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 U.S. 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 ADU that is illegal, 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, it might not be a good idea to spend money on an ADU conversion. 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’m going to run through the essential steps you should take to make sure 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.

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 the laws in different states and cities vary, 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.

The first thing you need to check is 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 homeowners to build ADUs and convert garages into ADUs. For instance, if you own a single-family home with residential zoning anywhere in California, there’s no question. You can convert your 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 L.A. you don’t have to cover it. It can even be on a driveway, as long as it is at least 18 ft. x 8.5 ft. 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, then 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.” But in cities with stricter limits and/or restrictions, homeowners will need a conditional use permit before they can build an ADU or convert a garage to an ADU.

In 2020, Portland, Oregon became the first city in the U.S. 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. There are cities and counties in California and other states that have stricter conversion requirements if you are planning to rent the unit out. (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 that you don’t have to substantially rebuild the existing structure.

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.

Foundations

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

You will need to make sure 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.

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’re going to have to add insulation and make sure that 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 more than 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 are going to refigure the framing of your garage structure. For example, you are going to need to install windows and doors. You will also need some internal partitioning for a bedroom and bathroom. 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 will need to 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, make sure 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 L.A., 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’ll 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 labor costs for the conversion. You must also take insulation, heating (including water heating), plumbing, and energy costs into account.

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 kinds of costs 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 L.A., for example, this includes:

  • A check that all the items they required 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 L.A. County Building and Safety Officer.
  • Copies of building description bank slips from the L.A. 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 the 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 (C.O.).

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.

Overview

The process of converting a garage to an ADU isn’t difficult. It is also cost-effective and, because you are starting off with an existing structure, it will be a lot 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 U.S. Unless you really do need and use your garage for your vehicles, this is precious 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 will result in a sound ROI.  

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