San Mateo | ADUs and SB 9
San Mateo, California ADUs and SB 9: Units Per Lot, Size, Height, Setbacks, Parking, Fees, Junior ADUs and Two-Unit Projects
An accessory dwelling unit, or ADU, is a second home on a lot that has or will have a main home. A junior ADU, or JADU, is a unit of no more than 500 square feet inside a house. A setback is the distance a building must keep from a lot line. San Mateo's rules for both are Chapter 27.19 of the Zoning Code, which the City Council rewrote in February 2026. Its rules for SB 9 projects, two main homes on a house lot, are Chapter 27.21, written in 2022. This page sets out both chapters and marks where each differs from the state statute. The San Mateo overview has the summary.
San Mateo ADUs and SB 9 in six facts
- A San Mateo lot with a house may have one attached or conversion ADU, one detached new ADU and one junior ADU, each approved with a ministerial permit. Source: Zoning Code 27.19.060
- The City Council adopted Ordinance No. 2026-02 on February 2, 2026, rewriting the ADU chapter after a state housing department letter of October 18, 2024. Source: Ordinance No. 2026-02
- Up to 800 square feet of ADU living area is exempt from the floor area ratio of a lot with a house, and no off-street parking is required for any ADU or junior ADU. Source: Zoning Code 27.19.060
- A detached ADU may be 18 feet high to the top plate and 24 feet to the roof peak, and new ADUs need 4 feet from side and rear lot lines. Source: Zoning Code 27.19.060
- A lot with an existing multifamily building may add up to eight detached ADUs, capped at the number of homes already on the lot. Source: Zoning Code 27.19.070
- All R1-A, R1-B and R1-C land is in a two-unit overlay district that allows two primary homes on a parcel and up to four units in all. Source: Zoning Code 27.21.060
How the ADU Chapter Got Its Current Text
The recitals of Ordinance No. 2026-02, the opening statements of the ordinance, tell the history. The earlier chapter was "adopted on March 21, 2022," and "on October 18, 2024, the City received a letter from the California Housing and Community Development Department outlining provisions in the City of San Mateo Zoning Ordinance that were inconsistent with State Accessory Dwelling Unit regulations." The Planning Commission heard the new text on December 9, 2025. The ordinance "was introduced on January 26, 2026, and adopted on February 2, 2026," with five votes in favor and none against. The City's single family ADU handout, updated October 7, 2026, says the chapter "was updated effective March 4, 2026."
The chapter sorts ADUs with defined terms. A "Conversion ADU" is made from an existing structure. A "New Construction ADU" is any other. A "State Exempt ADU" is one "created pursuant to Government Code Section 66323," the state list of units a city must approve, and a "Nonexempt ADU" is one that falls outside that list. The City's design standards apply to Nonexempt ADUs only.
ADUs on a Lot With a House
| Topic | Rule in Section 27.19.060 |
|---|---|
| Units on a house lot | One attached or conversion ADU, one detached new construction ADU and one junior ADU |
| Size | No fixed maximum; the first 800 sq ft is outside the floor area ratio, and a larger ADU uses the floor area left on the lot. An ADU may not be larger than the main home, but a State Exempt ADU of up to 800 sq ft is allowed when the main home is 800 sq ft or less |
| Conversion ADU | Not subject to size requirements; may grow up to 150 sq ft beyond the existing footprint for entry and exit |
| Height, attached | 24 ft to the top plate line and 32 ft to the roof peak |
| Height, detached | 18 ft to the top plate line and 24 ft to the roof peak |
| Side and rear setbacks | 4 ft for new construction; none for a converted garage or accessory structure rebuilt in the same place, or for a detached ADU in the rear one-third of the lot that meets the accessory building rules |
| Front setback | The setback of the zoning district; none for a State Exempt ADU |
| Parking | None required; a converted or demolished garage need not be replaced |
| Rental and sale | No rental for less than 30 consecutive calendar days; no separate sale except as Government Code 66341 allows |
Source: San Mateo Zoning Code, 27.19.030 and 27.19.060, as amended by Ord. No. 2026-02. Heights are measured from existing grade. The code defines the plateline as "the line established by the horizontal girder which supports the trusses or rafters of a roof."
- The three units. A lot "may include any combination of the following three types of ADUs, provided that each meets the applicable development standards": "One Attached or Conversion ADU," "One Detached New Construction ADU," and "One JADU, provided it is located within the walls of an existing or proposed Single-Family Dwelling Structure."
- Floor area. "Up to 800 square feet of ADU living area, including New Construction State Exempt ADUs, is exempt from the maximum FAR for each lot with a Single-Family Dwelling Structure. This exemption does not apply to JADU living area." FAR, the floor area ratio, is building floor area divided by lot area; the R1 limits are on the zoning districts page.
- What counts as living area. The chapter applies "only to ADU Living Area." A garage, carport or shed attached to an ADU follows the rules of the zoning district.
- The rear third of the lot. A detached ADU needs no side or rear setback when it "is proposed in the rear one-third of a lot" and "otherwise meets all requirements of accessory buildings and structures in required yards of R1 districts." Chapter 27.70 limits such a building in R1 to 9 feet to the plate line and 16 feet to the roof peak, on one habitable floor.
- Entrance. "An ADU must include a separate exterior entrance from the primary residential dwelling unit."
- Address. The building permit application "must include application for a separate address assignment."
Design Rules for Nonexempt ADUs
A Nonexempt ADU is checked against five written design standards. Staff applies them with no hearing.
- Daylight plane. The ADU must "meet the daylight plane requirements of the R-1 zoning district as applied to the nearest adjacent side and rear lot lines." In R1 that is a plane that starts 12 feet above each side property line and slopes into the lot at 45 degrees.
- Windows near a lot line. "Provide and maintain obscured glazing on second-story windows up to five feet from the finished floor when the windows are located within five feet of a lot line with an abutting residential lot."
- Outside stairs. An exterior staircase "shall have a setback of at least five feet from the nearest lot line."
- Roof decks. "Rooftop decks or terraces above any portion of an ADU are prohibited."
- Second-story decks. They "shall not be larger than 50 square feet in size," must stay out of the zoning district's setbacks, and "shall not face adjacent residential properties."
An owner whose design misses a standard may use Section 27.19.045, a discretionary review by the Zoning Administrator, in which the reviewer uses judgment and may approve or deny. The section lists five findings, among them that "there are exceptional circumstances applicable to the property that would make compliance with the standard(s) an undue hardship." This review "shall not be used to increase the maximum allowable floor area of a lot." The decision "may be appealed to the City Manager or designee, which may be a hearing officer."
Parking
The 2026 text is short: "No off-street parking shall be required for any ADU or JADU." The main home keeps its own parking requirement, "except when an existing garage, carport, covered parking structure, or uncovered parking space is converted to an ADU or JADU or demolished in conjunction with construction of an ADU. When this occurs, no replacement off-street parking is required for the primary residential dwelling unit."
Parking may be added by choice. "A maximum of up to two voluntarily provided parking stalls may be provided for each ADU or JADU," the driveway of a single-car garage may widen to 20 feet, and uncovered ADU parking "may be located in the required front, side, and rear yard setback areas." A related change came in September 2026: Ordinance No. 2026-09 amended the driveway chapter so that a driveway need not be removed and the curb restored when a garage is converted to an ADU.
ADUs on Multifamily Lots
Section 27.19.070 covers a lot with an existing or proposed "Multifamily Dwelling Structure," which the chapter defines as "a structure with two or more attached primary residential dwelling units." An attached duplex therefore follows these rules.
- Detached units, existing building. "Up to eight New Construction Detached ADUs may be allowed, provided that the number of ADUs shall not exceed the number of existing primary residential dwelling units on the lot."
- Converted space. ADUs made from non-living space inside the building "may be at least one, and not more than 25 percent of the number of existing primary residential dwelling units."
- Proposed building. A lot with a proposed multifamily building may have two new detached ADUs.
- Size. An ADU "shall not have a larger living area than the smallest primary residential dwelling unit on a lot," with a State Exempt ADU of up to 800 square feet always allowed. The 800 square foot floor area exemption "shall be applied individually to each new Detached ADU."
- Height and setbacks. 18 feet to the top plate line and 24 feet to the roof peak, with "a minimum setback of four feet from side and rear property lines."
- Separation. New detached ADUs sit at least four feet from the main building and "may be attached to each other."
Junior ADUs
- Where. One per lot with a single-family home. It is built "within the walls of the proposed or legally existing Single-Family Dwelling Structure, including an attached garage." Detached garages and other detached structures "are not permitted to be converted into a JADU."
- Size. "The living area of a JADU shall not exceed 500 square feet."
- Kitchen. An efficiency kitchen with a "Cooking facility with appliances" and a "Food preparation counter and storage with cabinets that are of reasonable size in relation to the size of the JADU."
- Bathroom and doors. A junior ADU may share a bathroom with the house. When it does, or when it shares utility controls such as a thermostat, "an interior entry access between the JADU and the Single-Family Dwelling Structure is required." It always has its own outside entrance.
- Owner occupancy. "If the JADU has shared sanitation facilities with the existing structure, owner-occupancy shall be required." The same subsection waives it "if the JADU has its own sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization." This matches Government Code 66333 as amended effective January 1, 2026.
- Deed restriction. Before the building permit, a deed restriction, a limit recorded against the property title, is recorded stating that "Sale of the JADU separately from the single-family residence is prohibited" and giving its approved size.
- No parking and no new utility connection. "No additional off-street parking is required for a JADU," and separate water, sewer and power connections are not required.
The Permit and the Fees
Section 27.19.040 says "If the ADU is in full compliance with the provisions of this Chapter, a ministerial, non-discretionary permit will be issued." Ministerial means staff checks the plans against written rules, with no hearing. The City's ADU page adds that "A planning application with public notification is not required for applications that meet the objective development standards of the ADU Ordinance." The page lists the Planning Division at (650) 522-7212 and the Building Division at (650) 522-7172.
| Fee | Amount, July 1, 2026 to June 30, 2027 |
|---|---|
| ADU permit, new construction | $2,831.57 each |
| ADU permit, remodel | $2,031.12 each |
| ADU permit, eligible manufactured, pre-fabricated or pre-approved unit | $1,455.43 each |
| Junior ADU permit | $1,454.02 each |
| Wastewater capacity charge, ADU | $5.87 per square foot |
| Development impact fees, ADU of 750 sq ft or smaller | None |
Sources: City of San Mateo, Adopted Comprehensive Fee Schedule 2026-2027 and Development Fees summary; Zoning Code 27.19.050. Plan review, school and other agency fees are not in this table.
- Impact fees. Impact fees are charges on new construction that pay for public facilities. "No impact fees shall be charged for ADUs 750 square feet in size or smaller. Any impact fees charged for an ADU over 750 square feet shall be charged proportionately in relation to the square footage of the primary residential dwelling unit." This follows Government Code 66311.5.
- Park fee. The park impact fee in Section 13.05.070 does not apply to "one-family detached dwellings or secondary units allowed under Chapter 27.19."
- Utilities. "For Conversion ADUs, a separate water connection, a sewer service connection, or power connection is not required." For a new detached ADU "the City will require payment of a connection fee or capacity charge that is proportionate to the ADU's burden."
- School fees. The handout notes that "School District Developer Impact fees may also apply." The school districts set those; this guide did not read their schedules.
Where the ADU Texts Differ
- The parking table. Section 27.64.160 still lists "1 parking space for an accessory dwelling unit with up to 1 bedroom" with five exemptions. Chapter 27.19, amended in 2026, requires none. The newer and more specific chapter is the one the City's handout follows.
- Deadlines. Chapter 27.19 states no number of days. Government Code 66317 requires a completeness notice "not later than 15 business days after the permitting agency received the application" and a decision "within 60 days from the date the permitting agency receives a completed application," after which "the application shall be deemed approved." The statute controls.
- Appeals. Section 27.19.040 says "the issuance of a ministerial ADU or JADU permit may not be appealed," and the chapter has no section on appealing a denial. Government Code 66317 says the agency "shall provide a process for the applicant to appeal that decision in writing." Ask the Planning Division how it handles such an appeal.
- An old term. Section 13.05.070 speaks of "secondary units allowed under Chapter 27.19." Chapter 27.19 itself no longer uses the word secondary.
- A stale web entry. The City's Policy and Zoning Code Update Efforts page still lists the February 2, 2026 adoption hearing under upcoming meetings.
SB 9 Two-Unit Development
SB 9 is the state law that lets an owner build two main homes on a lot in a single-family zone with no hearing. San Mateo carries it out through an overlay district on all R1 land. An overlay district adds a second set of rules on top of the base district. Chapter 27.21 says its purpose is "to allow up to two detached or attached primary housing units on one parcel." The statute is Government Code 65852.21, last amended effective June 30, 2026.
- How many units. "A maximum of four units, with a maximum of two primary dwelling units (attached or detached)." The other two are ADUs or a junior ADU. After an urban lot split the limit is two units of any kind on each new parcel.
- The application. An SB 9 planning application with a title report no more than a year old, a legal description, plans, and "A completed property owner tenant disclosure affidavit."
- Who decides. The application "shall be approved ministerially if the Zoning Administrator or designee makes all the following findings," which are four checks against written standards.
- Size. City standards may not "physically preclude either of the two units from being at least 800 square feet in floor area." Where the lot's floor area is used up, "an SB 9 unit of up to 800 square feet in size with at least 4-foot side and rear yard setbacks and up to 16-feet in height shall be permitted."
- Setbacks. The front setback of the zoning district, and "A minimum setback of four (4) feet shall be required from the side and rear lot lines." No setback applies to an existing legal structure that is converted or rebuilt in the same place.
- Parking. "There shall be a minimum of one standard size off-street parking space (either uncovered or covered) for each SB 9 unit." No space is required within one-half mile walking distance of a major transit stop, such as a rail station, or of a high-quality transit corridor, which the chapter defines as "a corridor with fixed bus route service with service intervals no longer than 15 minutes during peak commute hours," or within one block of a car share facility.
- Sale and rental. Two primary units on one parcel "shall not be sold separately until an urban lot split application has been approved by the City and a final parcel map recorded." A deed restriction bars rentals of less than 30 consecutive days.
- Lots that are excluded. Section 27.21.050 excludes a parcel where the project would demolish or alter rent-restricted housing or "Housing that has been occupied by a tenant within the last three years," a parcel withdrawn from the rental market under the Ellis Act, the state law on leaving the rental business, "within 15 years prior to an SB 9 application submittal," and a parcel "designated an historic district or property."
- Design standards. New primary units "shall comply with the Interim Objective Design Standards as adopted by City Council resolution." This guide did not read that resolution.
| Topic | San Mateo Chapter 27.21 | Government Code 65852.21 |
|---|---|---|
| Review of a home over 800 sq ft | A discretionary design review (SFDDR or SPAR) | The project "shall be considered ministerially, without discretionary review or a hearing" |
| Height of a detached unit | 16 ft to the top plate line and 24 ft to the roof peak | No standards "that do not apply uniformly to development within the underlying zone"; R1 houses may reach 24 ft and 32 ft |
| Demolition of 50 percent or more of the walls or roof | Discretionary review and a special use permit | No wall limit in the current text |
| Grounds to deny | Impact on "public health and safety or the physical environment" | Impact "upon public health and safety" |
| Parking | One space per unit; none within one-half mile walking distance of a major transit stop or high-quality transit corridor, or within one block of a car share facility | The same limits |
| Deadline | None stated in Chapter 27.21 | 60 days from a completed application, then "deemed approved" |
Sources: San Mateo Zoning Code, 27.21.030 and 27.21.070 (Ord. No. 2022-10) and 27.18.050; Gov. Code 65852.21 as amended by Stats. 2026, Ch. 56 (SB 1426).
A city rule that is stricter than the statute allows is void to that extent, and the statute controls. The City chapter was written in 2022, and the statute has been amended since. Four rows of the table show City text that is narrower than the current statute. The first is the one with the most effect on a project: Section 27.21.070 says "SB 9 unit(s) in excess of 800 square feet shall be subject to a Single-Family Dwelling Design Review (SFDDR) or Site Plan and Architectural Review (SPAR) discretionary review process." The statute sets no size above which a hearing or discretionary review returns. Ask the Planning Division which size, height and review rules it applies to an SB 9 home today.
The parking exemption in Section 27.21.070 cites "Public Resources Code Section 2155(b)" for a high-quality transit corridor. The statute cites Section 21155; the City text has a digit missing.
Limits of This Page
Chapters 27.19 and 27.21 were read from the City's bulk copy of the code, whose index page prints "Current through" October 8, 2026. City Council agendas of September 8, September 22 and October 5, 2026 list no ordinance on ADUs or SB 9. The City's reply to the state housing department letter, the multifamily ADU handout, the pre-approved plans guide, the handout on unpermitted ADUs, the interim objective design standards for SB 9 homes and the school district fee schedules were not read. Ask the Planning Division whether a newer ordinance is in force.
Best For
- ✓ A house lot that can fit a detached ADU with 4-foot side and rear setbacks, with 800 square feet outside the floor area limit.
- ✓ A garage conversion, which needs no setback, no replacement parking and no new utility connection.
- ✓ An apartment property with yard space for detached ADUs, up to eight and no more than the homes already there.
Usually Fails When
- ✕ A two-story Nonexempt ADU is drawn with a roof deck or with clear second-story windows within five feet of a lot line.
- ✕ An SB 9 project is planned on a house that a tenant occupied within the last three years.
- ✕ An SB 9 home larger than 800 square feet is scheduled as a staff approval before the City confirms which review applies.
What To Verify Before Spending Money
- → How much floor area is left on the lot once 800 square feet of ADU living area is set aside.
- → Whether the ADU is State Exempt or Nonexempt, which decides the front setback, daylight plane and design rules.
- → The size, height and review rules the City applies to SB 9 homes after the 2026 state amendments.
Where to Go Next
Related on VanPlex: the San Mateo overview, the California ADU law page and the California SB 9 page.
Frequently Asked Questions
How many ADUs can I build on a house lot in San Mateo, California?
How big can an ADU be in San Mateo?
Does San Mateo require parking for an ADU?
How tall can a detached ADU be in San Mateo?
What does an ADU permit cost in San Mateo?
Do I have to live on the property to have a junior ADU in San Mateo?
How many ADUs can an apartment building add in San Mateo?
Can I rent my San Mateo ADU on a short-term rental site?
What if my ADU design does not meet the San Mateo standards?
Can I build two houses on one R1 lot in San Mateo under SB 9?
Does San Mateo give public notice of an SB 9 project?
Official sources (checked October 2026)
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