Zoning, building permits and septic rules in Millbrae, California
Millbrae is a city in San Mateo County, California. Millbrae adopts its own zoning, so inside its limits Millbrae’s rules apply, not the county’s. 7 rules below are quoted from official sources and independently re-checked, most recently on October 22, 2026. Rules for unincorporated land are on the San Mateo County page.
Answers from the rulebook for Millbrae
Millbrae’s own rules
Local zoningPresent (with conditions)
Land inside Millbrae city limits. Millbrae city adopts and administers its own zoning ordinance (MMC Title 10, Chapter 10.05, Zoning).
Use and dimensional rules come from the district each parcel is in; this claim only establishes that the city has its own zoning ordinance.
Districts are listed in MMC 10.05.0300: R-1LL, R-1, R-2, R-3, C, DTECRSP, I (and a combining district under 10.05.0310).
“the city is divided into zoning districts which are designated as follows:”
Zoning districtsPublished (with conditions)
Zoning districts inside Millbrae city limits
District boundaries are shown on the Zoning Map incorporated by MMC 10.05.0320; the zoning map itself was not opened.
District standards for R-1 (single-family residential) and R-2/R-3 were not opened.
“single-family large lot or "r-1ll" district; b. single-family residential or "r-1" district; c. duplex/triplex residential or "r-2" district; d. multifamily residential or "r-3" district;”
Building heightSpecified
Structures in the single-family large lot (R-1LL) district inside Millbrae city (hillside large-lot zone, MMC 10.05.0500 et seq.)
If the uppermost five feet is not a roof sloping toward an adjacent street at a minimum of three inches per foot, the maximum height is twenty-five feet.
Chimneys, silos, cupolas, flagpoles, towers, church steeples and similar appurtenances may exceed the limit only upon a height exception, with front, side and rear setbacks increased one foot for each additional foot of height (MMC R-1LL height subsection 2).
Other structures exceeding the maximum allowable height are addressed in the subsection that follows (not opened).
maximum building height: <= 30 feet (25 feet where the roof slope toward an adjacent street is less than three inches per foot)
“maximum height of structures shall be thirty feet, provided the uppermost five feet shall consist of a roof which slopes toward any adjacent street at a minimum slope of three inches per foot. if the slope is less than three inches per foot, the maximum height shall be twenty-five feet.”
Accessory dwellingsAllowed
Accessory dwelling units (ADU) and junior ADUs on lots zoned for single-family or multifamily residential use inside Millbrae city
Limited to one ADU per lot on lots zoned for single-family residential use (MMC 10.05.3240(b)(1)).
An ADU cannot be constructed on any easement on the lot (MMC 10.05.3230(e)).
No ADU is permitted if the public works department finds inadequate water or sewer service to the property (MMC 10.05.3260(a)).
An ADU may be required to have a new or separate utility connection, including a separate sewer lateral, and a proportionate connection fee may apply (MMC 10.05.3260(b)).
Occupancy of an ADU is not allowed until the city approves occupancy of the single-family dwelling (MMC 10.05.3220(b)).
Where this article conflicts with state law, state law controls (MMC 10.05.3210(b)); state ADU statute was not separately checked.
“accessory dwelling units are permitted uses in all areas zoned to allow single-family or multifamily dwelling residential use.”
Mandatory utility hookupConditional
Accessory dwelling units inside Millbrae city (not the primary single-family home)
Permitted only if the public works department finds adequate water and sewer service to the property.
A separate utility connection or separate sewer lateral may be required, with a proportionate connection fee or capacity charge (MMC 10.05.3260(b)).
“no accessory dwelling unit shall be permitted if the public works department determines that there is a lack of adequate water or sewer service to the property.”
Short-term rentalsLimited
Junior accessory dwelling units (JADU) inside Millbrae city only; the city's general short-term rental rules were not checked
JADUs may not be rented for fewer than thirty consecutive calendar days (MMC 10.05.3240(a)(5)).
The city's general short-term rental ordinance is MMC chapter 7.30 (not opened).
minimum rental term for JADU: >= 30 consecutive calendar days
“junior accessory dwelling units may not be rented for fewer than thirty consecutive calendar days. junior accessory dwelling units may not be used as short-term residential rentals pursuant to the short-term residential rental ordinance in chapter 7.30 mmc.”
Permit processPartial
Accessory dwelling unit and junior ADU permit applications inside Millbrae city only; general building permit process not opened
Permit is ministerial, and incomplete applications are returned with an explanation of required information (MMC 10.05.3220(a)).
If submitted with a new single-family dwelling, the ADU application is not acted on until the single-family application is approved (MMC 10.05.3220(b)).
Application fee and city building inspection and permit fees apply (MMC 10.05.3220(c)-(d)).
ministerial review period: <= 60 days (from receipt of a complete application)
“shall be ministerially considered and approved, without discretionary review or a hearing, within sixty days of receipt of a complete application that meets the requirements of this article”
Still unanswered
We looked and could not find an official answer yet. Silence is not permission; ask Millbrae or San Mateo County directly.
Building permit — New construction permits inside Millbrae city
Owner-builder exemption — Owner-builder permit exemption inside Millbrae city
Building code adopted — Building codes for construction inside Millbrae city
Inspections — Building inspections inside Millbrae city
Certificate of occupancy — Certificate of occupancy for new dwellings inside Millbrae city
18 more unanswered
Minimum dwelling size
Minimum lot size
Setbacks
Lot coverage
Living in an RV
Camping on your land
Tiny homes
Manufactured homes
Who permits septic
Septic permit
Septic inspection
Off-grid sanitation
Well permit
Agricultural exemption
Subdividing land
Driveway access
Building in a floodplain
Permit fees
Each answer was extracted from an official source and independently re-checked against that source by a second reviewer. That is an evidence check, not legal advice. Rules change; confirm with the authority before you buy or build.
Unanswered questions are usually a sign that Millbrae publishes no rule on the point, not that none exists. San Mateo County’s page lists the county offices to call.
Off-grid gear
Going off-grid in Millbrae means bringing your own water and power. The kit we’d start with:
