Hillsborough ADU Hillsborough ADU Checklist Hillsborough ADU Floor Plans

Hillsborough ADU Zoning and Building Standards

Creating an Accessory Dwelling Unit (ADU) in Hillsborough is an exciting way to maximize your property’s potential while adhering to local zoning and building standards. Understanding these regulations is crucial for a smooth construction process.Our team of experts can assist you through the entire ADU development process, from initial planning and design to obtaining necessary permits and ensuring compliance with all local zoning and building standards in Hillsborough.

Hillsborough ADU Zoning and Building Standards

Property Requirements

Properties in Hillsborough must meet specific criteria to qualify for an ADU. These include minimum lot sizes, zoning regulations, and adherence to building codes.

Building Size

The size of your ADU is subject to local regulations which determine the maximum allowable square footage based on your property’s zoning.

  • Single-Family Zoned Lots:

Single-family lots can accommodate ADUs with specific requirements regarding size, setbacks, and design to maintain the character of the neighborhood.

  • Duplex Family Zoned Lots:

Duplex zoned lots have distinct requirements allowing for the addition of ADUs while considering the increased density and impact on infrastructure.

  • Multi-Family Zoned Lots :

Multi-family zoned lots offer more flexibility for ADUs, including both attached and detached options to enhance the living space.

  • Attached ADUs:

Attached ADUs are integrated into the existing structure, offering seamless additional living space.

  • Detached ADUs:

Detached ADUs provide a separate living space, maintaining privacy for both the primary residence and the ADU.


Hillsborough requires the property owner to occupy either the primary residence or the ADU, ensuring owner involvement and oversight.

Number Of Units Allowed

The number of ADUs allowed per lot varies based on zoning, with single-family lots typically permitting one ADU and multi-family lots allowing more flexibility.

Hillsborough Building Height

Building height restrictions ensure that ADUs remain compatible with neighborhood aesthetics and do not overshadow existing structures.

  • Single-family: Single-family ADUs have a height limit that typically does not exceed two stories or 16 feet, depending on specific zoning regulations.
  • Duplex family: Duplex ADUs may have different height restrictions, allowing for additional design flexibility while maintaining neighborhood consistency.
  • Multi-family: Multi-family ADUs can vary in height, often allowing for taller structures to accommodate increased density and multiple units.

Hillsborough Building Size

Building size regulations for ADUs ensure that new units are appropriately scaled to fit within the existing neighborhood context.

  • Single-Family Freedom: Single-family ADUs are usually limited to a maximum of 1,200 square feet or 50% of the primary residence’s floor area, whichever is less.
  • Duplex house: Duplex ADUs must comply with size restrictions that balance the need for additional living space with maintaining overall property aesthetics.
  • Multi-Family Mix: Multi-family properties offer more flexibility, often allowing larger ADUs to accommodate multiple families or shared living arrangements.

Fire Safety Regulations

ADUs must adhere to strict fire safety regulations, including smoke alarms, fire extinguishers, and adherence to local building codes.


Parking requirements for ADUs vary, but typically include provisions for at least one additional parking space, unless exemptions apply.

Utility Connections

ADUs must have proper utility connections, including water, sewer, and electricity, to ensure they meet health and safety standards.


Setback requirements dictate the distance an ADU must be from property lines, ensuring privacy and adequate space between structures.

Zoning Regulations

Zoning regulations for ADUs in Hillsborough are designed to ensure that new units are compatible with existing neighborhood characteristics.

Hillsborough ADU Zoning Regulations

Single Family

Single-family zoning typically allows for one ADU per lot, with specific requirements for size, height, and setbacks.

Duplex Family

Duplex zoning permits ADUs while considering the increased density and infrastructure needs, often with more flexible regulations.


Multi-family zoning offers the most flexibility, allowing multiple ADUs per lot with varying size and height restrictions.

RF Zone Minimum Lot Area in Hillsborough

In Hillsborough, RF Zone minimum lot sizes range from spacious single-family homes (RF-20) to sprawling estate properties (RF-40), ensuring ample space for your ADU and lifestyle.

  • RF-20: RF-20 zoning requires a minimum lot size of 20,000 square feet.
  • RF-30: RF-30 zoning requires a minimum lot size of 30,000 square feet.
  • RF-40: RF-40 zoning requires a minimum lot size of 40,000 square feet.

Room Specifications

Each room in an ADU must meet specific requirements to ensure safety, functionality, and comfort.

Hillsborough ADU Room Specifications

Room TypeSpecifications
Bathroom– Minimum of one toilet
– One sink
– One shower or bathtub
– Proper ventilation
– Adequate lighting
Kitchen– Stove or cooktop
– Refrigerator
– Sink
– Counter space
– Storage cabinets
Sleeping Areas– Minimum room size
– Egress window
– Smoke detector
– Adequate lighting
– Proper ventilation
Storage– Closet space
– Secure door
– Adequate shelving
– Dry and cool area
– Accessible location

Hillsborough Requirements

Hillsborough has specific requirements for the development and construction of ADUs, ensuring they meet local standards and regulations.

Detached ADU Hillsborough: Detached ADUs in Hillsborough must comply with specific size, height, and setback requirements to ensure they blend seamlessly with the primary residence and neighborhood.

Attached ADU Hillsborough: Attached ADUs must be integrated into the existing structure, adhering to design standards that maintain the architectural integrity of the primary residence.

JADU Hillsborough: Junior Accessory Dwelling Units (JADUs) in Hillsborough are smaller units within the primary residence, typically limited to 500 square feet, and must meet specific safety and building code requirements.

Rental Requirements

Hillsborough has established rental requirements to ensure that ADUs are safe, habitable, and accessible to renters while protecting property owners.

  • Proof of Income: Tenants must provide proof of income to demonstrate their ability to pay rent, typically through recent pay stubs or a letter from their employer.
  • Credit Score: A minimum credit score requirement ensures that tenants have a history of managing their finances responsibly, which can help reduce the risk of non-payment.
  • Rental History: A good rental history, including references from previous landlords, is often required to demonstrate reliability and good tenancy.
  • Application Fee: An application fee is usually required to cover the cost of processing rental applications, including background and credit checks.
  • Security Deposit: A security deposit is collected to cover any potential damages or unpaid rent, typically equivalent to one or two months’ rent.
  • Proof of Renter’s Insurance: Tenants may be required to provide proof of renter’s insurance to protect their belongings and cover liability for any damage to the property.
  • Pet Policy: The pet policy outlines any restrictions or requirements related to pets, including types, sizes, and number of pets allowed.
  • Smoking Policy: The smoking policy defines whether smoking is allowed on the property and, if so, where it is permitted, to ensure the health and comfort of all residents.

Development Standards for Hillsborough ADU in Table Form

ADU development standards in Hillsborough vary based on the type of residence and specific zoning regulations.

ADU TypesDevelopment Standards Specifications
Single-family– Maximum 1,200 square feet
– One additional parking space required
– Must match architectural style of primary residence
– Height limit of 16 feet
– Must comply with fire safety regulations
Duplex– Maximum 1,000 square feet per unit
– One additional parking space per unit
– Height limit of 18 feet
– Must have separate utility connections
– Adherence to setback requirements
Multi-family– Multiple ADUs allowed
– Size limit based on lot area
– Height limit of 20 feet
– Must comply with building codes
– Additional fire safety measures
Junior ADUs (JADUs)– Maximum 500 square feet
– Must be within primary residence
– No additional parking required
– Must have separate entrance
– Adherence to building safety standards

Hillsborough Property Designations for ADU

Certain property designations can affect the feasibility and requirements for constructing an ADU.

  • Flood Zones: Properties in flood zones must comply with additional regulations to ensure the safety and integrity of the ADU, including elevation and floodproofing measures.
  • Geohazard Zones: Geohazard zones require specific building techniques and materials to mitigate risks associated with earthquakes, landslides, and other geological hazards.
  • Easements: Easements can impact the placement and design of an ADU, requiring careful planning to ensure compliance with access rights and utility lines.

ADU Permit Requirements and Fees

Obtaining the necessary permits is a crucial step in the ADU development process, ensuring that the unit meets all local building codes and regulations.

Elegant ADU In Hillsborough

  • Building Permit: A building permit is required for the construction of an ADU, ensuring that the structure complies with local building codes and safety standards.
  • Electrical Permit: An electrical permit is necessary for any electrical work associated with the ADU, ensuring that all installations are safe and up to code.
  • Plumbing Permit: A plumbing permit is required for any plumbing work, including the installation of fixtures and connections to existing water and sewer lines.
  • Other Permits: Additional permits may be required depending on the specifics of the ADU project, such as mechanical, grading, or demolition permits.

Design Your Dream ADU Today!

Embark on the journey to create your perfect ADU in Hillsborough with our expert guidance and support. From initial planning to final construction, we ensure a seamless and successful project that meets all local standards and exceeds your expectations.


Yes, but specific setbacks and design considerations apply. Contact the Hillsborough 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.