply. Contact the Millbrae Planning Department for detailed guidance on navigating unique lot shapes.

Some lenders offer ADU loans, but traditional home loans can also be used for ADU construction, especially if they add value to your property.

Yes! The State of California offers various ADU incentive programs, including grants for low-income homeowners and developers. Research state and local programs to see if you qualify for financial assistance.