Cities | San Mateo

San Mateo, California Multiplex Rules 2026: Zoning, Lot Sizes, ADUs, SB 9, Multiple-Family Homes, Measure T Height Limits and Fees

This guide covers the City of San Mateo, California, a city of 103,337 people in San Mateo County. San Mateo County has a separate code for land outside city limits, and this guide does not use it. The City's zoning rules are Title 27 of the San Mateo Municipal Code, which the City publishes in its own online law library. Three facts matter for a project here. A voter measure, Measure T, sets the top heights and densities through a table in the General Plan, and that table overrides the older zoning numbers. The ADU chapter was rewritten in February 2026, and the SB 9 chapter dates from 2022 and is narrower than the current state statute in four places. And the Below Market Rate program starts at five homes. The seven guide pages below cover each topic in depth.

San Mateo Guide Pages

Start With State Law

Five state laws set the minimum San Mateo must allow. SB 9 (2021) allows two main homes on a lot in a single-family zone, a split into two lots, or both, approved by staff with no hearing. ADU law sets the accessory dwelling units (ADUs), second homes on a lot, and junior ADUs a city must allow; a junior ADU is a unit of 500 square feet or less inside a house. SB 684 and SB 1123 allow maps of up to 10 lots with 10 homes. The state density bonus gives extra homes to projects that include affordable homes. And SB 79, the 2025 law on housing near transit, applies here: HCD, the state housing department, says in its advisory that the law "will take effect on July 1, 2026, in Alameda, Los Angeles, Sacramento, San Francisco, San Mateo, Santa Clara, and San Diego Counties."

California cities are either general law cities, which follow the state's general rules for city government, or charter cities, which have a charter approved by their voters. San Mateo is a charter city. Its City Charter gives the City "all powers necessary or appropriate to a municipal corporation and the general welfare of its inhabitants which are not prohibited by the constitution," and the transportation fee chapter cites "the Charter City authority provided by the Constitution of the State of California." The state housing laws above apply to charter cities by their own words: Government Code 65852.21 and 66411.7 define a local agency as a city "whether general law or chartered," and the ADU statutes use the same words.

The City has written three of these laws into its code: ADUs in Chapter 27.19, SB 9 two-unit projects and urban lot splits in Chapter 27.21, and the density bonus in Chapter 27.15. A search of Titles 26 and 27 found no mention of the small-lot map statute or of SB 79. On SB 79 the City's own FAQs say "The City is not implementing SB 79 in the Zoning Code at this time."

San Mateo in Numbers

The US Census Bureau's Vintage 2025 city file estimates 103,337 people in the City of San Mateo on July 1, 2025. The 2020 estimates base is 105,669, so the estimate is 2,332 lower, a change of 2.2 percent (VanPlex arithmetic), and the 2024 estimate is 102,856. In that file San Mateo is the 70th largest city in California, after Vacaville (103,850) and ahead of Burbank (102,988). San Mateo County has 743,568 people, so the City holds 13.90 percent of them (VanPlex arithmetic: 103,337 divided by 743,568).

The City's housing plan gives the scale of what is planned. The recitals, the opening statements, of Ordinance No. 2025-04 say the Housing Element "plans for a total of 10,286 units, over and above the RHNA allocation of 7,015 units for the eight-year planning period of 2023-2031." RHNA is the regional housing needs allocation, the number of homes the state assigns a city to plan for. The City's Housing Element page says the element "was adopted by the City Council in May 2024 and was certified by the California Department of Housing and Community Development in July 2024." This guide did not read the Housing Element itself, so no permit count is given here.

The Ways to More Homes Compared

Sources: San Mateo Zoning Code Chapter 27.19, Chapter 27.21, Chapter 27.22, 27.06.020 and Ordinance No. 2024-4; City of San Mateo SB 79 FAQs; Gov. Code 65915 and 66499.41.

Zones That Allow Homes

Title 27 has five base residential districts. R1, split into R1-A, R1-B and R1-C, permits "One-family detached dwellings." R2 permits two-family dwellings. R3, R4 and R5 permit multiple-family dwellings, which the code defines as buildings "for occupancy by three or more families," and set the home count by parcel area per home: 2,500 square feet per home on a 5,000 square foot parcel, down to 1,250 in R3 and 870 in R4 and R5 on a parcel of 15,000 square feet. Three downtown districts, R4-D, R5-D and R6-D, have a floor area ratio (building floor area divided by lot area) of 3.0. Homes are also permitted in the C1, C2, C3, CBD, E1 and E2 districts. Above all of these, Section 27.02.220 says the densities and heights in Table LU-1 of the General Plan "shall control." See the zoning districts page.

Lot Sizes and Dividing Land

A new house parcel needs 10,000 square feet in R1-A, 6,000 in R1-B and 5,000 in R1-C, with widths of 75, 60 and 50 feet. Title 26 allows a parcel map, the surveyed drawing of the new lots, decided by the Zoning Administrator, for four or fewer lots; other divisions need a tentative map heard by the Planning Commission and a final map approved by the City Council. Every new lot needs 30 feet of street frontage, and "The creation of new panhandle lots shall not be permitted." The City's SB 9 chapter is the exception: an urban lot split may create two parcels where "Each newly created parcel shall be at least 1,200 square feet in gross area," reached by a strip or easement 10 feet wide. See the lot size and subdivisions page.

House Lots: ADUs and SB 9

Chapter 27.19 was rewritten by Ordinance No. 2026-02, adopted February 2, 2026, after a state housing department letter of October 18, 2024. A house lot "may include any combination of the following three types of ADUs": one attached or conversion ADU, one detached new ADU and one junior ADU. "Up to 800 square feet of ADU living area" is exempt from the lot's floor area ratio, a detached ADU may be 18 feet to the top plate and 24 feet to the roof peak, new ADUs need 4 feet at the side and rear, and "No off-street parking shall be required for any ADU or JADU." A lot with an existing multifamily building may add up to eight detached ADUs.

Chapter 27.21 places all R1 land in a two-unit overlay that allows two primary homes and "A maximum of four units" with ADUs. It was adopted in 2022, and the statute was last amended effective June 30, 2026. The City text sends a home over 800 square feet to discretionary design review and allows denial for an impact on the physical environment; the statute requires a staff decision with no hearing and names public health and safety. Ask the City which rules it applies today. See the ADUs and SB 9 page.

Three or More Homes

Every apartment building needs site plan and architectural review. Section 27.06.020 lets the Zoning Administrator, a staff position, decide projects of "Up to ninety-nine (99) residential dwelling units that meet objective design standards" and up to five homes that do not. Larger projects go to the Planning Commission. The Below Market Rate Program applies to "all development projects that include five (5) or more residential units or parcels": projects of 5 to 10 homes pay a fee, and projects of 11 or more set aside 15 percent of the homes. On the 39 Housing Opportunities Overlay sites, a project with 20 percent lower income homes is approved by right. The City's SB 79 FAQs apply that state law within one-half mile of the Downtown, Hayward Park and Hillsdale Caltrain stations. See the three or more homes page.

Parking, Setbacks and Height

An R1-B or R1-C house needs a 15-foot front yard, 20 feet to a garage, 5-foot side yards and a 15-foot rear yard that becomes 25 feet above a single story. Height is 24 feet to the plate line, the girder line that supports the roof framing, and 32 feet to the roof, under a daylight plane that starts 12 feet high at each side property line and slopes in at 45 degrees. The floor area ratio is 0.5 on the first 6,000 square feet of an R1-B or R1-C parcel. Section 27.64.160 asks a house under 3,000 square feet for "2 garage spaces" and a multiple-family home for 1.5 to 2.2 spaces by bedroom count. An R3 building needs 15 feet at the front and rear and 6 feet at the sides for one and two stories. See the parking, setbacks and height page.

Measure T and the Voter Height Limits

San Mateo voters have set the top building heights and densities by ballot measure since 1991. The last of the older measures, Measure Y, limited the city to 55 feet and 50 units per acre. On November 5, 2024 voters approved Measure T "by a vote of 59% (25,271 votes) to 41% (17,283 votes)." It repealed Measure Y, kept the old limits outside ten study areas, and made Table LU-1 of General Plan 2040 the limit inside them: up to 5 to 8 stories and 100 to 130 homes per acre in the Residential High and Mixed-Use High designations. Until January 1, 2031, "voter approval shall be required to modify the density ranges, intensity or FAR, or building heights" in the table. See the Measure T and height limits page.

Permits and Fees

San Mateo has three approval bodies: the Zoning Administrator, the Planning Commission and the City Council. The Zoning Administrator decides within 21 days of a complete application, decisions take effect after 10 calendar days unless appealed, and an approval "shall expire two years from the date of approval." A project of up to 99 homes that meets the design standards is in Tier III of the City's application guide, which requires a neighborhood meeting before filing and notice within 500 feet. The fee schedule for July 1, 2026 to June 30, 2027 sets a $6,000 initial deposit for a multi-family Zoning Administrator application and a $580 appeal fee. The Development Fees summary lists a park fee of $29,461 per home in a 2 to 4 unit building, a transportation fee of $3,706.73 and a childcare fee of $3,373.46 per multi-family home, and a wastewater charge by bedroom count. See the permits and fees page.

How Current the Code Is

The City's law library asks readers not to load it page by page with scripts: "Please do not scrape. Instead, bulk download the HTML or XML." This guide followed that request. It read one bulk copy of the code and tested ten section addresses on the public site, which all loaded.

Source: City of San Mateo, Law Library, Municipal Code index and bulk download copy, read October 11, 2026.

The law library also holds the text of each ordinance since 2010. These are the ones that shape the housing rules on these pages.

Sources: City of San Mateo ordinances 2023-12, 2024-4, 2024-10, 2025-03, 2025-04, 2025-05, 2025-11, 2025-12, 2026-02, 2026-06, 2026-07 and 2026-09, each from its Legislative History section; City Council agenda of September 22, 2026.

Recent City Council Actions

• December 1, 2025. The Council adopted Ordinance No. 2025-11, which added the Residential Tenant Protection Program as Chapter 10.100, and Ordinance No. 2025-12, which amended the tenant relocation chapter.
• February 2, 2026. The Council adopted Ordinance No. 2026-02 on ADUs, with five votes in favor and none against.
• May 18, 2026. The Council adopted Ordinance No. 2026-06, a new Historic Preservation chapter that requires a Certificate of Appropriateness, an approval document, for work on listed Landmarks and district Contributors, and Ordinance No. 2026-07, which created a Historic Resources Commission.
• September 8, 2026. The Council adopted Ordinance No. 2026-09 and, according to the report for the next meeting, "introduced an ordinance amending Title 27" to create four high density districts. At that hearing "The Council also directed staff to remove floor area ratio (FAR) requirements from the R6 and R7 residential districts."
• September 22, 2026. The agenda listed that ordinance for adoption on the consent calendar, the list of routine items the Council passes with one motion. The minutes were not posted when this guide was written.
• October 5, 2026. The agenda listed a resolution on a Below Market Rate alternative plan for a 441-unit project at 1700 S. El Camino Real. It listed no ordinance on zoning, ADUs or SB 9.

Adoption dates and votes above come from the Legislative History section at the end of each ordinance. The items for September 22 and October 5, 2026 come from agendas, which show what was proposed.

Where the Code, City Documents and State Law Differ

The guide pages flag each difference where it arises. The main ones:

• The R3 to R5 density tables stop at one home per 870 square feet of parcel, 50 homes per acre. Table LU-1 allows up to 130, and Section 27.02.220 says the table controls.
• Sections 27.06.040 and 27.08.030 still send buildings over 55 feet to the City Council. Section 27.06.050 says buildings that exceed the General Plan height limit. A staff report of September 2026 calls the 55-foot text inconsistent.
• Section 27.04.130, a definition, now carries the site plan review rules. An ordinance of July 2025 printed them under the wrong section number, and the definition of community care facility was removed from the code.
• The SB 9 chapter sends a home over 800 square feet to discretionary review. Government Code 65852.21 requires a staff decision against written standards, with no hearing.
• The SB 9 chapter allows denial for an impact on the physical environment. The statutes name public health and safety.
• The SB 9 chapter excludes any parcel in a historic district from a lot split. Government Code 66411.7 now bars only work that alters a contributing structure or an exterior structural wall.
• The parking table in Section 27.64.160 lists one space per ADU. Chapter 27.19, amended in 2026, requires none.
• The ADU and SB 9 chapters state no number of days for a decision. The statutes give 60 days.
• The Developer Resources page says the Below Market Rate program applies at 6 or more units. The adopted guidelines and the fee schedule start at 5.
• Title 26 bans new panhandle lots and asks for 30 feet of frontage. The urban lot split section allows a flag lot on a 10-foot strip and says its standards prevail.
• Table LU-1 prints 51 to 99 homes per acre for Mixed-Use Medium II. Table 5 of Measure T prints 51 to 90 for the same designation.
• The Planning Application Guide puts a tentative map without exceptions with the Zoning Administrator. Chapter 26.48 has the Planning Commission act on tentative maps.

Limits of This Guide

The Municipal Code, Charter and ordinances were read from the City's bulk copy, whose latest update is dated September 28, 2026 and whose code index prints "Current through" October 8, 2026. Titles 1, 2, 3, 5, 7, 10, 11, 13, 15, 17, 23, 24, 25, 26 and 27 were converted to text and searched. Title 8 (Animals and Fowl) was left out. Three City Council agendas were read, for September 8, September 22 and October 5, 2026, with two agenda reports and one draft ordinance. Earlier agendas, all minutes and all Planning Commission agendas were not read, so this guide cannot say that the ordinance on the September 22 agenda was adopted. The Council may have acted on a housing rule after October 5, 2026.

These documents were not read: the zoning map and the General Plan land use map, the attachments to Measure T, the Housing Element, the Downtown Specific Plan, the Rail Corridor Plan, the Bay Meadows Specific Plan and the Hillsdale Station Area Plan, the objective design standards, the interim design standards for SB 9 homes, the regional SB 79 map, the 1989 Historic Building Survey, the building permit fee tables, and school district fee schedules. Ask the Planning Division which standards and fees apply to your lot and whether a newer ordinance is in force.

Where to Go Next

Related on VanPlex: the California SB 9 page, the California impact fees page and the San Jose guide.

Frequently Asked Questions

Official sources (checked October 2026)