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Hillsborough, California: ADU Regulations

Are ADUs allowed in Hillsborough:


Date Updated:


Number of ADUs allowed:

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

Types of ADUs allowed:

ADUs in Hillsborough are referred to as Second Units and they may be ATTACHED or DETACHED or created from the CONVERSION of an existing detached accessory structure.

Lot size:

No min/max provided the ADU can comply with lot setbacks.

ADU size:

Attached – the lesser of 1,400 sq.ft. or 50% of the primary dwelling
Detached – 1,400 sq.ft

Lot Coverage: 

Lot Size in sq.ft. | Max Square Foot Area of Lot Permitted To Be Covered

17,500 to 21,780 | 3,850 plus 15 sq.ft. for each 100 sq.ft in excess of 17,500

21,780 to 32,670 | 4,495 plus 10 sq.ft. for each 100 sq.ft. in excess of 21,780

32,670 to 43,560 | 5,585 plus 6 sq.ft. for each 100 sq.ft. in excess of 32,670

43,560 and over | 6,239 plus 5 sq.ft. for each 100 sq.ft. in excess of 43,560


Must comply with setbacks applicable to primary dwelling on the lot.
Second-story ADUs built above an existing nonconforming structure must have a minimum 5’ setback and have no openings facing respective property line or lines

Note: Fire Department regulations should be assessed on a project by project basis.


Must be consistent with existing general plan. Roof slope must match that of the dominant roof slope of primary dwelling. The exterior lighting must be down-lit or as otherwise required by the building or fire code.
All second-story ADU windows and doors less than 30’ from interior property lines shall either be (for windows) clerestory with the bottom of the glass at least 6’ above the finished floor, or (for windows and for doors) utilize frosted or obscure glass).
No outdoor showers.
Non-conforming units built prior to January 10, 1983 are allowed, however, any alterations or remodeling are limited to the interior of such structures.

Note: Historic Neighborhoods or Districts should be assessed on a project by project basis


ADUs with one or two bedrooms must provide 1 unenclosed parking space. ADUs with three or more bedrooms must provide 2 unenclosed parking spaces.

Parking for ADUs may be provided in tandem with parking for primary dwelling.
No additional parking is require for ADUs located within one-half mile of a public transit stop or car share vehicle pickup location, or Conversion ADUs. (17.36.035)

Fire Safety:

Refer to state requirements.

Note: Fire Department regulations should be assessed on a project by project basis.

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.


Detached ADU may not exceed 22’ from existing grade. Nonconforming ADUs within a setback may not exceed 15’ from existing grade.


ADUs must have independent exterior access from the primary dwelling not facing a public-right-of-way.


Evergreen landscape screening must be planted and maintained between the second unit and adjacent parcels as follows: At least one (1) 15 gallon size plant shall be provided for every five (5) linear feet of exterior wall. Alternatively, at least (1) 24” box size plant shall be provided for every ten (10) linear feet of exterior wall. For a ground-level second unit, plant specimens must be at least 6 feet tall when installed. As an alternative for a ground level ADU, a solid fence of at least 6 feet in height shall be installed. For a second-story second unit, plant specimens must be at least 12 feet tall when installed.

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 Residential Design and Design Review Board. Upon Planning Division approval, construction drawings may be submitted for Building Permit application.

Note: Historic Neighborhoods or Districts should be assessed on a project by project basis.


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. Click for more information.

 Note: Drainage and sewer connections should be assessed on a project by project basis.


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 Regulations

Potential Rental Income:

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

Important Note: Rules and regulations are updated and changed frequently. It’s one of our jobs to be up to speed and navigate those regulations. Let us know if our website isn’t up to date or if you have any information that could help. Cheers!