Introduction to ADUs
Interested in building an ADU in San Mateo? Whether you’re interested in increasing your property value, generating extra income, or housing loved ones, review this page to start learning about ADUs in San Mateo. To get more detailed information that’s specific to your property, visit Symbium Build, your free one-stop ADU resource.
What’s an ADU
ADUs are known by many names, including accessory dwelling units, granny flats, in-law units, second units, and casitas. No matter what you prefer to call them, ADUs are separate, complete homes for at least one person that are located on the same property as another, primary home. That means that ADUs have a place to live and sleep, a kitchen, and in most cases, their own bathroom.
Different types of ADUs
ADUs can take a few different forms.
Detached ADUs are located in free-standing buildings that are separate from the main single-family home or multi-family building.
Attached ADUs involve adding on to a single-family home.
Conversion ADUs are constructed by converting part of an existing single-family or multi-family home, or by converting all or part of an accessory building (like a garage, shed, or studio). This includes garage conversions.
Junior ADUs (JADUs) involve converting part of the single-family home into a new dwelling unit that’s no more than 500 square feet and that may share a bathroom with the main home.
Benefits of ADUs
There are many benefits to ADUs! They include:
Rental income. The amount you can rent your ADU out for will vary depending on the location of your property, the size of the ADU, and the amenities the ADU provides.
Increase in property value. A rough estimate is that your property value will increase by 100 times the ADU's monthly rental value. Meaning, if you can rent out your ADU by $1,000/month, your property value will increase by roughly $100,000.
Housing for relatives, friends, and/or caretakers. ADUs are often used to house adult children returning home, retired parents, and people who assist with child and/or elder care.
Opportunity to downsize while staying in your community.
San Mateo ADU Regulations
Some basic information about San Mateo ADU regulations is below. To get more specific information about how these regulations relate to your property, visit Symbium Build.
Allowed locations. Generally, you can have an ADU on your property if residential uses are allowed on it, although exceptions may apply. Also, you must have an existing or proposed single-family home on your property or an existing multi-family building (which is a fancy way to say a duplex, triplex, or other type of apartment or condo building). To learn instantly if your property is in a location where an ADU is allowed, visit Symbium Build.
Minimum lot size. The good news is that your property can be any size and be eligible for an ADU. There are no minimum lot size restrictions.
Number of ADUs allowed. If you have a single-family home, you can for the most part have one or two ADUs on it. The number of ADUs allowed on multi-family properties depends on the type of ADU constructed and how many dwelling units are already on the property. To learn more about how many ADUs are allowed on your property, visit Symbium Build.
Maximum square footage. The maximum size of your ADU depends on what type of ADU you’re interested in building. Meaning, the maximum size of a detached ADU on your property will be different than the maximum size of an attached ADU on your property. To learn more, visit Symbium Build.
Maximum height. Generally, you should be able to build an ADU that’s at least one-story and 16 feet in height. To learn more, visit Symbium Build.
Minimum setbacks. Attached and detached ADUs will need to be built at least 4 feet from your property’s side and rear property lines, while conversion ADUs and junior ADUs need to be set back a sufficient distance to ensure fire safety. Visit Symbium Build to find out how far your ADU will need to be located from your front property line.
Parking. Generally, conversion ADUs and junior ADUs don’t require any parking. Attached and detached ADUs will require 1 parking space, unless your project qualifies for an exemption from the parking requirements. To check if you need to provide parking for your ADU, visit Symbium Build.
San Mateo ADU FAQs
How long does it take to plan, design, and build an ADU? Generally, the amount of time it will take will vary based on several factors, including the design selected, whether your ADU is built on-site or built ahead of time in a factory, and the availability of your architect or builder. To learn more, visit Symbium Build.
How much does an ADU cost? The cost of an ADU depends on lots of things, including the size of the ADU, the design of the ADU, whether the ADU is built on-site or ahead of time in a factory, and the cost of materials and labor. Visit Symbium Build to help you estimate the cost involved.
Can ADUs be rented? Yes, your ADU can be rented out, although most cities don’t allow short-term rentals of ADUs. This means you can probably only rent your ADU out for a period of month or longer.
Does the owner need to live on the property that the ADU is built on? Generally, the property owner will only have to live on the property if a JADU is built on it.
Does building an ADU impact property value? An ADU will increase the value of your property. The exact amount is difficult to predict, but a very rough estimate is that your property value will increase by 100 times the ADU's monthly rental value. Meaning, if you can rent out your ADU by $1,500/month, your property value will increase by roughly $150,000.
Does building an ADU impact property taxes? Your property taxes will slightly increase based on the added value of the ADU. The base value of your property will not be reassessed. As an example, if an ADU will add $100,000 in value to your property, and your property tax rate is 1.1%, then your property tax would increase by $1,100 ($100,000 x 1.1%).