Due to the State of California’s affordable housing crisis, 2018 legislation and recent case law require California jurisdictions to approve Accessory Dwelling Units (ADUs or Second Units) ministerially. In other words, there should be no discretion applied, and applications must be processed within 120 days.

Aside from State Law, Hillsborough has committed to providing housing choices to meet the needs of current residents and their families along with people who provide important services to our town- like our teachers and police and safety officers. Additionally, adopting “objective design standards” will promote more balanced expectations between applicants and neighbors, further streamline the ADU entitlement process, and maintain the unique character of the community.

The proposed “objective design standards” include references to height, setbacks, architecture and landscape for ADUs.

The City Council Adopted the ordinance on Monday January 14, 2019 you may find a copy of the Adopted Ordinance and Ordinance 753 on Discretionary Exceptions and Additional Benefits.

Please contact Elizabeth Cullinan at 650-375-7416 or ecullinan@hillsborough.net with any questions and comments regarding this item.

ADU Ordinance Presentation

Please see the chart below for a summary of the proposed standards compared with today’s standards, along with the objective of each set of standards:


Proposed Requirements Existing Requirements/Guidelines Intent


A detached ADU may not exceed 22 feet from existing grade.
​Nonconforming ADUs within a setback may not exceed 15 feet from existing grade.
The current height limit is 32 feet from existing grade. Promote ADUs as ancillary in appearance to primary residence and consistent with limit for height envelope.
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. Currently there is no prohibition on openings. Promote privacy.


The materials and colors of the exterior walls, roof, and windows and doors must match the appearance of those of the primary dwelling. The roof slope must match that of the dominant roof slope of the primary dwelling. The exterior lighting must be down-lit or as otherwise required by the building or fire code. These concepts are currently guidelines and advisory rather than prescriptive. Promote architectural compatibility and positive aesthetics.
The second unit must have an independent exterior entrance not facing a public-right-of-way. Currently there is no requirement that the entrance not face a public right-of-way. Preserve single family character/appearance of community.
All second-story ADU windows and doors less than 30 feet from interior property lines shall either be (for windows) clerestory with the bottom of the glass at least six feet above the finished floor, or (for windows and for doors) utilize frosted or obscure glass. Currently there is no respective requirement. Promote privacy.
ADUs shall comply with ministerial historic resource standards. No local requirement. Compliance with State Law.
Outdoor showers are prohibited. Currently there is no respective requirement. Promote privacy and reduce conflict.


Limit of one driveway unless otherwise required by Central County Fire Department. This concept is currently policy/advisory rather than prescriptive. Preserve single family character/appearance of community and maximize landscape opportunities.
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. These concepts are currently guidelines and advisory rather than prescriptive. Promote privacy and preserve landscape screening opportunities.

Neighbor Notification

Due to the ministerial nature of the process, notification is required prior to construction only. Notification would be required prior to design review determination for exceptions as noted below. Currently there is no respective requirement. Promote neighbor preparedness for construction activities.


Exceptions to the above requirements, up to the standards permitted for primary residences, would be considered using the standard Administrative Review discretionary design review process. Currently there are no exceptions to ADUs permitted under the Municipal Code. Allow architectural solutions/enhancements appropriate to the unique character of a property.