City of San Mateo planning staff on a community webinar outlined proposed updates to the city’s accessory dwelling unit (ADU) ordinance that would align the local code with recent state guidance from the California Department of Housing and Community Development (HCD) and add several locally driven changes.

The proposed changes include allowing detached ADUs up to 18 feet to the top wall plate (maintaining a 24‑foot roof‑peak limit), clarifying junior ADU (JADU) access and occupancy rules, eliminating required off‑street parking citywide for ADUs while encouraging voluntary parking solutions, and carving a ministerial path for small second‑story balconies. Staff said the Planning Commission is scheduled to hold a public hearing Oct. 28, followed by a City Council hearing on Nov. 17.

Why it matters: The updates would change what homeowners and ADU designers can build on residential lots across San Mateo. Staff said the changes respond to state law and to local feedback that current standards can constrain typical two‑story construction and create confusion about parking and minor design features.

Liz Gelliardi, associate planner who presented the webinar for the Community Development Department, said the changes respond both to HCD comments and to locally requested adjustments. “This 18 foot allowance that we’re looking at would allow for more standard 8 over 8 foot ceiling heights with, 1 foot floor joist spacing between and an additional foot for some wiggle room,” Gelliardi said, describing how the extra two feet to the wall plate makes two‑story ADUs easier to design in practice.

State compliance and local options

Gelliardi told webinar attendees that California’s ADU statutes (the statewide ADU laws enacted and updated since 2016) and HCD guidance require cities to permit certain ADU configurations and to use objective, ministerial standards in many cases. As part of the update, staff said San Mateo will explicitly align local standards with HCD guidance, including allowing, on single‑family parcels, one detached ADU, one conversion ADU and one junior ADU, and folding recent multifamily ADU updates into the code.

Local proposals explained

Height: Under the proposal detached ADUs would be allowed up to 18 feet to the top wall plate and 24 feet to the roof peak. Gelliardi said the change is intended to “allow for two‑story ADUs as required by state law but still be sensitive to existing scale of single‑family neighborhoods.” Current San Mateo rules measure both top wall plate and roof peak; single‑family homes remain allowed higher limits (for comparison, single‑family plate/peak limits remain unchanged under existing zoning).

Unit size: Staff proposed no change to the city’s approach to maximum ADU size. San Mateo currently does not set a fixed ADU maximum; instead ADU area is regulated through floor‑area ratio (FAR) with an 800‑square‑foot exemption consistent with state minimums. Since the 2022 ordinance, staff reported about 300 ADU permits issued with an average ADU size of roughly 600 square feet.

Parking: Gelliardi said state law already provides several parking exemptions (for example, ADUs created from converted garages and ADUs within a half‑mile of transit). Staff proposed eliminating the city’s remaining off‑street parking requirement for ADUs citywide and instead adding incentives for voluntary off‑street parking provision—such as allowing an existing driveway/curb cut to remain when a garage is converted to an ADU. “The proposal that is being brought forward for the update is to eliminate all required off street parking for ADUs citywide,” Gelliardi said during the webinar.

Ministerial review and the ADU discretionary path (“ADU Doctor”): San Mateo will keep a ministerial (building‑permit level) review path for ADUs that meet objective standards and retain an existing discretionary review option called the ADU Doctor for projects that depart from those standards. Staff said 19 ADU Doctor requests have been filed out of roughly 300 ADU submittals since 2022, commonly for buildings within 4 feet of a property line, two‑story plate‑height waivers and second‑story balconies. Under the proposal, small second‑story decks and balconies (limited to 50 square feet and meeting underlying zoning setbacks) could be approved ministerially; larger decks would remain discretionary.

Junior ADUs and conversions

Staff clarified several JADU rules in line with HCD guidance: JADUs remain limited to 500 square feet, must be within the primary structure or attached garage (cannot be detached), and generally require owner occupancy of one of the units on the parcel. Gelliardi also noted that interior access to the main house is required only when a JADU shares a bathroom and utility controls; otherwise interior shared access is optional.

Public questions and staff clarifications

Attendees asked about specific project scenarios. A participant identified as Rob, a resident, asked whether placing an ADU over a new two‑car garage would require the ADU to be two bedrooms because the garage footprint could hold two car spaces. Gelliardi deferred project‑specific advice and asked the resident to contact staff for an individual review. Zach Dahl, the city’s director of community development, added that staff are “looking at an option where we could allow for a 2 car up to a 2 car garage to be located within the footprint of an ADU” to help incentivize parking, but said the details would be addressed in the final code language.

On notification and public engagement, staff said discretionary ADU Doctor applications follow the city’s standard notice—mail to properties within a roughly 500‑foot radius at least 10 days before a decision—and neighbors may submit written comments during that notification window.

Resources and next steps

Gelliardi encouraged homeowners to consult the San Mateo County ADU Resource Center and the City of San Mateo ADU webpages for guidance and one‑on‑one assistance. She said the city posted a planning commission study session recording from Aug. 26 and will post the webinar recording and slides on the ADU webpage; the formal hearings are scheduled for the Planning Commission on Oct. 28 and City Council on Nov. 17.

No formal vote or ordinance adoption occurred during the webinar; staff presented the draft updates and took questions.

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Editor’s note: Direct quotes in this article come from the webinar presentation and Q&A by Liz Gelliardi, associate planner, and Zach Dahl, director of community development. The webinar recording and staff report on the ordinance update are available on the City of San Mateo public meeting portal (posted to the city ADU webpage).