ADU Regulations in San Mateo County, California (2026)

Regulations at a glance

Local rules & notes

Front Setback Waiver: The city cannot preclude an 800 sq ft ADU with 4-foot side/rear setbacks by enforcing front yard setback restrictions. Subjective Design Standards Prohibited: HCD flagged city requirements for 'similar exterior materials' and 'maximize privacy' as non-objective and therefore unenforceable. Unpermitted Structure Amnesty: The permit status of an existing structure cannot be used to deny an ADU unless it is a health and safety threat.

For the official ordinance and latest updates, check your city or county planning department.

Frequently asked questions about ADU regulations in San Mateo County

• What is the maximum ADU size in San Mateo County, California?

  In San Mateo County, the maximum ADU size is typically 1000 square feet, depending on whether the unit is attached or detached. California state law allows up to 1,200 sq ft for detached ADUs and 800 sq ft for attached in many jurisdictions; San Mateo County may follow state defaults or have local limits. Verify with San Mateo County planning for your specific lot.

• What are the setback requirements for ADUs in San Mateo County?

  In San Mateo County, typical setback requirements are 4 feet from the side property line and 4 feet from the rear. These may be reduced to 4 feet under California Government Code § 65852.2 where applicable. Check with San Mateo County planning for overlay zones or design standards that could affect your site.

• What is the maximum height for an ADU in San Mateo County?

  In San Mateo County, the maximum height for an ADU is generally 16 feet. State law often allows up to 16 feet for detached ADUs and up to 25 feet when within 4 feet of the property line in some cases. Confirm with San Mateo County or county planning for your zone.

• Is ADU approval ministerial in San Mateo County?

  In San Mateo County, ADU approval is typically Ministerial (by-right) per Gov Code §65852.2. Ministerial (by-right) review means the city must approve qualifying applications without discretionary review, per California Gov Code § 65852.2. Your project must still meet objective standards; San Mateo County planning can confirm the exact process.

• Are there parking requirements for ADUs in San Mateo County?

  In San Mateo County, parking for ADUs is generally false. California law often waives parking for ADUs within a half-mile of transit or in certain historic districts. Check with San Mateo County planning for your address.