Accessory Dwelling Units


Accessory Dwelling Units (ADUs) require permits submitted for review to the Building Division. Please email the Building Division for building permit submittal requirements or call 650-259-2330.

Please contact the Planning Division staff at 650-259-2341 if you have questions about Millbrae's ADU requirements.

Development Impact Fees for ADUs

Accessory Dwelling Units of less than 750 square feet are exempt from Development Impact Fees. However, Accessory Dwelling Units of 750 square feet or more are charged Development Impact Fees proportionately in relation to the square footage of the primary dwelling unit based on the Single Family Residential Fee, which is $75,274.42 effective January 1, 2023. All building permits for ADU construction issued on or after August 8, 2020 are required to pay these fees prior to the date of the final inspection, or the date the certificate of occupancy is issued, whichever occurs first. However, if the fees are paid at final inspection or occupancy, the City will require that an impact fee agreement be recorded as a lien.

An example of the calculation methodology for Accessory Dwelling Units of more than 750 fifty square feet:

  • ADU Total Size - 800 Square Feet
  • Primary Dwelling Unit Size - 1,600 Square Feet
  • Percent Proportion of ADU to PDU - 800/1600 = 50%
  • Development Impact Fee Total - 50% of $75,274.42 = $37,637.21

School District Fees for ADUs

All building permits for ADUs that exceed 500 square feet of new floor area are required to pay school developer fees to both the San Mateo Union High School District and the Millbrae Elementary School District as permitted by Government Code Section 53080. When the permit is ready to issue, the contractor or the owner will be given a copy of the completed application to deliver to the school district. They must return to the Building Division with receipts from both school districts before a permit can be issued.

Additional ADU & Housing Information

ADUs showing attached, detached, interior, and garage conversion