Secondary dwelling units

A secondary dwelling unit (SDU) is a dwelling unit on a residential lot which provides complete independent living facilities for one or more persons, and includes permanent provisions for living, sleeping, eating, cooking, and sanitation independent of the main dwelling. SDUs are referred to as an “accessory dwelling units” (ADUs) in State law, and sometimes called a “in-law unit”. SDUs have been the focus of recent State legislation, as they can provide needed housing without materially changing the visual character of a neighborhood. The Planning and Building Divisions are committed to making the SDU review process as predictable and streamlined as possible for our customers.

Requirements and guidelines


New SDUs or conversions of existing buildings into SDUs are required to meet a number of standards and guidelines, such as: minimum lot size, maximum unit size, and aesthetic similarity to the main residence. Please review the Secondary Dwelling Unit FAQ in full prior to preparing a SDU application. The SDU section of the Zoning Ordinance is also available for review.

In addition to all of the SDU-specific regulations, the overall parcel has to comply with the total floor area and building coverage limits. As a result, applicants will need to know the size of any existing structures.

Review process


SDU applications that meet all Zoning Ordinance requirements may be reviewed and approved by staff. Prior to starting such an application, staff recommends coming in to the counter (see hours at top right of this page) to discuss the SDU regulations and review the application components as listed to the right in the "Related documents" section. In addition to those forms, project plans (including a site plan, floor plans, floor area and building coverage diagrams, and elevations) are required with SDU applications.

Following the Planning Division's preliminary review, applicants who are proposing fully-compliant SDUs will coordinate with the Building Division to confirm the items required for submittal of a building permit application. In general, the following guidelines are applicable to SDU building permits, depending on the specific proposal:
Please note that new SDUs require a number of standard fees, including the TIF (Transportation Impact Fee) and school fees charged by the local school district. The Building Division can provide detailed fee quotes prior to an application.

SDU applications that do not meet all Zoning Ordinance requirements may be reviewed and acted on with a Use Permit by the Planning Commission (with the exception of the density and subdivision requirements, which may not be modified). In particular, please note that a use permit is required to develop an SDU on a lot less than 6,000 sf in size, and/or a unit that is dissimilar in architectural style from the main residence. If you do not know your lot's area, please consult the City's interactive GIS map for an approximate lot size. Please contact the Planning Division if you are considering any such application for discretionary review. 

Common issues


Please review the following list of common application issues, in order to ensure timely review and action:
  • The forms listed in the "Related links" section above and to the right are required elements. Failure to submit these forms, or failure to fully complete them (e.g., not providing responses to the Historic Evaluation Form questions), will result in an application being designated incomplete.
  • SDUs are required to have colors, materials, textures and architecture similar to the main dwelling. Pre-fabricated and/or modular units, or those that are delivered to the building site constructed, may require a use permit to authorize a unit if the style is not similar to that of the main residence. Please make sure your SDU application clearly relays how this requirement has been met, and provide supplemental photographs and narrative descriptions if needed.
  • Because the tenancy requirement allows only one of the units on a lot to be rented out, rental properties where the owner does not live at the property will not be eligible for a secondary dwelling unit. Applicants for new SDUs must provide a signed tenancy acknowledgement form stating they recognize the requirement and will comply. (Short-term exceptions to the tenancy requirement can be granted by the Community Development Director, provided certain criteria have been met, and other tenancy exceptions can be made through a use permit. See a planner if you are thinking of applying for either type of tenancy exception.)
  • Legally-built existing accessory structures and buildings may be converted to a SDU (even when SDU setback, height, or daylight plane requirements are not met), provided the structure/building was built before January 1, 2017. To document the legality of the structure, please visit the County of San Mateo Assessor to obtain a "property assessment record" for your property.

Staff reports


The following represent recent Commission/Council staff reports on the topic of SDUs: