Each jurisdiction has issued their own interpretation and procedures to apply the state mandated regulation SB9. Here is what you need to know about building more units per SB9 in San Mateo City

At CityStructure, our vision is a world where every homeowner has a clear understanding of the development potential of their property. This is why we give you free access to the Untapped Development Potential for any property.

In San Mateo City, SB 9 became effective on January 1, 2022 and allows for the ministerial (staff-level) approval of certain housing development projects containing up to two (2) residential units (i.e. duplexes) on single-family zoned parcels.  

SB 9 also allows for the ministerial approval (staff level) of lot splits in single-family zoned areas and the creation of up to two (2) residential units on each newly created lots.

Qualifing Criteria for SB9:

  1. The property is zoned single-family residential (i.e. R1-A, R1-B, or R1-C).
  2. The project does not require the demolition or alteration any housing unit that has been occupied by a tenant within the last three 3 years.
  3. The project site cannot have been withdrawn from the rent controlled rental market within the past fifteen 15 years (subject to the Ellis Act).
  4. The project does not require the demolition of more than 25% of the existing exterior walls of a structure.
  5. Rental of any unit created under SB 9 must be for a term longer than 30 days (i.e. no short-term rentals).
  6. The property, if located in a very high fire hazard zone, earthquake fault zone, floodplain, floodway, and/or a site with hazardous materials, meets certain conditions specified in state law.

Development Standards for SB9

Existing and statutory objective development standards are enforced, such that objective standards do not preclude the construction of up to 2 units of at least 800 square-feet (sq. ft.) each in floor area.

Building Setbacks per SB9

a. Minimum 4-foot rear and side yard setbacks; or
b. No setbacks are required for existing legal structures or structures constructed in the same location and dimensions of an existing legal structure.

Required Parking per SB9

a. One off-street parking space per unit is required; or
b. No off-street parking is required for projects located:
c. Within a ½ mile walking distance of a high-quality transit corridor or a major transit stop or;
d. Within one block of a car share.The Hillsdale, Hayward Park, and San Mateo Caltrain stations are considered major transit stops. El Camino Real is considered a high-quality transit corridor.

Unit Type per SB9

Units may be attached or detached, provided that the structure(s) meet building code safety requirements and are sufficient to allow separate conveyance.

Can I AirBnB the new unit per SB9

No.Establishment of short-term rentals are prohibited.

Urban Lot Splits regulation per SB9 in San Mateo City

Urban Lot splits carried out under SB 9 must meet the same criteria as listed above, in addition to the following:

Parcel Size

a. Minimum 1,200 sq. ft. parcel size (i.e., parcel being split must be at least 2,400 sq. ft. in size); and
b. Each new parcel may be no smaller than 40% of the original parcel’s size
c. Where a parcel does not front on a public right of way, the options are to allow a flag lot or to provide access to the public right-of-way via an easement through the other lot.


Newly created parcels shall have access to, provide access to, or adjoin the public right-of-way.


Each parcel is limited to residential use.


Correction of existing legal nonconformities will not be imposed as a condition of approval for any urban lot split.

Sequential Lot Splits.

Sequential splitting of a parcel established through a prior SB 9 urban lot split is prohibited.


The creation of new accessory dwelling units (ADUs) or Junior ADUs (JADUs) may be restricted on parcels created under the provisions of SB 9.


Owners must sign an affidavit that they intend to occupy one of the housing units as their principal residence for a minimum of three (3) years from the date of the lot split approval.

How much could you build on your property?

Get an instant free estimate if there is any untapped potential for your property based on the city zoning and state regulations.