Accessory Dwelling Unit (ADU) City Regulations

Are ADUs allowed in Hillsborough:


Number of ADUs allowed:

One ADU per lot and lot must contain a single-family residence.

Types of ADUs allowed:

Attached, detached and junior ADUs are all permitted.

Lot size:

Refer to state requirements.

ADU size:

A detached ADU is limited to 1,400 sq. ft. in floor area, whereas attached units cannot exceed 30% of the living area of the primary residence.


Setback requirements as specified in the Hillsborough Municipal Code for all residences can be found here:


ARUs with one or two bedrooms require only one parking space. However, units with three or more bedrooms must be provided two parking spaces. This is in addition to those required for the primary dwelling. Parking spaces may be unenclosed but cannot be in tandem. Please note that exceptions to the parking requirements may be granted for ARUs with two or more bedrooms.

Fire Safety:

Refer to state requirements.

Shape, materials and style:

New construction or exterior modification to existing structures should be compatible with the architectural style and details of the primary residence.


Must be in compliance with the applicable zoning district.

Design review:

The Planning staff can approve most new second units administratively. If staff approval is not possible, the application will need approval by the Town’s Architecture and Design Review Board. Upon Planning Division approval, construction drawings may be submitted for Building Permit application.


Either the ARU or the primary residence may be rented, but not both at the same time unless both are rented to the same party. Additionally, the ARU may not be sold separately from the primary unit.

Permit Fees:

There are no fees for the Planning Division review and there are no Building Permit fees for second units; Building Division plan-checking and other related fees do apply. For more information see:


The term “ARU” means a residential dwelling unit that provides complete independent living facilities for one or more persons and is located on the same lot as the main house (called the “primary dwelling”). The ARU must have permanent living, sleeping, eating, cooking, and sanitation provisions.

Lot Coverage:

The second unit must be constructed within the allowable maximum permitted floor area (FAR) and lot coverage of the applicable zoning district.

Conversion of Existing Space:

Converting existing living space into an ARU requires approval from the Planning Office and Building Department and must meet ARU guidelines. Upon approval, all necessary permits and inspections must be acquired and performed. Once the structure is approved by the Building Division, the ARU will be approved.

ADU Manual / Guide:

Potential Rental Income:

According to, an ARU in Hillsborough, CA can generate $36,000+ in possible rental income.

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