San Mateo staff propose ADU ordinance changes: taller two‑story ADUs, citywide parking exemption, ministerial tweaks

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…