The basics of accessory dwelling units

Before we start, it’s of course important to note what an accessory dwelling unit is. We’ll start with the Housing and Community department of California’s definition:

Accessory Dwelling Units (ADUs) have been known by many names: granny flats, in-law units, backyard cottages, secondary units and more. No matter what you call them, ADUs are an innovative, affordable, effective option for adding much-needed housing in California. HCD is the state’s leader on local ADU ordinances, which — while optional — have grown exponentially in number as more cities, counties, and homeowners become interested in ADUs as one solution to increasing the supply of affordable housing.

So why would you consider adding an ADU? What are the benefits? Here’s HCD California again:

ADUs are an affordable type of home to construct in California because they do not require paying for land, major new infrastructure, structured parking, or elevators. ADUs can provide a source of income for homeowners. ADUs are built with cost-effective wood frame construction, which is significantly less costly than homes in new multifamily infill buildings. ADUs allow extended families to be near one another while maintaining privacy.

Take a look at the graphic below: You can see there’s essentially four stages to an ADU project. We’ll spend timeon each of these areas throughout this guide, but we wanted to introduce it here because it offers a look at how we structured the guide.

Should I hire an ADU professional, contractor, or architect?

Depending on your bandwidth, willingness to learn, and the complexity of your project(s), you could perform some of the project management yourself. Or, you might discover that you would prefer the help of an expert. Either is okay!

If you end up looking for expert help, it’s important to know that many “ADU build services”, but can vary widely in quality and scope of work. Knowing how to choose a good ADU company can save you a lot of time and money, as the wrong ADU techniques can cause huge headaches and can harm you far more than they will help. maintains a list of ADU vendors, builders and contractors. However, these are not vetted – so make sure to do your own research and evaluate which company best fits your needs.

Do I have to permit my ADU?

Any residential dwelling in California must be permitted, so yes. The permitting process is in place as a way to confirm that buildings pass all necessary building codes to ensure safety and structural integrity.

ADUs are not always welcome in some cities

Although ADUs have grown in popularity, there are still some jurisdictions that don’t want increased density in their cities for one reason or another. However, there are plenty of ADU advocates to help you along your journey. HCD releases an annual ADU Handbook, similar to our own ADU Beginner’s Guide to help answer questions you have. You will also find plenty of partners and guides in our resources page at the end of this guide to answer further questions.

Casita coalition,, and local nonprofit agencies (like Napa Sonoma ADU) put out regular webinars about ADU’s.

While ADU guidelines may vary from city to city, the underlying principles remain the same: don’t try to skip steps. Instead, plan ahead for all the events needed to complete your project. To do that, follow ADU guidelines and build an ADU that fits your needs.

ADU Project Manager Guidelines

Basic Principles:

  • Understand the ADU permitting process of your local jurisdiction
  • Be familiar with all the partners you’ll need for your project (this includes organizations like PG+E)
  • Avoid tricks to get through permitting faster. A good rule of thumb is whether you’d feel comfortable explaining what you’ve done to an on-site inspector. Another useful test is to task, “Would I feel comfortable staying in this home?”
  • Think about what you type of ADU will fit your needs, budget and goals.

Things to avoid:

  • Creating an ADU without securing proper permits
  • Hiring contractors without ADU experience
  • Failing to account for all the expenses in an ADU project. Some contractors will give low initial estimates without incorporating all the expenses that come with an ADU.

💡 Local vs Statewide ADU Practices California is a homestead state, meaning the State can pass laws and ordinances, but it is up to the City to enforce these laws and to permit your ADU. Think of this relationship as the State being as the big brother to a local jurisdiction. The big brother can ask you to clean up your room, but the younger brother can choose to clean up the room and exactly how that room gets cleaned up. This means you (or your project manager) will have to be an advocate for your project when the local jurisdiction is not following state guidelines.

Know your ADU goals

Every ADU project is different, so take the time to really understand your ADU’s specific goals. This will not only help you determine which areas of the ADU to focus on, what your budget should be, and will help create talking points when speaking with ADU contractors and professionals.

What will your KPIs (Key Performance Indicators) be to measure the return on ADU investment? More simply, what is your barometer to measure the success of your efforts? You’ll want to have it documented, even if it’s this simple:

For my adu project ___________, my primary ADU KPI is ____________.

Here are a few common KPIs to get you started:

  • ROI %
  • Time to First Occupant
  • Family Members housed

You may have noticed that things like “total project cost” and “design type” weren’t on the KPIs list, and that’s intentional.

You say, “I came here to learn about how to build an ADU because I heard it would be a good investment and want to build it as cheaply as possible”.

These are both valid thoughts, but they should still fit into your goals for the project. If you have the choice between a:

  • $119,000 project that is not permitted and could be fined after construction
  • $150,000 project that is legally permitted and constructed within 3 months or

We hope you would choose the latter.

ADUs, when done well, help real homeowners achieve real goals for their success.

💡 Setting (and eventually achieving) the right goals is one of the most important things you can do as an ADU project manager. We even have a post all about setting ADU specific goals.

Now grab a cup of coffee, and let’s dive into Chapter 2: ADU Property Checklist