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All ADU Regulations

San Mateo, California

The City of San Mateo has updated their ADU ordinance to comply with new state laws. Below are the up-to-date requirements for permitting an ADU in San Mateo.
Updated Sep 28, 2023

San Mateo, CA

Like many cities across CA and beyond, ADUs are gaining momentum in San Mateo, CA. The number of ADU permits issued increased more than 80% in the past two years, with more and more homeowners recognizing the value of a backyard home. Whether you’re hoping to add living space for family, make rental income, or something else, an ADU is a great addition to your San Mateo property. Here are the ADU requirements in San Mateo as of 2023.

General ADU requirements

All standard ADU types – detached, attached, and conversion – as well as JADUs are permitted in San Mateo. The specific requirements for your project will depend on the ADU type. ADUs must be equipped for independent living and have permanent facilities for: Living Eating Sleeping Sanitation (i.e. a bathroom) Cooking Exterior access that is separate from the primary dwelling
ADU kitchens must also have: A sink with running water Countertops and cabinetry of a reasonable size A refrigerator A permanent range/cooktop that doesn’t need more than 120V or natural/propane gas JADUs may share bathroom/sanitation facilities with the primary dwelling, but they must have a separate entrance. Both ADUs and JADUs must provide a separate exterior entrance.
As for design requirements, San Diego requires attached ADUs to be in the same architectural style as the primary unit.


All properties zoned for single family or multi-family may be eligible to build ADUs. ADUs are only allowed on lots developed within multi-family apartment buildings, but are not allowed in multi-family buildings that are condominiums or townhouses.


During the course of permitting your ADU project, you can expect to pay ADU permit fees and school impact fees, and potentially additional fees. Permit Fees (as of 2023):
  • ADU (new) - $2,671.39 (per unit/flat fee)
  • ADU (remodel) - $1,916.15 (per unit/flat fee)
  • JADU: - $1,371.72 (per unit/flat fee)
School Impact Fees (as of 2023):
  • ADUs under 750 sq. ft. and JADUs under 500 sq. ft. are exempt from school impact fees.
  • ADUs above 750 sq. ft. and JADUs above 500 sq. ft. must pay school impact fees of $4.08 per sq. ft.

Number of units

The number of ADUs you’re able to build depends on how your lot is zoned:
  • Lots zoned for single family use may add 1 ADU or 1 Jr. ADU (JADU).
  • Lots zoned for multi-family may add ADUs up to 25% of the existing number of units (e.g.: 4 unit building would be allowed 1 ADU), or at least 1 ADU.

Building size

The maximum ADU you can build depends on your zoning and ADU type:
  • JADUs may reach a maximum of 650 sq. ft.
  • The maximum size for attached or detached ADUs is determined by the Floor Area Ratio (FAR). Units up to 800 square feet are exempt from a lot’s FAR. For larger ADUs, the maximum size is set by the FAR of the underlying zoning district.
  • For instances where existing floor area allowance on a lot has been utilized, only a “by right” ADU of up to 800 square feet is permitted.

Building height

Attached ADUs can reach a maximum height of 24 feet to top plateline (the top horizontal line of a building wall upon which the roof rests) and 32 feet to roof peak, as measured from existing grade.
Detached ADUs can reach a maximum height of 16 feet to top plateline and 24 feet to roof peak, as measured from existing grade.
Additionally, ADUs must be within a daylight plane, which starts at a height of 12 feet and extends into the parcel at a 45-degree angle from the nearest side and rear property lines.


Conversion of existing unfinished basement space can be used for an attached ADU.


San Mateo requires minimum setbacks of 4 feet from side and rear property lines, as well as 6 feet from any existing structures for detached ADUs. No additional setback is required if the ADU is constructed within the dimensions of an existing structure (e.g. converting your garage into an attached ADU).
An ADU must meet the front yard setback of the underlying zoning district. For ADUs on through lots, only one street frontage will be considered a front yard. Also, an ADU and associated appurtenances must be located outside of any recorded easements on a property.


There is a minimum of one standard off-street parking space required for each ADU in San Mateo. No parking for an ADU is required if:
  • The ADU is within ½ mile walking distance of public transit.
  • The ADU is located within an architecturally and historically significant historic district.
  • The ADU is part of the proposed or existing primary residence or accessory structure.
  • When on-street parking permits are required but not offered to the ADU occupant.
  • When there is a car share location within one block of the ADU. There is no additional off-street parking requirement for JADUs.

Fire safety

ADUs in San Mateo are not required to have fire sprinklers if the primary dwelling does not have fire sprinklers, except when a new ADU exceeds 1,200 square feet in size. Additionally, the following scenarios may require a fire sprinkler system:
  • When the ADU is more than 150 feet from a public right-of-way
  • When the water supply in the area is not adequate; or
  • There is a change of use (e.g. a garage is converted into a living space) For portions of an ADU located closer than five feet to a property line, fire rating walls may be required.

Utility connections

For detached ADUs, the City may require a new or separate utility connection. The new connection may be subject to a connection fee or capacity charges based on ADU size or number of fixtures.


Owner occupancy is not a requirement for permitting an ADU from January 1, 2020 until January 1, 2025. This is a great opportunity for rental property owners to increase their rental yield in a cost-effective way.


While long-term rentals are permitted for ADUs, short-term rentals (e.g. Airbnb for less than 30 days) are not permitted in San Mateo.


Do you have a utility easement over your property? Do you have special setbacks? Reach out to us and we’ll help you build a custom ADU to fit your unique property characteristics and local regulations!


How close to my house can I build an ADU in San Mateo?

When building a detached ADU, it should be at least 6 ft. away from existing structures, and 4 ft. away from side and rear property lines.

Can I convert my garage into an ADU?

Yes, if you own a single-family home in a residential-zoned area, you may convert your garage into an ADU. Depending on the condition that your garage is in, the project may take under 9 months to complete and cost less than building a detached ADU. Reach out to our team to get an assessment on the condition of your garage and whether it can be converted affordably.

What is the smallest ADU I can build in San Mateo?

Across California, the smallest ADU you may build is 150 sq. ft., regardless of type.

Does an ADU need a kitchen in San Mateo?

Yes, ADUs must accommodate permanent and independent living, and be equipped with a fully functional kitchen.

Can I build 2 ADUs on my property in San Mateo?

You may only build one ADU per lot for properties zoned for single-family residences. For properties zoned for multi-family residences, you may build ADUs up to 25% of the existing number of units.

What is the difference between an ADU and a JADU?

ADUs may either be attached to or detached from the primary dwelling, while JADUs are contained entirely within the primary dwelling. ADUs must be equipped with permanent facilities for independent living, but JADUs need only have an “efficiency” kitchen and may share some facilities with the primary dwelling. The maximum size allowed for a JADU is 500 sq. ft., while two-bedroom ADUs in San Mateo may be up to 1,000 sq. ft.

How much does an ADU increase property taxes in San Mateo?

Thankfully, your existing property will not be reassessed if you build an ADU in San Mateo! You can expect the value of your property to increase after you build your ADU. Since property taxes are often based on property values, this can increase your property taxes. However, any increases will only be based on the value of the ADU itself. Building an ADU will not trigger a reassessment of the value of your main home.

How many bedrooms can an ADU have in San Mateo?

There’s no explicit limit to the number of bedrooms for an ADU in San Mateo, however if your ADU were 1,000 sq. ft., you’d generally be able to fit 2 nicely sized bedrooms or 3 smaller ones if you get creative.

How long does it take to build an ADU in San Mateo?

The entire process, from start to finish, would depend on the type of ADU you’re building, the scope of your project, and the area you’re building in. In our experience in San Mateo, the permitting timeline has averaged at just over 4 months, with a range of 3-6 months.

Can I rent my ADU in San Mateo?

Yes, many homeowners we’ve worked with in San Mateo rent out their ADUs, given the high demand for housing and high rents. Short-term rentals of less than 30 days are not allowed in San Mateo, but you can rent out your ADU on platforms like Airbnb for 30+ days.

How many square feet can an ADU be in San Mateo?

The maximum size for JADUs is 650 sq. ft. in San Mateo. There is no specified maximum size for standard ADUs – the maximum size for attached or detached ADUs is determined by the Floor Area Ratio (FAR). Units up to 800 square feet are exempt from a lot’s FAR. For larger ADUs, the maximum size is set by the FAR of the underlying zoning district.

Can you build a 2 story ADU in San Mateo?

It is possible to build a two-story ADU in San Mateo for attached ADUs, as the maximum height is 24 ft. However, it’s unlikely that you’d be able to build a two-story detached ADU, due to the height limitation of 16 ft. for detached units.

Do I need solar on my ADU in San Mateo?

Newly constructed detached ADUs are subject to the Energy Code requirement to provide solar panels if the unit is newly constructed and non-manufactured. The panels can be installed on the ADU or on the primary dwelling unit.

Want to learn more about ADU regulations in your area?

Book a free consultation and estimate with a Cottage ADU Expert to find out what’s possible on your property!