Different names for an ADU with the caption "What Does ADU Stand For?"

There is a growing trend in the housing industry towards building ADUs. But what is an ADU? And what does it stand for?

ADU stands for “Accessory Dwelling Unit.” It is a technical term used throughout the housing industry for secondary housing structures on the same lot as a primary dwelling. You may know them by one of the more common names, like mother-in-law suite, backyard cottage, carriage house or basement apartment.

ADUs come in all shapes and sizes. So let’s take a look at some examples of ADUs, examine what they all have in common, and what you need to do to build your own ADU. Plus, we’ll talk about the advantages that come from ADUs, so keep reading.

Types Of ADUs

ADUs are not a new concept. In fact, ADUs are an iconic part of American living. From Happy Days to Parenthood, these secondary housing units have featured in prime-time TV shows for decades. But they aren’t just on TV. ADUs are everywhere–you’ve just probably never thought about them with that name.

Have you ever known anyone who lived in a converted garage? No? Then maybe you’ve known someone who lived in a backyard cottage. Both of these examples are types of ADUs.

ADUs can add additional living space for older family members who need more care or young adults who are not quite ready to leave the nest. Or they can become a valuable rental unit on your property.

So, let’s look more closely at some different types of ADUs.

Freestanding ADUs

People call freestanding ADUs by a whole host of interesting names, including backyard cottages, mother-in-law suites, and my personal favorite, granny pods. What all these have in common is that they stand alone, separate from the main house.

an ADU half way built in the backyard of a Saint Petersburg, FL property, with white water barrier around the walls and an exposed wood roof
Our friend Chad’s freestanding ADU build half-way done in Saint Petersburg, FL. He plans to live in it and rent out the main house for more income and the freedom to travel more!

Freestanding ADUs range in size from small units around 500 square feet up to larger structures of 1,200 square feet or more. They can be built either in a traditional stick frame fashion, or you can choose a prefabricated ADU.

Choosing a prefabricated model can save you time and money on your build. With the right builder, you may be able to have your freestanding ADU constructed in just a few months. If you are thinking about a prefabricated ADU, check out our guide for the best prefab home builders in your state.

Garage Conversion

If you don’t want the hassle of building something completely new, converting your garage to an ADU could be a good option.

A stone house with a detached stone garage.
Wouldn’t this detached garage make a charming carriage house?

Garages can be converted into ADUs in a number of different ways. The entire garage can be converted into a living space. Often, detached garages that have been converted to ADUs are called carriage houses. Or in if the garage has two stories, just the upper floor can be converted–think of Fonzie’s garage apartment in Happy Days. But the garage doesn’t need to be detached to convert it to an ADU. You can easily convert your unused attached garage space to an ADU.

Basement ADUs

Like your garage, your basement may be the perfect area for you to convert to an ADU for either personal use or rental income. Now, let’s be clear–a basement ADU is more than just a finished basement.

The requirements for a basement ADU will vary, but in Portland, OR, where basement ADUs are popular, you must have plumbing and ventilation for a separate kitchen and bath. It is also required that the entrance to the basement be either a side door or a back door, to limit the number of street-facing doors.

Components Of An ADU

All ADUs must contain certain elements to qualify as a separate living space, whether in a garage, a basement, or your backyard. As we mentioned, you can’t just claim your finished basement is a basement apartment. So let’s take a closer look at what all ADUs have in common.


A separate entrance is essential for a space to qualify as an ADU. You must be able to access the unit without going through the primary living space. This isn’t usually an issue for freestanding ADUs or garage conversions. However, with basement and attic spaces it can get more complicated.

Now that doesn’t mean your ADU can’t access your primary living space. The basement apartment can still have a door that connects to the main house. However, the basement must be able to maintain its own private space not dependent on the other living area.

Yellow ADU door on a yellow house.
An ADU without its own door isn’t an ADU.

Some areas, like Portland, OR, may have more specific guidelines on what is required for egress. So always check with your local authorities.


Just like it needs to have its own door, your ADU must have a fully functioning kitchen. I mean, think about it. A kitchen is an essential component of a home, and an ADU is just a second home on the property.

But you can’t just plug in a hot plate and call it done. The ADU must have a real kitchen. According to Fanny Mae, an ADU kitchen must contain cabinets, a countertop, a sink with running water and a stove or stove hookups. And as we said, it must be a real stove–microwaves and toaster ovens do not qualify.


A bathroom and bathing area are also required, which makes sense. It’s hard to say your backyard cottage is a separate living space if you need to run back to the main house every time you’ve got to go.

The exact bathroom requirements will vary, so you do need to do your research. And make sure you check your state as well as city or town requirements. As the folks at Smart Choice Plumbing point out, it is possible that your town may not require a bathroom while the state does. In that case, you would still need to follow the state guidelines.

Now, if you don’t want to add a full bathroom, some areas may allow you to build a JADU or junior accessory dwelling unit. These smaller units only need to have access to a bathroom in the main dwelling space.

Sleeping Area

The last component an ADU needs is a sleeping area. I mean, it’s not a home without a place to lay your head at night. Now just because the home needs a sleeping area doesn’t mean it has to be a full bedroom. In many areas, a studio layout is perfectly acceptable.

Again, you want to check the specific zoning requirements for where you are building. Many towns have specific square footage requirements for the specific areas of the ADU, like the bedroom and bathroom.

And keep in mind, this list is not an exhaustive list of what your city or town will require. Some jurisdictions may require specific ceiling heights, ventilation systems or even parking spaces. But without a separate door, kitchen, bathroom, and sleeping space, you don’t have an ADU.

Murphy bed folded down in a modern white minimalist apartment or ADU.
Who wouldn’t want a Murphy bed?

Can I Build An ADU Anywhere?

If you are considering building an ADU, the first thing to do is check out the regulations in your town and state. Every location will be different. Accessory Dwellings.org has compiled a great list outlining the requirements for different states and towns.

It can be tricky trying to make sense of it all. Take California, for example. Since the housing crisis, the State of California has been encouraging the development of more ADUs. The state has passed laws that are sometimes in conflict with the city ordinances limiting ADU construction.

Check out this article to learn more about how you find out about building an ADU where you live.

Does An ADU Add value?

There are many different reasons you may want to add an ADU to your home. But any construction comes with costs. And you are probably wondering if an ADU will add value to your property.

This is a hard question to answer. There are many factors that can affect that answer. Design choices and construction costs, of course, play a large role in determining the value the ADU will add. But beyond the total value the structure itself brings, adding an ADU can add value to your life in other ways.

Maybe you are considering building an ADU to house either an elderly family member or a younger family member who isn’t ready to leave the nest fully. There are many advantages to having a multi-generational home.

It is hard to place a value on the peace of mind you receive when your family members are close by. Grandparents can also help take on child care roles that benefit both the grandparents and children, not to mention the money you save on child care costs.

So when considering the value an ADU can bring to your family, don’t forget to look beyond the initial rental returns or rise in property value when you sell. There is often hidden value you don’t see at first glance.

Final Thoughts

Whether you decide to build a granny pod in the backyard or convert your unused garage into a unique rental space, there are as many different ways you can add an ADU to your property as there are names for them. And while ADU is a technical term used throughout the industry, you may want to use one of the more creative names in your rental listing.

Keep in mind, your ADU does need to contain all the components you expect in a traditional home–they just come in a smaller package, making them perfect for singles and couples. Before you start planning your ADU, check with your local zoning board to find out exactly what is required to be legal in your area.

And don’t forget to figure in the non-monetary value you can gain by building an additional unit. As we said, ADUs are an iconic part of our housing heritage, particular when multi-generational living was more common. It’s an idea worth bringing back.

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