Photo of a modern house with a large, landscaped yard. There is a red arrow pointing to the words "ADU here?"

You might wish to expand your property without necessarily adding on more to your house. One possible solution is building an accessory dwelling unit or an ADU. Ideally, you’d like the ADU in your backyard or elsewhere on your property. Is this legally allowed?

Whether you can build an ADU in your backyard or on your property varies on a state and city/town zoning, ordinances, and building code. Even if an ADU is allowed where you live, you often need a permit, and you might be limited on the size of the ADU, along with other specific requirements as well.

If you’re eager to learn more about building an ADU on your property, this article will be your guide. Ahead, we’ll explain the types of ADUs, whether you can build one in your yard, and which permits and zoning laws you must abide by.

What Is An ADU? ADUs 101

Let’s begin by talking about ADUs in detail.

As we touched on in the intro, ADU stands for “accessory dwelling unit.” ADUs go by many nicknames, such as secondary suites, backyard cottages, in-law units, and granny flats.

Regardless of what you want to call it, an ADU is a small house that’s on a residential property. The ADU is not the main house. It may or may not be connected to the main house.  

The size of an ADU varies. At the smallest, an ADU might measure 600 square feet. Bigger ones could be 1,200 square feet or over. Considering that the average apartment is 800 square feet, you’d have as much room in an ADU as you would in a typical apartment.

An ADU is designed as its own house. It should have entry and exit doors, a living room, a bathroom (possibly more than one), and a kitchen. This way, the residents in the ADU do not have to come into the main house (or vice-versa) to use the bathroom or kitchen.

ADU Types

Not all ADUs are the same. As you plan where yours will go, keep the following five types of ADUs in mind.

Basement Conversion ADU

A basement conversion ADU is one in which you don’t build anything extra on your property. Instead, you’d convert your basement into a comfortable living area. The construction work for a basement conversion ADU might entail adding flooring and insulation as well as adding doors and windows.

Photo of an attractive finished basement with carpeting, a couch and a kitchen area that could function as an ADU.
Converting your basement is one way to add another housing unit to your property.

Typically, to be considered a legal ADU, you will need a separate entrance, and a way to close it off from the main house. This way it can be treated as a physical separate apartment.

Garage Conversion ADU

In the same vein as a basement conversion ADU is a garage conversion ADU. If you don’t use your garage for much else besides storage, you can remove everything in there, transfer it all to a storage unit, and repurpose the garage into its own dedicated living space.

This is very popular here in Saint Petersburg, FL, where there is a general shortage of housing due to general population growth, having lack of land to build on (the city is a peninsula), and the influx of out-of-state people moving in.

More than likely, before your garage would be considered livable, it would need work, à la the basement construction we highlighted in the paragraphs above. Both garages that are attached to the home and those that are detached are eligible for a garage conversion ADU.

Above-Garage ADU

Here’s another garage-related ADU option. If you have a two-story house with a single-story garage, you can add on a unit above the garage.

the inside of a small above garage adu apartment, with white walls, a short cathedral ceiling, showing a bed against the wall and a brown leather couch
I just stayed in this AirBnB garage conversion ADU during my last trip to Denver for a wedding. Even though small, they make great short-term rentals or extra guest rooms!

Attached ADU

An attached ADU, also known as an addition ADU, is a separate section that’s still connected in some way to the main house or building.

Photo of an older red brick house with white trim, with an addition being built onto it. Additions are one way to build an attached ADU.
An ADU can be an addition to the existing house on a property.

Detached New Construction ADU

The last type of ADU is a detached new construction ADU. This ADU is like what we described first, as the ADU is its own mini house or apartment. The ADU is separate from the main property and would have all its own amenities to prevent crisscrossing between the properties.

Where Can You Build An ADU? In Your Own Backyard? On Your Property?

If a detached ADU or an addition is right for you, it’s time to determine where it will go. Going back to what we talked about in the intro, you’d prefer to build the ADU in your backyard. You have the space there for a detached new construction ADU or perhaps an attached ADU.

If you can’t build the ADU in your yard, you might have room somewhere else on your property. Are you allowed to do this?

More than likely, yes. The rules surrounding the legality of ADUs vary on a state by state and even a city and town basis, so we highly recommend looking up the rules in your neck of the woods.

As a whole, though, in the United States, ADUs are viewed quite favorably. In California, laws were passed in 2017 that made it harder for the government to put restrictions on ADUs. Those laws were further amended in 2020 to relax the rules even further.

For example, per the 2020 rules in California, you can typically build an ADU or a junior accessory dwelling unit (JADU) on your property. The ADUs that are exempt from state laws can be up to 800 square feet.

In New Hampshire and Vermont, local laws encourage building ADUs.

In other states or cities, ADU permit rules can be a lot more restrictive. For example, even though the state of California welcomes ADUs, Los Angeles doesn’t to the same degree. According to the County of Los Angeles website, you can’t have a garage conversion ADU unless you meet residential building codes for insulation, doors, and windows.

We have to mention Hawaii as well, as they do things a little differently. Besides ADUs, Hawaii also gives you the option to build ODUs or Ohana Dwelling Units.

Implemented into Hawaii’s 1981 zoning code, ODUs were promoted to make houses more affordable, save green space in the state, and increase the number of homes available in Hawaii without the need for government subsidy.

ADUs are less restrictive in Hawaii than ODUs, as the latter require zoning codes and permitting.

Permits, Development Standards, & Zoning Laws That Dictate ADU Placement

So by now you’ve checked the local rules and you’re happy to find that you can build an ADU on your property. Before you get started, make sure that you’re on the right side of the law!


In the states in which ADUs are embraced, permits are required less often. Many states can require you to get a permit, though. In those states, building and living in an unpermitted ADU is illegal. If you go without a permit, all sorts of bad things could happen.

Photo of a building permit and various papers with a tape measure lying across them.
If your locale requires a building permit, make sure you get one!

For instance, you might be fined, possibly thousands of dollars. In some instances, you could lose your homeowner’s insurance since building an unpermitted property is considered voiding the insurance.

In a worst-case scenario, your ADU could be demolished, or you could receive threats of demolition until you get the proper permitting.

The resource Accessory Dwellings has compiled all the regulations on ADU permits in cities and towns across the country, which is hugely helpful. Look for your neighborhood and see if you need a permit to build an ADU. If you do, please don’t forego one. 

Zoning Laws

Another legality to concern yourself with when building an ADU is the zoning law(s) in your city or town. Zoning laws dictate whether you can build one ADU or several, what type of ADU is allowable, and the size of it.

Some zoning laws only apply to the size of your ADU while others dictate the style of ADU. Thus, you need to familiarize yourself with all the zoning laws in your city or town before you even so much as pick up a hammer to start building.

If you’re part of a homeowner’s association, the HOA will have its own set of requirements your ADU must meet before you can start building. This narrows the scope of your ADU project even further.

Development Standards

Development standards are a third legal area to concern yourself with as your ADU plans get closer to reality. A development standard is a set of criteria concerned with land use or subdivision as well as building design.

As you might have guessed, development standards are not uniform across cities, towns, and states. If the development standards in your neighborhood restrict ADUs to no more than 500 square feet (even if that is rather small for an ADU), then so be it, that’s the allowable size of your ADU.

If you can’t build an ADU longer than 75 feet, you must abide by that rule as well. Usually, there will be other specific rules on the ADU’s maximum allowable height and width.

Double-check that your city or town doesn’t apply development standards to the size of the lot in question, as some do. In those instances, your lot must be 30 feet wide with 3,200 square feet available to be eligible for an ADU.

ADU Pros & Cons

If you’re debating whether you should add an ADU to your property, allow us to help you make up your mind with this pros and cons section.

The Benefits Of ADUs

Let’s start with the advantages of adding an ADU to your home, whether it’s part of your existing home or a new construction detached ADU.

More Affordable Housing

It’s no secret that house prices keep increasing and increasing. This leaves a lot of people out in the cold. Perhaps they live with their parents for longer than they want, or the best they can afford is an apartment. These people aren’t spending money for something they own like they would with a mortgage. They’re only paying to rent.

ADUs are often a far more affordable proposition for people in these situations. That’s why so many ADU state legislations have passed across the U.S. The cities and towns that freely permit ADUs are facing affordable housing crises (as is much of the country), so ADUs are a suitable solution.

Closeup photo of a key in a glass door with a golden house-shaped keychain.
ADUs may be the key to enough affordable housing for everyone.

No Need To Buy More Land

Another perk of ADUs is that they can be built without the need to acquire more land. Land shortages are already a growing problem in the U.S., and the issue will likely snowball in the years to come.

Since a property with an ADU is two houses on one lot, it helps the available land go further in terms of housing.

Keep Loved Ones Close By

ADUs are also a great way to keep those you love close to you. Whether it’s your own parents who live in the ADU, your in-laws, friends, or even adult children, you’re close enough that you can spend time together, yet everyone has their privacy.

Small & (Relatively) Easy To Build

Compared to building an entire home, which takes hundreds to thousands of manhours and costs significant amounts of money, building an ADU is a lot easier to do. That’s due to the smaller, more manageable size of these dwellings.

Can Increase Your Resale Value

According to a report entitled Understanding and Appraising Properties with Accessory Dwelling Units published by The Appraisal Journal, in a survey of ADU homeowners in Portland, Oregon, the home resale value of a property with an ADU jumped by 51 percent. 

The Downsides Of ADUs

You should mull over the following considerations before you commit to building an ADU.

Can Be Very Expensive

Although building an ADU is easier since it’s a smaller dwelling, that doesn’t mean the work is cheap. Better Homes & Gardens quotes the price of a new ADU at anywhere from $30,000 to $300,000 and up. You’d need a sizable budget (and probably some financing) for an ADU!

Must Follow Rules

As the last section exemplified, ADUs are not without their laws and restrictions. In some parts of the country, the rules are more relaxed, but that’s not true of all states. If yours is a state with tight ADU rules, reading up on all the permits and development standards can be headache-inducing.

Increased Property Taxes

Since an ADU technically expands your property, adding one to your home can raise your property taxes. Deductions are available as you use the various amenities on the new property, but the deductions might not be enough to completely offset the higher taxes.


Building an accessory dwelling unit or ADU on your property is usually allowable, and that goes for your backyard as well. But the rules do vary significantly based on which part of the country you reside in, so make sure to do your homework.

ADUs can increase your home’s curb appeal and provide affordable housing, but your property taxes could go up for your troubles. We hope this guide helped you decide whether an ADU is right for you!

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