Phase One
Getting Started
- Clarify what you want
- Find inspiration
- Estimate project cost
Project timeline
Getting Started is part of the Planning phase, which typically takes the first 1-3 months. Most ADU projects take 12-18 months to complete, but some extend to 24 months or more.
Step-by-Step
Think About What You Want
Getting Started Step 1
Start by thinking about your short- and long-term goals and concerns for your ADU project and what kind of ADU is best for your needs. Our ADU Exercises have a checklist to get you started.
Look for places to get inspired, such as:
- Talk to friends and neighbors who have built an ADU and reach out to any builders or designers you know for a casual chat.
- Check out our ADU Stories and sample floorplans to explore real-world ADUs in California.
- Check out opportunities to use pre-designed and/or pre-approved ADU plans via our Plans page.
Keep in mind your plans might change once you hire a professional. It’s also a good idea to talk with your neighbors about your project, and to think about what it means to share space with tenants (and if that impacts your design and layout).
Key Resources
Consider different types of plans
Getting Started Step 2
There are multiple kinds of ADU plans for you to consider:
Site-built is a traditionally constructed ADU built from scratch on your property (AKA “stick-built”). This option allows for a lot of customization and smaller changes throughout the construction process.
Prefab, modular, and panelized are partially or mostly built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide a “turnkey” service that helps with permitting and all on-site construction, from laying the foundation to utility hookups. If not, you’ll need to hire additional professionals to help.
Using a prefab design can save time and make clearer what your completed project will look and feel like, but they also typically offer less customization, can be difficult to permit and finance, and often cost the same as site-built ADUs. People are often surprised at the cost, but transportation, crane, foundation, and sales tax can end up being 20-40% of the total cost. Some companies do offer a smaller price point.
To explore prefab or modular ADU options, you will want to get in touch with companies directly. They will visit, evaluate your property, and make recommendations. You can find prefab contractors by researching online.
Using a pre-designed plan can save you significant time and money over starting with a fully custom design. Some designs may be pre-approved by your local, state, or federal agencies, which could make you eligible for faster permitting and review.
Check our Plans page for more information about HEART’s free ADU plans, local ADU Plans Galleries, and other sources.
Looking for info about types of ADUs (conversion, detached, JADU, etc.)?
free ADU Plans
Save time and money with a pre-designed ADU plan! HEART of San Mateo County offers free design and construction plans that can be customized for your property. (Check with your jurisdiction to confirm these plans work for your property.)
Make an informal sketch
Getting Started Step 3
Make a rough sketch of your property, including the existing home, any other structures, and space for a possible ADU. Use our ADU Exercises to help you with this step. (Note: You can update this sketch or make new versions as you continue to learn about your property and what’s possible.)
Key Resources
Estimate project cost
Getting Started Step 4
It’s never too early to start thinking about and planning for your project budget. A very rough placeholder for you to use is $450-600 per square foot, including construction and other costs (design, fees, etc.). The real number can vary widely and depends on many factors. 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, all customized to real local numbers.
See Budgeting & Finance for more about design, permitting, and construction costs and an overview of financing options.
Key Resources
Frequently Asked Questions
Here are a few of the most frequently asked questions about getting started. See the content above for more guidance, resources, and tips for all steps of the process.
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.
ADU Layouts that fit your needs
Browse real-world ADU floorplans
