Understand the legal requirements and steps necessary for building your accessory dwelling unit (ADU).
So, you’re thinking about building an accessory dwelling unit (ADU) on your property. Great choice! ADUs provide additional private living space, increase your lot’s value, and can provide a great source of passive income. Plus, they’re a relatively easy project to get off the ground.
We say relatively easy because there are ADU legal requirements you do have to follow. Even though it’s going up on your own property, an ADU isn’t like a shed or a play fort that you can just assemble without asking anyone’s permission. You’ll have to find out the ADU laws in your area and make sure you abide by them. It’s just like building a mini house because, well, that’s what it is!
The first thing you need to find out is a simple yes / no: Are ADUs legally allowed in my area? Can you even build one in the first place?
If the answer is yes, then great! Keep reading.
ADU legal requirements differ depending on the state or town you’re in, but before you dive into the specifics of learning your area’s specific ADU laws, let’s get a sense of what kinds of ADU rules there are out there.
This article will give you a primer on the kinds of rules and regulations your local area will have in its ADU ordinance, as well as how to go about finding the specific laws in your area.
What is an ADU?
An accessory dwelling unit (or ADU) is a fully self-contained living unit that shares the same lot with a primary residence or main house. It can be detached, like a backyard cottage, or attached to your main home. ADUs can also be created by converting another structure, like a garage. If you’ve ever heard the terms in-law unit, casita, granny flat or carriage house–yep! That’s an ADU.
ADUs have all the elements that make up a little home–areas for cooking, bathing, sleeping, and living. ADU housing law also requires them to be fully equipped with all the necessary mechanical, electrical and plumbing aspects of a dwelling.
Because of their small size, sometimes design choices like a loft bed area are included to maximize the space. (Similar to how a lot of tiny homes are designed.) There is room for creativity in designing and building your ADU, though of course it’s important to make sure you’re following your local area’s ADU laws first and foremost and work within that frame.
Your ADU will need to comply with ADU building codes, zoning laws and more. Some areas allow you to have multiple ADUs on a lot, some places allow only one or none at all.
So, when it comes to ADU legal requirements in your area, what do you need to know?
ADU Zoning
Once you figure out if ADUs are legal in your area, the next question to consider is whether that area you live in is zoned for them. In most cases, ADUs are allowed in areas zoned residential or areas zoned for mixed-use, both residential and commercial. (So yeah, that means in commercial-only areas no ADUs are allowed. But that does beg the question, why are you living in a strip mall nail salon anyway?)
ADU zoning regulations also dictate how many ADUs you can build on a property. Sometimes only one is allowed, sometimes several. Sometimes you can do something like have one detached ADU and one JADU or “Junior” ADU that is attached to your home.
ADU Building Codes
The building and design considerations you’ll need to have for your ADU are similar to building a house. But don’t let that scare you–it’s a MUCH simpler and easier process.
Size and lot requirements for ADUs
The ADU building code where you live will specify a max limit on the square footage, roof height, and some other basic building considerations. These ADU requirements can vary whether you are building a freestanding backyard cottage, attaching a unit to your main house, or doing a conversion, like of a garage or basement. Generally, the ADU size requirement falls somewhere between 500 square feet and 1,500 square feet. The height limit for detached ADUs is usually around 16 or 24 feet, and attached ADUs it’s usually whatever the main house is.
You will also have to make sure you follow regulations around aspects like setbacks (the ADU’s distance from the house, property line or other nearby structures), and parking. Sometimes adding a parking space is legally required if you build an ADU, sometimes not. Also, exemptions to ADU law around things like parking can be granted sometimes in cases where a property is located close to a transit stop, for example, so be sure to look closely.
ADU design guidelines
Sometimes ADU housing law will require you to follow certain design rules. The most common of these are regulations that your ADU must blend in with the neighborhood by using similar building materials, exterior paint colors, and roof slope measurements as the main house on the lot. Especially if you live in a historic neighborhood, expect these types of design rules to apply.
There may be other details specific to your area, for example roof decks being prohibited, or a rule that you have to put a sprinkler system or a solar panel on your ADU. Your local ADU ordinance will lay all these out.
Permit process
Just like with building a traditional house, you do have to submit a building proposal for your ADU. This gives your local government a chance to review your plans and make sure everything is approved before you start to build. How long this permit approval process takes depends on where you live. In California, ADU state law mandates a maximum 60-day turnaround for cities to approve ADU permits.
Note: Some cities, mainly in parts of California, offer pre-approved ADU design plans that you can get for an easy, plug-and-play option that is guaranteed to give you no headaches in the permit process. If you just want a simple backyard ADU for utility, a pre-approved plan could be a great choice, since it’s guaranteed to meet all the ADU requirements.
How to Find Out Your Local ADU Laws
Before you design or build your accessory dwelling unit (ADU), it’s necessary to have a full understanding of your local area’s specific ADU requirements. Depending on where you live, you’ll turn to your city, county, or state law.
(It is a good practice to learn the ADU laws in both your local municipality and state, FYI. If you live in California, for example, certain aspects of state ADU law override local regulations. Be sure to check out California's ADU Handbook in that case.)
Note: If you live in an area without housing codes, that also means no ADU regulations. Areas like this are rare in the U.S. but do exist. If you find yourself in one, build that dream corn cob village you have always wanted! Please be safe, though, and have fun.
Here’s how to find out the local laws in your area.
Step 1: Turn to the internet
Where else? You’re already here.
- Start with your local city or county’s official .gov website. Within a section like the housing department or planning division there may be information about the ADU rules and regulations or a link to the city’s official accessory dwelling unit ordinance. Especially in ADU-friendly states like California, some cities have all the information handy and laid out for you on a specific web page, or a downloadable ADU guide or fact sheet.
- Dig further into the specifics by looking at the official municipal code. It can get weedy, but hey, some people like this kind of thing. Searching for your city and keywords like “official municipal code” and “accessory dwelling unit ordinance” can get you to the official laws around ADUs in your area as your legislators wrote them. These documents can help ensure you’re following the letter of the law to the T.
- A lot of ADU construction companies publish guides to the ADU requirements in different cities on their websites. These articles, which are easily available on search engines, can be helpful to start with or use as a framework but often aren’t completely accurate, up-to-date, or comprehensive.
Step 2: Call or visit your city’s housing department
In most areas, you’ll need to get a building permit in order to build an ADU on your property. The office that deals with these permits will also have people there who can walk you through your area’s specific ADU requirements or direct you to the most up-to-date information.
Even if you prefer to just search online, it’s worth giving this office a call to double-check that the information available online is up-to-date, because the laws around ADUs are changing fast. They can also give you a sense of how long the permit process will take and any other questions you may have.
Step 3 (Optional): Hire a builder to figure all this out for you
Depending on your budget, priorities, and emotional tolerance for paperwork, it might be worth it to outsource this process. Many ADU builders, which are becoming increasingly easier to find, are willing to sort out the rules and regulations in your area, submit building permit applications, and do other logistical legwork in the service of building your ADU.
As you can see, ADU legal requirements are pretty straightforward and minimal, for the most part. Now that you’ve got your head around all the aspects you’ll need to account for, it’s time for the fun part–designing the ADU of your dreams! We wish you the best of luck navigating your ADU-building journey. And when you're done &ready to rent it out, you know where to find us.
