A small home makes a big difference
Supporting your accessory dwelling unit (ADU) project, no matter where you are in the process
UPCOMING EVENT: ADU FAIR
Thinking about building an ADU? Join us for a FREE event on Saturday, May 10, 2025.
Whether you’re curious about design, cost, financing, or ROI, you’ll have the chance to:
- Speak directly with industry pros
- Get expert insights
- Tour factory-built homes
free feasibility consult
Wondering what kind of ADU you can build on your property? Or what your first steps should be?
Residents of our member jurisdictions can sign up for a free one-hour personalized virtual consult with an ADU specialist. Our team will assess your unique project concerns and provide tailored information on the steps you can take to bring your ADU vision to life.
Consults are sponsored by the ADU Resource Center of San Mateo County and are provided by our partner, Hello Housing.
Do you have questions or would you like more information?
Ready to sign up today?
The San Mateo County ADU Resource Center
The ADU Resource Center provides information, resources and tools to make it easier and less expensive for homeowners to build ADUs in San Mateo County. The Center is funded by member jurisdictions of San Mateo County, the San Francisco Foundation, the Silicon Valley Community Foundation and Home for All.
IN THE NEWS Read the press release to learn more about the launch of the ADU Resource Center.
Your free go-to guide to all things ADU
Our ADU Guidebook provides step-by-step guidance through the ADU process, from inspiration to move-in.
Building an ADU is an investment for your family and your community
A home as you age
Housing friends and relatives
Environmental Benefits
Rental Income
Safety
Community support and Workforce Housing
Your One-Stop Shop for Support
We Help Homeowners Build Accessory Dwelling Units
Get Inspired
ADU Stories
Read about people in your community who have built ADUs
Floorplans Inspiration
See layouts for dozens of real ADUs from across California
“
I love it. It’s the perfect amount of space. I wouldn’t trade it for anything.”
Veronica, Pacifica, ADU owner
“
It provides peace of mind, knowing that our tenants are nearby to keep an eye on things when we’re away. We have no problem renting out the second unit, since it’s located near major employers.”
Surinder, Menlo Park, ADU owner
Frequently Asked Questions
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 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.
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.
