Phase Two
Learning the Rules
- Learn what you can build on your property
- Understand local rules and regulations
- Meet with local staff to discuss your project
Project timeline
Learning the Rules is part of the Planning phase, which typically takes 1-3 months. Most ADU projects take 12-18 months to complete, but some extend to 24 months or more.
Step-by-Step
Learn about your property
Learning the Rules Step 1
First, you’ll gather some basic information about your property – what’s your Assessor’s Parcel Number (APN)? How big is your lot and how is it zoned? Use the San Mateo County Assessor’s Office Maps, and our Exercises to record some basic information on your property.
Key Resources
Learn what you can build
Learning the Rules Step 2
Next, you’ll need to understand what you can build on your property. Local and state laws will impact what is possible for your ADU, like its size and height, and may include additional requirements about parking, fire safety, and more. See Local ADU Rules and the FAQs below to find out what applies and check our free ADU Guidebook for more details.
You’ll be looking for rules including (but not limited to) the following:
- ADU size (floor area)
- ADU height
- Setbacks (distance from the edge of your property to the main home or ADU)
Keep in mind: Understanding how local and state rules apply to your project can feel like a lot at first. Remember: Local staff are here to help. Contact Planning staff to ask all your ADU questions! You may also want to check out our Glossary to get familiar with key terms.
Coastal zone rules
Make sure you note if your property is in the Coastal Zone, in which case your project will require an additional permit and be subject to slightly different rules.
Check your local rules
We have summaries of ADU rules in San Mateo County jurisdictions, conveniently collected in one place. (You should always confirm latest rules with local staff.)
Meet with Local staff
Learning the Rules Step 3
One of the best things you can do is to talk to local staff early in the process about potential issues and rules that might apply.
If your property is in a community that is a member of the ADU Resource Center (see our Contact page for more details), you can reach out to Resource Center staff to be pointed in the right direction.
Otherwise, reach out to local staff. You can ask questions at your local Planning or Permit Counter, or you can call or email your Planning Department. Depending on where you live, you may be able to schedule an appointment. For contact information, see the Contact page.
If you decide to move forward, it’s a good idea to return to meet with staff so they can review the final design and provide advice before submitting your application. While not required, this extra step can make sure you get your permit as soon as possible.
Use our Exercises to help plan your conversation and take notes. This is also a good time to contact any utility service providers (waste, sewer, gas, electricity, etc.) to confirm requirements, timelines, and fees. See our Contact page for contact information.
Key Resources
Adjust project budget
Learning the Rules Step 4
If your basic project details (size, number of bedrooms) change based on learning the rules and meeting with expert staff, it’s a good idea to adjust your estimated project budget.
Key Resources
Frequently Asked Questions
Here are a few of the most frequently asked questions about learning the rules. See the content above for more guidance, resources, and tips for all steps of the process.
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.
Special Zones FAQs
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.
