San Mateo Ordinance 2026-02 took effect March 4, 2026 and amended Municipal Code Chapter 27.19. The City’s ordinance text applies ADU and junior ADU (JADU) rules in zoning districts that allow residential uses. The lot’s existing or proposed dwelling type determines which unit-count branch to check.
Single-family lots
On a lot with an existing or proposed single-family dwelling structure, the ordinance allows a combination of:
- one attached or conversion ADU;
- one detached new-construction ADU; and
- one JADU within the walls of the single-family dwelling.
Each unit still has to meet the applicable development standards. This count is not a statement that every combination fits a given parcel.
Multifamily lots
For a proposed multifamily dwelling structure, the ordinance says up to two new detached ADUs may be allowed. For an existing multifamily property, it describes up to eight new detached ADUs, capped at the number of existing primary dwelling units, and conversion ADUs in non-living space at a count of at least one and no more than 25% of the existing primary dwelling units. Additional units allowed by State law may also apply.
Confirm whether the property is classified as single-family or multifamily, the current consolidated Chapter 27.19, the zone, existing or proposed unit count, and remaining floor-area allowance. The ordinance text is current evidence of the 2026 amendment, but a parcel and design review are still necessary.