Photo of a clean and empty garage interior with graphics of furniture and the caption "How to Convert A Garage to an ADU."

With the escalating housing shortage in US urban areas, the popularity of accessory dwelling unit (ADU) garage conversions is on the rise. It’s not surprising since garage conversions are the most cost-effective way homeowners can add an ADU to their property. It sounds easy enough, but what are the actual steps in the process?

In brief, here is what you’ll need to do:

  • Research laws and permits in your area
  • Assess and repair the existing structure, if needed
  • Decide how large your ADU will be
  • Estimate costs
  • Have plans drawn up
  • Build
  • Have your completed project inspected

Undoubtedly, one of the most important steps you need to take when you convert your garage to an ADU is to make sure it’s legal.

If you already own an illegal ADU, there are ways to make it legal. But that’s a topic for another day.

One thing I’ve learned while researching the topic of converting garages to ADUs is that hundreds of thousands of conversions undertaken over the decades are not permitted. This means that they are illegal, technically.

And I firmly believe that unpermitted conversions present both a financial and legal risk. This risk is even greater if you want to rent out your ADU for a passive income.

The next essential step is to make sure the idea will work. If your garage is old, neglected, and run down, spending money on an ADU conversion might not be a good idea.

We also wrote up an article on some pros and cons of deciding to do a garage ADU here if you’d like to check it out.

If you do decide to go ahead, there are steps you should take to make it work.

I will run through the essential steps you should take to ensure your new garage to ADU conversion is safe, legal, feasible, and viable. Then, we can dive in and see what you should do to get started.

Benefits of Converting a Garage To an ADU

I understand that construction is one of the most significant investments for most American families. So, you want to ensure that you’re making the right decision before investing your hard-earned money in any construction project.

Garages have long been the underappreciated workhorses of our homes. Often, we use them as a storage space for rarely used belongings or as a refuge for our vehicles.

That said, how will you benefit from converting your garage to an ADU? Well, here are the main benefits of ADU garage conversion:

Increased property value

Converting your garage to an ADU will increase your property’s value. This is because ADUs are in high demand as they provide additional living spaces. You can rent out your garage conversion ADU or use it to house extended family members.

Flexible use of space

You can tailor your ADU to suit different needs. Whether you want to use it as a guest suite, a home office, an art studio, or even a dedicated rental unit, you have the flexibility you need with the space.

Save on construction costs

Garage conversions help save excavation and other construction costs since the project’s outer shell is already in place.


The materials used to build older structures may not be on the market. Therefore, your garage conversion ADU will have a unique outer shell that matches your primary residence.

Sustainable living

Converting a garage is a resource-efficient alternative to new construction, reducing the demand for new building materials and minimizing environmental impact.

Expanded living arrangements

The ADU will offer an invaluable solution to accommodate multigenerational families or an independent living space for aging parents.

Now that you know the benefits of converting your garage to an ADU, let’s dive into the process of achieving that.

1. Make Sure Your Garage Conversion Is Legal

Before you embark on an ADU garage conversion, you need to check what your local authority allows and what permits you need.

An increasing number of cities and states now have laws to improve the quality and safety of all forms of ADUs, so it’s important to check out your local rules and regulations.

Typically, legislation and local ordinances that govern ADUs cover everything from design and dimensional standards to material specifications.

While laws vary in different states and cities, most local authorities insist that an ADU incorporates a small kitchen area and bathroom. It’s important to take this into account when you do your initial budget because these infrastructure costs can mount up.

Photo of a gray kitchenette countertop complete with small appliances such as might be used in a garage ADU.
Remember that an ADU kitchen doesn’t have to be large to be functional.

First, you need to check if your home (and garage) are in a single-family residential zone and whether your local authority allows ADUs. If they allow ADUs, find out what you can, cannot, and must do to ensure that the conversion is legal.

Some states make it really easy for property owners to build ADUs and convert garages into ADUs. For instance, if you own a single-family home with residential zoning anywhere in California or Florida, there’s no question. You can convert your detached or attached garage into an ADU.

Plus, there are usually fewer requirements for a garage ADU conversion than for ADUs built from scratch. But be aware that this may not be the case if you plan to add a second floor to a garage rather than remodel what exists.

In some cities, including Los Angeles, you don’t have to provide parking if you convert an existing garage into an ADU— unless you increase the floor area. If you need to add a parking spot in LA, you don’t have to cover it. It can even be on a driveway as long as it is at least 18 feet x 8.5 feet in size.

In New Hampshire, many cities allow ADUs with limits and restrictions. But state legislation states that if local zoning ordinances don’t mention ADUs, homeowners can build them legally, as “a matter of right.”

More specifically, the New Hampshire law states that one ADU is allowed on properties with single-family dwellings, “and no municipal permits or conditions shall be required other than a building permit, if necessary.”

However, in cities with stricter limits and/or restrictions, homeowners will need a conditional use permit before building an ADU or converting a garage to an ADU.

In 2020, Portland, Oregon, became the first city in the US to allow homeowners to build two ADUs on a single property. The law includes garage conversions. Most city by-laws only allow one.

There is an important caveat. Some cities and counties in California and other states have stricter conversion requirements if you plan to rent the unit out. If you plan to go through this route, check out our article, “How Much Can You Rent Out Your ADU For?

So, be sure to contact your local authority to be sure you know what is legal and what isn’t.

2. Check The Condition Of Your Existing Garage

Many people planning an ADU opt for a garage conversion to save money. After all, unless you plan to add a second story to your garage, you will have an existing structure to start with. But you need to be sure you don’t have to rebuild the existing structure substantially.

If you do have to redo foundations or framing or replace sections of the roof, you might be better off starting from scratch somewhere else on the property. Think ROI.

Start from the foundation and work your way up. If you don’t have building expertise, it makes sense to bring in a professional to inspect the structure for you.


Most garages are built on a concrete slab to accommodate vehicles. However, the building codes for garages and habitable structures are different. 

You will need to ensure the slab is thick enough for the required specifications of your planned ADU.

Also, make sure that it hasn’t degraded over time. In some instances, it might be necessary to reinforce the slab by underpinning it. This involves digging a trench around the outside of the structure, laying steel rebar in the trench, and then pouring a continuous concrete beam around the garage.

Photo of a crack in a gray concrete house foundation.
If you see cracks like this in your garage’s foundation, you’ll have to make some repairs before you can continue with your ADU.

If the concrete has cracked or sagged, you may need to add another layer of concrete on top. In any case, you’re going to need a plastic moisture barrier between the foundation slab and the new floor of your ADU.

To minimize new construction, you can use a self-leveling underlayment designed for flooring finishes, including ceramic tile. Drytek Levelex is a good option. Knowing different garage conversion ideas comes in handy here.

Framing & Siding

If the exterior siding and interior cladding of your garage are intact, and there are no signs of sagging or collapse, chances are the existing framework is sound.

If there are signs that there may be problems, you might have to remove the siding or cladding to check.

Bear in mind that you will, in any case, have to open up parts of the existing structure, including walls, for inspection if you aren’t replacing them.

Garages may not have insulation in the walls. You will have to add insulation and ensure the walls are airtight, so assess their condition now.

Garage dimensions may also be smaller than what the building code specifies, which will require alterations. That scenario isn’t too bad. But if your garage is over three decades old, you might find that asbestos siding is still in place. Ideally, this should all be replaced.

This is a good time to think about how you will refigure the framing of your garage structure.

For example, you are going to need to install windows and doors. You will also need internal partitioning for a bedroom and a full bathroom. Meanwhile, living areas usually work well with open-plan kitchen areas. This will help you assess the existing structure and help you later when you establish a budget.

Roof Structure

Check the beams as well as the roof sheeting or tiles. Many garages have open beams, so a roof inspection will be relatively simple. If there is a ceiling, look for signs of dampness.

If the roof is leaking, you will obviously have to repair it. Additionally, you must ascertain whether leaks have undermined any other part of the structure, including the foundation and wall framing. 

Electrical and Plumbing

Can you easily route the electrical and plumbing from the city or utility companies to the garage? Does it have hookups or service for either of these already?

How about the sewer line? Do you know how it runs through the property?

Visualize how a house is built and all the components that go into that, and try to map out how each of these will connect to your new ADU where the existing garage is located.

3. Plan For Size

It’s often feasible to plan a garage conversion that simply involves the existing structure. But if your one-car garage is a minimal 20 feet x 10 feet, ensure this will meet your needs.

These dimensions certainly fit the concept of a tiny home. But even though the authorities will usually accept them for an ADU conversion, you might want to go bigger.

There will, though, be pros and cons to increasing the footprint of an existing garage structure when converting it. For instance, in LA, it will mean you must provide at least one parking spot for cars.

Photo of cars parked along a neighborhood street in Palm Desert, California.
If you turn your garage into an ADU in California, you have to provide another parking space.

4. Do A Thorough Cost Estimate

Apart from estimating the cost of any improvements to your existing garage structure, you will need to include plans (see below), as well as all the materials and construction costs for the conversion.

You must also consider insulation, heating (including water heating), plumbing, and energy costs.

Utilities will likely be the most expensive upgrades you will have to tackle, even if the local council doesn’t require a separate water or sewage disposal system for the ADU.

Often, homeowners are able to tie water, gas, and electric utilities to the existing meter in a house.

Alternatively, you may be able to install separate meters for the ADU. Even though this will probably be more expensive, it’s a good choice if you are going to rent your ADU out to a tenant.  

All of this will help you to determine whether your planned garage to ADU conversion will be feasible and ultimately viable and whether it will give you a good return on investment (ROI). 

Always add a contingency budget in case of any unforeseen issues.

I know that it’s difficult to estimate any kind of cost without plans. A useful tip is to get a company that offers quick-turnover, non-certified plans to draw up a quick computer-based plan that you can use for estimating purposes. You can work from this when you are ready to draw up plans for the city.

5. Prepare Site & Building Plans

Most councils will call for a completed site plan review for ADUs. In LA, for example, this includes:

  • A check that all the items they require are on the site plan, including a land use application checklist.
  • Floor plans that include the area where the ADU conversion will be.
  • Copies of building permits for the existing garage from the LA County Building and Safety Officer.
  • Copies of building description bank slips from the LA County Assessor for the existing structure.

Cities and counties will usually ask for copies of the original building permits to make sure that the existing structure is legal. If your garage was built without regulatory permits, you may not be able to legally convert it to a habitable ADU.

You will also need plans that show the layout of your ADU as well as elevations. Once the city has approved these, they will issue a building permit for your conversion.

6. Have Your ADU Inspected

Photo of a tiny brown model house with a person behind it looking through a magnifying glass.
You’ll need one final inspection before anyone moves into your new garage ADU, usually called the Certificate of Occupancy (CO).

Inspections for garage conversions take the same logical sequence as they do for new builds. The usual inspections will be:

  • Existing parts of the structure that will remain as is, e.g. the walls or roof structure
  • Foundations
  • Structural tie-downs that hold walls to the concrete foundations
  • Insulation
  • Electrical and plumbing installations, including solar and any off-grid installations

Once the local authority gives you the final stamp of approval, they will issue a certificate of occupancy, and you, members of your family (parents or children), or new tenants can move in.

Crucial Questions to Consider Before Converting Your Garage to an ADU

Before converting your garage into an accessory dwelling unit, it’s essential to contemplate some critical questions to ensure a successful project.

Converting a garage into an ADU is a significant undertaking that involves not only financial and legal considerations but also practical, aesthetic, and logistical decisions. Therefore, here are the key questions to consider before embarking on this journey.

How Much Do You Need for the Project?

Finances come in handy when embarking on any construction process. 

Luckily, garage conversions are cheaper than attached and detached ADUs. This is because you’ll not have to incur expenses for some construction practices like excavation.

Also, garage conversions offer a higher ROI.

That said, you should ensure you have enough capital to build the ADU from start to finish. Of course, it depends on the remodels needed for the conversion. 

Some of the things to remember when outlining your budget include:

  • The size of the ADU (The cost is determined based on square footage)
  • Required remodels
  • Permitting costs (Find out how to apply for an ADU permit)
  • Utility requirements
  • The needed materials and labor
  • Potential unexpected costs

What Is the Duration of the Project?

Knowing the duration for converting your garage into an ADU is vital for budgeting. You want to know how much you’ll pay for labor and other logistics.

In most cases, detached and attached ADUs take between 10 and 18 months to build. This incorporates the initial stage of talking to a designer and obtaining the necessary permits for the project’s completion.

Luckily, garage conversions are much faster. If your garage is in excellent condition, you will have a completed garage conversion ADU in seven to nine months. 

Some key factors to consider when it comes to assessing the duration include:

  • The current condition: Is your garage in a damaged condition? Does it require just a few repairs? Such questions will help you determine the duration.
  • Expansion work: Expanding the garage into part of your backyard means more duration than remodeling the garage to an ADU.

Can You Build an ADU Over the Garage

If you don’t want to fully convert your garage into an accessory dwelling unit, you might have contemplated building an ADU over it.

While building an ADU over your detached garage can be advantageous if you want to retain your garage for vehicle storage, the process is more costly. 

You should expect to spend double the cost of a garage conversion ADU. This is because you’ll have to build a new roof and make the flooring, among other utilities.

Additionally, you should consider the maximum height of buildings based on your zoning ordinances. You don’t want to exceed the stipulated building height for your zone.


The process of converting a detached garage to an ADU isn’t difficult. It is also cost-effective, and because you are starting with an existing structure, it will be much quicker than building an ADU from scratch. Just be sure that the garage is worth converting.

All in all, a garage to ADU conversion can be an ideal way to create a small, separate home on a single-family residential property in most parts of the US.

Unless you really do need and use your garage for your vehicles, this is precious living space that you could easily convert into affordable housing.

If you follow the steps we have provided, we are certain that your garage conversion will be fulfilling and result in a sound ROI.  

If you’ve got your heart set on building an ADU, you may need designs and plans. It’s your lucky day because our articles on Where To Find ADU Plans and 8 Different Resources To Help Design Your Perfect ADU are here to help!

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