San Mateo ADU Zoning And Building Standards

An increasing number of people in San Mateo are building Accessory Dwelling Units (ADUs). It’s important for homeowners to fully understand the zoning laws and building standards that govern the building of these units. With these rules, you can be sure that they will be built in a way that is safe, efficient, and doesn’t bother the people who live nearby.

You can rest easy knowing that your San Mateo ADU will be built and planned to high standards if you work with DevArt8 Construction. with an emphasis on functionality, comfort, and good looks. We’ll help you make your dream home come true while making sure it follows all zoning laws and building codes. From the beginning to the end, the ADU process will be simple and stress-free.

San Mateo ADU Zoning and Building Standards

Property Requirements

For San Mateo residents, building an ADU has certain rules that depend on the type of zoning that your land has. The standards cover things like the size of the lot, fences, and living on the property yourself. These rules are very important for figuring out if your home is right for an ADU and what kind of unit you could build.

Building Size

To find out what size building you can have on your San Mateo ADU, you need to know what zone your property is in and whether the ADU is connected or separate. Knowing these size limits will help you plan the layout of your ADU and make sure it follows the rules in your area.

  • Single-Family Zoned Lots:

In San Mateo’s single-family zones, you can build an ADU that is either separate from the main house or connected to it. The largest size that an ADU can be depends on things like the size of the lot and the main house’s floor space. These types of lots give you the most options for where and how big your ADU can be.

  • Duplex Family Zoned Lots:

In San Mateo, duplex family-zoned lots can be used to build an ADU, but there are a few extra things that need to be thought about. Compared to single-family homes, the ADU may not be able to be as big or in the same place, and there may be special rules to follow to keep the style and look of the duplex as it is now.

  • Multi-Family Zoned Lots:

The rules and regulations for ADUs built on multi-family zoned lots in San Mateo are different from those for single-family zones. ADUs can be different sizes and shapes from duplexes and single-family homes. Parking and shared space might have more rules.

  • Attached ADUs:

These are connected to the main house and can’t be bigger than what is allowed by the zoning of the current buildings.

  • Detached ADUs:

ADUs that are detached are separate buildings from the main house and must meet certain size and spacing rules.


However, in San Mateo, the owner-occupancy standards for homes with ADUs rely on the type of ADU and the lot’s zoning. For some ADUs, the owner may have to live in either the main house or the ADU, but for others, this is not necessary. It is very important to know the rules about owner occupancy that apply to your case.

Number of Units Allowed

A home in San Mateo can have a certain number of ADUs. This number depends on the property’s zone and the type of main residence. One ADU can be built in a single-family home, and many ADUs can be built in multi-family and twin houses. To plan for your ADU job, you need to know what the rules are for your specific land.

Saratoga Building Height

Certain types of ADUs in San Mateo have different height standards based on their zoning and the type of ADU being built. It is guaranteed that these height limits will make ADUs fit in with their surroundings and not cause problems for nearby homes.

  • Single-family: Depending on the zone, the tallest an ADU can be built on a San Mateo property zoned as single-family is generally between 16 and 20 feet. This makes it possible to build both a one-story ADU and a two-story one with a smaller floor plan.
  • Duplex family: For ADUs on duplex family homes in San Mateo, the height limits may be a little stricter than for single-family homes. To keep the scale and spirit of the twin building, the highest point is usually between 14 and 16 feet.
  • Multi-family: For ADUs on duplex family homes in San Mateo, the height limits may be a little stricter than for single-family homes. To keep the scale and spirit of the twin building, the highest point is usually between 14 and 16 feet.

Size of the San Mateo Building

The zoning for the land and the type of ADU being built decide the largest size of an ADU building that can be built in San Mateo. These rules about size make sure that the ADUs are the right size for the land compared to the main house. This also keeps the neighbourhood looking nice and even.

  • Single-Family Freedom: In San Mateo, the biggest ADU that can be built on a single-family-zoned property is usually the biggest one that can be built. Up to 1,200 square feet of connected ADU space is allowed, and most of the time, these units are bigger than half of the home’s original living space.
  • Duplex house: In San Mateo, ADU sizes may be a little stricter on duplex house lots that are zoned for families compared to single-family homes. Depending on the zoning and size of the existing duplex, the largest separate or connected ADU that can be built on a duplex property is generally between 800 and 1,000 square feet.
  • Multi-Family Mix: ADUs in San Mateo’s multi-family zones can be very different depending on the zoning designation and the details of the main multi-family property. In some situations, ADUs may only be able to take up a certain amount of the building’s floor space. In other situations, they may be limited to a certain area.

Fire Safety Regulations

San Mateo has special fire safety rules for ADUs that protect the health and safety of the people who live there and the homes next door. These rules cover things like building materials that don’t catch fire, smoke and carbon monoxide monitors, and the right doors or windows for getting out of a building. Following these rules is important for getting the permits you need and making sure the area is safe for people to live in.


According to San Mateo, the parking rules for ADUs are different based on the type of ADU, its position near public transportation, and the area where it is zoned. There are times when it’s not necessary to provide more parking, and there are times when off-street parking spots may need to be made available. It is very important to know the parking rules in your area in order to plan for your ADU project.

Utility Connections

In San Mateo, converting utility lines to the ADU means getting the right permits and working with local utility companies. What is needed to link to waste, water, electricity, gas, and water will depend on the type of ADU and where it is located. To make sure the process of connecting services goes smoothly and quickly, it’s important to know about these standards early on in the planning process.


In San Mateo, the spacing requirements for ADUs say how far away the unit must be from the property line and any other buildings on the land itself. These rules are different depending on the zoning of the land and the type of ADU that is being built. Following the rules for Setbacks buildings will help you make sure that the ADU is put correctly on your land and stays at the right distance from nearby homes.

Zoning Regulations

There are different rules for ADUs in San Mateo depending on the type of land and the zoning designation. These rules cover things like the types and sizes of ADUs that are allowed, as well as height limits and distance requirements. It’s important to know the rules for zoning your land so you know what kind of ADU you can build and how to make it follow the rules.

San Mateo ADU Zoning Regulations

Single Family

The rules for ADUs are usually the least strict on San Mateo homes that are zoned as single family. They have big dimensions and less strict setback rules, and they allow detached and connected ADUs. But it’s important to check your property’s exact zone name to make sure it follows the rules.

Duplex Family

Duplex Family lots in San Mateo may have stricter rules for ADUs than single-family homes. The rules might put limits on the ADU’s size and shape, and they might demand stricter setback rules to protect the style and space of the present duplex building.


In San Mateo, ADU rules can be very different in places with multiple families, depending on the type of zoning and the specifics of the multi-family property. These rules may say what kind of ADU is allowed or how big and tall the building can be. They may also say how far back the building has to be and how many spaces can be parked so that it doesn’t interfere with the nearby multi-family development.

RF Zone Minimum Lot Area In San Mateo

  • The RF-20 lot sizes have at least 22,000 square feet of living space, which is a lot of room for an ADU and outdoor living areas.
  • The main goal of the RF-30 lot is to provide at least 30,000 square feet, which is enough space for bigger ADUs and bigger gardens.
  • This RF-40 lot is at least 40,000 square feet of beautiful land that’s perfect for fancy ADUs and a lot of different outdoor spaces.

Room Specifications

It’s important to think about the needs and ideas for each type of room when you’re designing your San Mateo ADU. In your ADU, this table gives you a quick look at the most important requirements you need to know about for the kitchen, bathroom, sleeping area and store space.

San Mateo ADU Room Specifications

Room TypeSpecifications
Bathroom– Minimum floor area: 24 sq. ft.
– Waterproof flooring and walls
– Proper ventilation
– Ground Fault Circuit Interrupter (GFCI) outlets
– Accessible plumbing connections
Kitchen– Minimum floor area: 50 sq. ft.
– Durable, easy-to-clean flooring
– Adequate counter space and storage
– Proper ventilation for cooking appliances
– Dedicated electrical circuits for appliances
– Accessible plumbing connections
Sleeping Areas– Minimum floor area: 70 sq. ft.
– Minimum width: 7 ft.
– Egress window or door for emergency escape
– Proper ventilation and natural light
– Comfortable and efficient layout
Storage– Adequate size for ADU occupants’ needs
– Easily accessible location
– Durable and organized shelving or cabinetry
– Proper ventilation to prevent moisture buildup
– Secure and lockable for valuable items

San Mateo Requirements

In order to build an ADU in San Mateo, you have to follow a set of rules that are special to the type of ADU you want to make. Some of these needs are sizes limits, borders, parking, and the owner living in the home. To make sure you follow the rules and complete your job successfully, you need to know the standards for the type of ADU you want to build.

Detached ADU San Mateo : In San Mateo, detached ADUs are separate buildings from the main house. They have to stay within certain sizes set by the property’s zoning. They have to be set back from the land line and other buildings. The parking rules for separate ADUs might be different depending on a number of things, such as how close the ADU is to public transportation.

Attached ADU San Mateo : In San Mateo, attached ADUs are connected to the main house by an open wall or an addition. Most of the time, connected ADUs can only take up a small amount of the main house’s living room and must follow the property’s setback rules. There may be times when the connected ADUs need to be occupied by the owner.

JADU San Mateo: It is a special type of ADU that is made by turning a bedroom in the main house into an ADU. JADUs are a certain size and need a kitchen that works well and two different entries. Also, compared to other types of ADUs, they may have less strict rules about driving and living there yourself.

Rental Requirements

If you want to rent an ADU in San Mateo, you should think about the different rules that apply. These rules will help lower the risk and make the rental process easy for both you and your renters. If you’re thinking about renting out an ADU in San Mateo, it’s important to keep the following things in mind. This will lower the risk and make the renting process easier for both you and your renters.

  • Proof of income: Verify that the tenant’s monthly income is at least 2-3 times the rent amount to ensure their ability to pay.
  • Credit score: Request a credit report to assess the tenant’s financial responsibility and history of paying bills on time.
  • Rental history: Contact previous landlords to gauge the tenant’s reliability, cleanliness, and adherence to lease terms.
  • Application fee: Charge a reasonable fee to cover the costs of background checks and credit reports.
  • Security deposit: Collect a refundable deposit to protect against potential damages or unpaid rent.
  • Proof of renter’s insurance: Require tenants to obtain renter’s insurance to safeguard their belongings and limit your liability.
  • Pet policy: Clearly state your pet policy, including any breed or size restrictions, and consider requiring a pet deposit.
  • Smoking policy: Specify whether smoking is allowed on the property and any designated smoking areas.

Development Standards For San Mateo ADU

The most important requirements for building different types of ADUs in San Mateo have been listed below. This should help you understand the unique requirements of your project. The rules will help you plan and build a friendly ADU by telling you about things like size and height limits, owner usage, and parking.

ADU TypesDevelopment Standards Specifications
Single-family– Max. size: 1,200 sq. ft. or 50% of primary dwelling
– Height limit: 16-20 ft.
– Setbacks: 4-5 ft. from property lines
– Parking: Varies based on location and ADU size
– Owner-occupancy: May be required
Duplex– Max. size: 800-1,000 sq. ft.
– Height limit: 14-16 ft.
– Setbacks: 4-5 ft. from property lines
– Parking: Typically required for each ADU
– Owner-occupancy: Usually required
Multi-family– Max. size: Varies based on zoning and primary building
– Height limit: Varies based on zoning and primary building
– Setbacks: Varies based on zoning and location on the lot
– Parking: Often required, amount varies by zoning
– Owner-occupancy: May be required
Junior ADUs (JADUs)– Max. size: 500 sq. ft.
– Must be created within the primary dwelling
– Requires an efficiency kitchen and separate entrance
– Parking: Often not required
– Owner-occupancy: Typically required in primary dwelling or JADU

San Mateo Property Designations For ADU

When planning your San Mateo ADU, it’s important to think about the property’s names, which could change how the building process goes. There are geohazard and water zones, as well as zones and boundaries. These labels might have an effect on how the ADU project is planned, built, and approved.

  • Flood zones: Find out if your home is in an area that has been marked as a flood zone. If it is, you may need to get extra permits and think about how to build your home so that it doesn’t flood.
  • Geohazard zones: Know if your property has any geohazard zones, such as places that are likely to have landslides or earthquakes. These areas may need special planning and building methods.
  • Easements: Check to see if there are any entry or utility easements on your land that might affect where you can put your ADU and how it looks.

ADU Permit Requirements And Fees

Getting the right licences and paying the fees are part of the process of building an ADU in San Mateo. These rules make sure that the ADU project follows the local building, safety, and zoning laws. To make sure your project doesn’t get held up, learn about the different types of permits and how much they cost.

San Mateo ADU Permit Requirements and Fees

  • Building Permit: Getting a building permit is necessary for all ADU projects. It covers the whole building process and makes sure that it follows all building rules and standards.
  • Electrical Permit: If you want to do any electrical work on your ADU, like adding outlets, wires, or lights, you need an electrical work permit.
  • Plumbing Permit: You need a plumbing permit for all plumbing work done in your ADU, such as installing and connecting water and sewer lines and appliances.
  • Other Permits: This depends on what kind of work you’re doing on your ADU. You may need more permits, like an HVAC mechanical permit systems or a grading permit for work on the site.

Design Your Dream ADU Today!

Now that you know everything there is to know about San Mateo’s ADU Zoning and Building Standards, you can start making your dream ADU. Our professionals are ready to help you with the whole process, from the beginning to the end. Set up a meeting with us right away to take the first step towards creating your perfect San Mateo home.


Yes, but specific setbacks and design considerations apply. Contact the Woodside Planning Department for detailed guidance on navigating unique lot shapes.

Some lenders offer ADU loans, but traditional home loans can also be used for ADU construction, especially if they add value to your property.

Yes! The State of California offers various ADU incentive programs, including grants for low-income homeowners and developers. Research state and local programs to see if you qualify for financial assistance.

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