Planning Commission studies broad ADU ordinance updates; commissioners split on discretionary review and size caps

San Mateo City staff presented proposed updates to the city’s accessory dwelling unit (ADU) ordinance at a Planning Commission study session on Aug. 20, seeking commissioner feedback before drafting a formal ordinance for later hearing. Liz Gagliardi, Associate Planner, presented state‑law context and staff‑initiated changes the city proposes in addition to required updates identified by the State Department of Housing and Community Development (HCD).

Staff said the two categories of change are: (1) updates required for state‑law compliance (front‑yard setbacks, multifamily ADU standards, clarifications on junior ADU size) and (2) staff‑initiated changes to improve local functionality. The staff‑initiated proposals the commission discussed in detail were: separate height standards for one‑ and two‑story detached ADUs (proposed top plate limits of 12 ft for one‑story and 18 ft for two‑story, with roof peaks up to 18 ft and 24 ft respectively), an optional 1,200‑square‑foot maximum size for new ADU construction (in addition to floor area ratio rules), removing required off‑street parking for ADUs citywide, and replacing the ADU discretionary review path (the “ADU Doctor”) with expanded objective, ministerial standards while leaving standard variance procedures for exceptional circumstances.

On height, staff explained state law requires allowing detached ADUs up to 16 feet in top plate height (18 feet near transit) and that staff’s proposal would raise the two‑story top plate from 16 to 18 feet citywide to accommodate typical two‑story construction (8 ft + 8 ft ceilings) while proposing a lower one‑story plate to avoid oversized single‑story massing in small lots. Commissioners questioned how daylight‑plane measurements and lot slope would interact with plate measurements and how grade is measured; staff clarified that height is measured from existing grade to the highest point on the structure and the most restrictive daylight‑plane standard would apply when a structure encroaches into required yards.

On maximum ADU size, staff proposed adding a 1,200‑square‑foot cap for new construction ADUs (existing conversions of accessory structures would not be constrained by the same cap). Commissioners were split. Some supported a 1,200‑sq‑ft cap — noting it would help keep ADUs accessory in scale — while others favored retaining the existing approach tied to FAR and the municipal code rule that an ADU may not exceed the primary dwelling’s floor area. Staff noted that of the city’s ADUs permitted since 2022, only four had exceeded 1,200 square feet.

On parking, staff proposed exempting all new ADUs from required off‑street parking citywide, clarifying that voluntary parking (driveways, retained curb cuts) would be allowed and encouraged; staff argued most of the city is already within the half‑mile transit exemption in state law and that a citywide exemption would be simpler and more equitable. Public commenters urged retaining parking minimums in neighborhoods where curb space is scarce; staff and commissioners discussed potential incentives to encourage voluntary off‑street parking (allowing driveways/curb cuts to remain, waiving certain front‑yard hardscape limits in constrained areas, or allowing covered parking within an ADU footprint as some cities do).

On review process, staff proposed converting objective standards so qualifying ADUs would be approved ministerially, eliminating the ADU Doctor discretionary alternative and reserving the variance pathway for exceptional circumstances. Commissioners and some public commenters urged keeping a discretionary review or “escape valve” so applicants with unusual parcel constraints (slopes, nonconforming accessory structures, daylight‑plane conflicts) could seek relief without the difficult variance findings. Staff noted that most ADU Doctor applications submitted since 2022 were either revised to meet ministerial standards or withdrawn; commissioners agreed the goal should be to expand objective ministerial approvals but keep a discretionary avenue for rare, site‑specific exceptions.

Public comment: Laurie Watanuki urged retaining current height limits, keeping the one‑off parking space requirement per unit and preserving ADU design review. Eric (no last name provided), an architect, urged flexibility for plate heights (cited common 14‑ft plates in older bungalows) and clarified how FAR and the municipal code interact. John (identified as a former commissioner) supported streamlined ministerial approvals and said ADUs are an important contributor to local housing production.

Next steps: staff said they will hold a virtual community workshop on Sept. 25, return a draft ordinance to the Planning Commission in October 2025, and aim to bring a first read to City Council in November 2025 with final action before year‑end if the schedule holds.

The session ended without a formal commission vote because the item was a study session; staff will draft code language and return with a formal ordinance for hearings.