Frequently Asked Questions
Many pages on this site feature FAQs – here are all the FAQs together in one place, categorized by topic.
ADU 101
What do I do first?
This site walks you through each part of the ADU process, from gathering initial inspiration and learning what you can build through construction and becoming a landlord or moving in.
You can also use our Process-At-A-Glance resource for an overview of the process and some initial issues to consider as you get started.
Start with Getting Started and walk through the pages of this site one-by-one for a detailed guide on what to do next.
What’s the difference between an ADU and a JADU?
Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is usually smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from studios under 500 square feet to over 1,000-square-foot homes with multiple bedrooms.
Junior Accessory Dwelling Units (JADUs) are within the footprint of your home (or attached garage) and less than 500 square feet. They can share a bathroom with the main home and have an efficiency kitchen (sink, cooking appliance, fridge, and small counter). Construction costs for JADUs are typically much lower. In most cases, the property owner must live on site in either the main home or the JADU.
State law now allows homeowners to have both a JADU and a regular ADU on their property.
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
Getting Started
What do I do first?
This site walks you through each part of the ADU process, from gathering initial inspiration and learning what you can build through construction and becoming a landlord or moving in.
You can also use our Process-At-A-Glance resource for an overview of the process and some initial issues to consider as you get started.
Start with Getting Started and walk through the pages of this site one-by-one for a detailed guide on what to do next.
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the city, and submit the application. Depending on what permits are required, how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to six months to get permits. Construction usually takes six to twelve months.
Do I need to tell my neighbors?
You’re not required to tell your neighbors about your ADU, but it’s always a good idea to communicate with them early in the process. Your project will run more smoothly if they are kept informed, and they may have great ideas for your project!
If you live in a Neighborhood or Homeowners Association, talk with your representative or board early in the process. They can’t prevent you from building or renting an ADU, but they may have guidelines you’ll need to know for design and construction. Depending on where you live, a letter certifying review from your HOA may be required before your city will issue permits for your ADU.
What early considerations should I keep in mind?
If your property is in a special zone or area, like a Historic District, California Coastal Zone, Flood Zone, or Fire Hazard Zone, you may need to consider additional guidance and rules. It’s important to understand these rules early on – see the Learning the Rules page for more details.
What should I know about setting up utilities for my ADU?
New or separate utility connections may be required for ADUs, but not JADUs. You may need to upgrade services and/or meters, which can add considerable time and cost to your project. Your local Planning and Building staff can provide more information and help you find contact information for your neighborhood service providers. You can find contact information for your local Planning Department on our Contact page.
What if I have an onsite septic system?
If your property has an onsite septic system, you will likely need to apply for additional permits from the San Mateo County Health Department. It is important to know this early on in the process and meet with SMC Health to see if you can accommodate an ADU on your property. If you can, the permit process can add months and significant costs.
Learning the rules
Am I allowed to build an ADU?
In almost all cases, yes! ADUs and JADUs are allowed in any single-family or multi-family zone. If residential buildings are allowed, ADUs are almost always allowed too (with limited exceptions for safety, traffic, and water).
State law now allows a single-family property to have a JADU, an internal conversion or attached ADU, and a detached ADU. Some places allow for more. Multifamily properties – like duplexes, triplexes, and apartment buildings – can have two detached ADUs or up to 25 percent of the number of units, depending on your property. Talk to local staff for more information if interested in building ADUs on a multifamily property. Developers building new homes or multi-family housing can include ADUs at the same time, but should confirm with local staff beforehand.
What’s the difference between an ADU and a JADU?
Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is usually smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from studios under 500 square feet to over 1,000-square-foot homes with multiple bedrooms.
Junior Accessory Dwelling Units (JADUs) are within the footprint of your home (or attached garage) and less than 500 square feet. They can share a bathroom with the main home and have an efficiency kitchen (sink, cooking appliance, fridge, and small counter). Construction costs for JADUs are typically much lower. In most cases, the property owner must live on site in either the main home or the JADU.
State law now allows homeowners to have both a JADU and a regular ADU on their property.
What if I have an onsite septic system?
If your property has an onsite septic system, you will likely need to apply for additional permits from the San Mateo County Health Department. It is important to know this early on in the process and meet with SMC Health to see if you can accommodate an ADU on your property. If you can, the permit process can add months and significant costs.
Can I put an ADU in my garage?
Homeowners can convert legally built structures (garage, barn, art studio) into an ADU. JADUs can be converted from an attached garage (but not detached).
If you demolish your garage or other enclosed structure and build an ADU in its place, the ADU can be in the same footprint if it’s the same size and height of the structure it’s replacing.
You do not need to replace the parking space if converting the garage to an ADU, but you may need to replace it if converting to a JADU. Demolition permits for an existing detached garage can be processed at the same time as the ADU permits. Note that garage conversion ADUs may require significant moisture barriers and other design elements in order to meet building codes.
How large can my ADU be?
According to state law, you can at least build up to an 800 square feet ADU, as long as rear and side setbacks are at least 4 feet and it is not above 16 feet tall. Otherwise, size limits depend on your property. No room behind or next to your main home? You can build it in your front yard instead.
Generally, ADUs can range from 150 square foot efficiency units up to 1,000 square feet. Junior ADUs (JADUs) can be up to 500 square feet. See Local ADU Rules for more details on height, setbacks, and other rules.
I don’t think I can fit an ADU on my property – what can I do?
According to state law, rules about setbacks, lot coverage, and open space requirements cannot restrict you from building an 800 square foot ADU, as long as the ADU has setbacks of at least 4 feet and is not above 16 feet tall. Front setbacks also cannot restrict you from building an 800 square foot ADU, which means an ADU can be in a front yard – but only if rear or side placement isn’t possible.
Do I need to live in the main house to build an ADU or JADU?
JADU owners need to live in the main home or the JADU – and this will need to be recorded in a deed restriction for the property.
Can I rent my ADU as an AirBnB or other short-term rental?
No. ADUs and JADUs cannot be rented for fewer than 30 days at a time.
Will I need to add parking?
Parking is much less of a concern than it used to be. JADUs do not require a new parking spot. Check your Local ADU Rules to see what parking may be required.
No parking is required if the ADU is:
- within ½ mile walking distance to transit (including a ferry);
- within an architecturally or historically significant district;
- on-street parking permits are required and not provided to the occupant of the ADU;
- located within one block of car-share access; or,
- built as part of a new home project.
Budgeting & Finance
How much does it cost to build an ADU?
In general, it is helpful to avoid having a fixed budget total in your head as you explore your options. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $450-$600 per square foot for construction (“hard costs”) and design and fees (“soft costs”), depending on your design and the materials you chose.
The San Mateo County ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time.
How am I going to pay for an ADU?
Many homeowners use a mix of options to finance their ADU, including savings, funds from family, and/or loans. It is strongly recommended that your financing is in place before construction starts. Be sure to factor in potential rental income since that will help you repay loans. See our ADU Exercises for more details on financing options.
What if I don’t have a lot of money available right now to build an ADU?
If you have equity in your home, a cash-out refinance or home equity loan/line of credit (HELOC) might work for you. Financing is typically unavailable for homeowners with lower income and insufficient home equity, but in the past the California Housing Finance Agency (Cal HFA) ADU Program has provided a grant of up to $40,000 to qualified homeowners – check their website for the most up-to-date information. Local financial agencies may also offer financing support or specialized loans, such as the San Mateo Credit Union.
How will building an ADU affect my taxes and property value?
Adding an ADU will likely affect your property taxes and the resale value of your home. However, your primary house will not be reassessed, and your property taxes will only increase based on the added value of your ADU. For example, if you build an ADU that adds $150,000 to your property value, and your tax rate is 1%, your taxes will increase by 1% x $150,000, or $1,500 per year.
Building a JADU will have a significantly smaller impact on assessed value. In some cases, your taxes will not increase at all. Home sharing will also not increase the assessed value of your home. Generally, garage conversions will not raise your tax bill as much as new construction, but they will also not add as much value.
Each property will require a one-on-one analysis to determine the added value of an ADU, so contact the San Mateo County Assessor’s office once you have an idea of your plan. They will be able to provide you with a rough estimate of tax implications.
Adding an ADU may impact your income taxes as well. This can be rather complicated, and it’s best to discuss these with a tax advisor.
Can I eventually sell my ADU separately from the main building?
No. You will need to record in a deed restriction for the property that the ADU cannot be sold separately from the primary home. Contact local staff to find out what local rules apply.
What about the money I could make renting the ADU?
Rental income is a major benefit of having an ADU or JADU on your property – for many people, it provides flexibility in their budget or an opportunity to grow their savings. In many places, you cannot rent your ADU for less than 30 days at a time (e.g., AirBnB, Vrbo), and shorter-term rentals are not permitted. Review your local ADU rules to see if your property is subject to restrictions. The San Mateo County ADU Calculator can help you estimate how much rental income could be generated by your new unit.
Design
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the city, and submit the application. Depending on what permits are required, how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to six months to get permits. Construction usually takes six to twelve months.
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
Can I put an ADU in my garage?
Homeowners can convert legally built structures (garage, barn, art studio) into an ADU. JADUs can be converted from an attached garage (but not detached).
If you demolish your garage or other enclosed structure and build an ADU in its place, the ADU can be in the same footprint if it’s the same size and height of the structure it’s replacing.
You do not need to replace the parking space if converting the garage to an ADU, but you may need to replace it if converting to a JADU. Demolition permits for an existing detached garage can be processed at the same time as the ADU permits. Note that garage conversion ADUs may require significant moisture barriers and other design elements in order to meet building codes.
How do I find an architect or designer?
Most homeowners choose to work with some type of design professional to plan their ADU and help throughout the process. Bringing on a professional early in the process is often key to getting your ADU approved quickly, managed efficiently, and built cost-effectively. Relevant experience and fit will be critical.
There are a variety of types of designers, and they may be an architect, builder, “designer,” design/build, or a modular/prefab company. If you’re hiring a local individual or team, they’ll likely start the process by visiting your home and talking to you about your ideas and goals. If it seems like a good match, they will prepare a proposal detailing their services and fee. Professionals typically charge for an initial consultation or proposal. Check with local staff for local requirements.
See our Exercises for a list of questions to ask a potential architect or designer and our Glossary to be clear on terms.
When do I show the City/County my design?
Once you have a design established with your architect/designer, it’s a great idea to discuss it with Planning staff so they can point out any issues before you prepare your application. For contact information, see Local ADU Rules.
This is also a good time to reach out to utility agencies (water, sewer, gas, etc.) to inquire about their infrastructural requirements and confirm connection and service fees.
Some utilities will be coordinated by your local government and others by outside agencies. You can find contact information for your local Planning Department on our Contact page, and local staff can direct you to your area’s services providers.
How much does it cost to design an ADU?
Design costs cover the work of your architect or designer and may or may not include engineering, surveying, and other professional services necessary to prepare for permitting. Together, these services may total around 10% of your total project budget – the exact amount depends on the details of your property and project. See the Budgeting and Finance page for more details.
Does San Mateo County have pre-approved ADU plans?
Check our Plans page for more details, including these two sources:
- Some communities in San Mateo County have an ADU Plans Gallery that shows locally pre-approved plans, plus prefabricated options (which are considered pre-approved at the state or federal level).
- HEART of San Mateo County offers free ADU plans that can be customized for your property. Explore plans online. Depending on where you live, some Planning Departments offer a quicker application process for these plans. To see if you’re eligible for expedited review, check the GLADUR Pre-Review webpage and talk to local staff.
Permitting
What permits are required for ADUs and JADUs?
Application requirements vary by where you live, but applications typically include:
- Site plan (depicting existing and proposed structures)
- Building/architectural plans (floor plans, elevation plans, and details)
- Structural plans (foundation plan, framing plans, and associated details)
- Other items (Title 24 energy calculations, deed restriction, address assignment form, or other materials required by your location)
- Deed Restriction filed with the County
- Supplementary materials
Some local Planning Departments have an ADU permit application or submittal requirements checklist that outlines all the required documentation and materials that must be submitted. Contact local staff (Link) to learn about your specific application process and requirements.
How much does it cost to get permits for an ADU?
ADUs can be charged a variety of fees from your local government, the State, and other agencies. Some fees are based on details of the project while others are fixed. The total amount depends highly on your property and plans, but local staff can most likely provide an estimate once you have a good idea of your project details. See the Budgeting and Finance page for more details.
Are there water and sewer issues I should consider?
You may be required to upsize service and or meters to meet capacity requirements. Check with local staff to learn more about local requirements for utilities.
Can my ADU be stopped because of other noncompliance issues on my property?
In most cases, state law no longer allows cities and counties to comment on pre-existing zoning issues unrelated to the ADU. For example, you should not receive comments about correcting the main house or a fence unrelated to the ADU, unless there is an obvious public safety issue.
Special zones
What if my property is in a historic district?
Properties in Historic Districts may require additional review for ADU projects and are very likely subject to design review for consistency with historic criteria. Contact local staff to confirm the process and submissions required for ADUs in historic districts.
What if I’m in a flood zone?
Hazard requirements depend on property elevation and location and may factor into ADU plans. Talk to staff early on to see what additional requirements your team will need to include.
What if I’m in a high-fire severity area?
Wildfires are a reality throughout our region, which is why it is important to understand the risk in your area. If your property is in a Fire Hazard Severity Zone or Fire Protection District there may be additional requirements or reviews. Find out about your location and talk to staff early on to learn how where you live might impact your ADU.
State agencies offer several resources and guidelines to help.
- Visit the CalFire Fire Hazard Severity Zone webpage to learn about Fire Hazard Zones and find maps. Plug your address into the Fire Hazard Severity Zone Tool to look up your property and identify your Fire Hazard Zone.
- Visit www.readyforwildfire.org to learn about design and landscaping choices that can help prevent fire risk.
- Review the Disaster Ready Guide and Board of Forestry Code to make sure your ADU and property are fire safe.
What if my property is in a Community Services District?
If the property is within the architecturally controlled area of the Granada Community Services District, Highlands Recreation District, or Ladera Recreation District the district must approve your plans before you can get a permit.
Construction
How do I find a contractor?
If you are not using a design/build firm, you will need to find a contractor to take over for the construction phase of your ADU.
First, you’ll solicit bids. You will want to get at least three bids for comparison.
When you have bids, you can begin selecting your contractor.
Before you hire a contractor, make sure to check their license and insurance and when they present you with a contract, review everything carefully.
How much does construction cost?
Construction costs (also called “hard costs”) for your ADU will vary significantly depending on personal preferences, site conditions, location, and many other factors. Hard costs typically total around 85% of your total project budget.
See the Budgeting and Finance page for more details and visit our Stories to see real-world cost examples for ADUs built in San Mateo County. Also check out our ADU Calculator to create a construction cost estimate based on your location.
How long does construction take?
Traditional construction will take 6-12 months, though this will vary depending on the specifics of the project. Stages of construction include:
- Site preparation: 1-2 months
- Foundation: 1 month
- Walls, roof, doors: 1-2 months
- Plumbing & electrical: 1-2 months
- Insulation & drywall: ½-1 month
- Fixtures & finishes: 1-2 months
- Final touches: ½-2 months
What are my responsibilities during construction?
While your contractor will lead the construction process, you will have the following responsibilities:
- Keep in touch with your contractor and set up a schedule for checking in.
- Regularly walk through the construction area to monitor the quality of the work and make sure the work is progressing the way you expect.
- Be prepared to make decisions about the details—light fixtures, appliances, and other materials—in a timely manner so your contractor can stay on schedule.
- Follow the contract you agreed to, including any changes as described specifically in a change order form.
- Although your contractor will usually arrange the required inspections, it is your responsibility as the property owner to make sure that the inspections are conducted as required.
Renting & Move-in
When is my ADU ready for move-in?
As soon as the final inspection is complete, your ADU is ready for move-in! Make sure utility services are set up, an address is established, and other preparations are in place. See above for more responsibilities of being a landlord.
What do I need to know about becoming a landlord?
Renting an ADU comes with many responsibilities, including understanding local and state housing laws, executing a lease, finding and managing a tenant, and maintaining a rental property. It’s important to understand the laws as they may affect things like future rent increases, changing use over time, evicting tenants, and moving family into the unit.
See our Exercises for help with your lease terms.
Can an ADU or JADU be used as a short-term/vacation rental?
No. Generally, J/ADUs are not allowed to be rented for less than 30 days. This discourages the listing of ADUs on popular websites like Airbnb and Vrbo and promotes them as a means to increase housing stock for the diverse needs of county residents. You may be required to file a deed restriction agreeing that the unit will not be used for short-term rentals.
