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How to Convert Your Garage into an Accessory Dwelling Unit

ADU stands for Accessory Dwelling Unit. An ADU is just an additional living space occupying the same lot as a detached single-family home.

Turning your garage into an ADU is an increasingly popular option for adding to the total living space of your home.

These spaces are ideal for housing additional family members, such as children or visiting relatives, or renting out for additional income.

If you are interested in converting your existing garage into an accessory dwelling unit, there are a few factors to consider.

How Much Will it Cost to Convert my Garage to an ADU?

Converting a garage into an additional dwelling unit is by far the cheapest method of obtaining one. However, it may still cost between $50,000 and $100,000.

ADUs and granny flats tend to be even more expensive per square foot than a traditional home, because the cost is based on materials and labor rather than empty space. Since ADUs have all of the infrastructure of a house with less internal space, the proportion goes up.

There are many effective ways to minimize the cost of converting your garage into an additional dwelling unit. Having a clear vision for the final product from the get-go, as well as a realistic evaluation of the current state of your garage, is by far the best way to avoid unexpected costs from popping up by surprise.

You can also take out a home loan to finance the construction of an ADU, just like you can with any other form of home renovation. 

How Long Will it Take to Convert my Garage to an ADU?

If you are converting an existing garage into an additional dwelling unit, you can expect it to take between 6 and 12 months from the time of your first talks with your contractor to the finalization of the habitable space.

If your garage is in particularly good shape, you may be able to skip pouring a new foundation, reinforcing the frame of the walls, and some other steps, which can save time and money.

If you are planning on building an ADU from scratch in the space your garage once was, you can expect it to take a bit longer, typically 8 to 14 months. Given the length of time required, make sure that you have a clear vision for the project from the beginning.

Changing decisions and reworking a build in progress will quickly slow things to a halt and waste yet more time and money.

Can I Build a Granny Flat Above my Garage?

A granny flat is a detached living space for one or two persons located on the same lot as an existing home. Granny flats are very popular for adding a second story living space on top of your garage.

This is a good way to maximize space efficiency, however there are also a number of additional challenges in adding a second story to your garage.

Most importantly, a one story garage was never meant to support a second story. Adding one will likely require adding additional or superior infrastructure to the garage.

This can be especially challenging as the second story needs support from the center, which would force a beam to run down the middle of the garage.

You will also need to match the new building elements to the old elements, which can be more difficult the older your garage is. The second story will also need a new roof, which a conversion would not require.

If the garage is attached, you will also need to match the granny loft to the rest of the house. This can potentially complicate the installation of plumbing and electricity.

When Should I Tear Down My Garage and Start From Scratch?

As a general rule, so long as there is even a tiny piece of the original garage that can be salvaged, it will be cheaper and easier to build on that instead of starting over.

Even old and worn down garages often still have solid, intact framing that can be reinforced for the ADU.

Of course, that only applies if you actually want to keep the structure of your garage. If you would rather wipe the area clean and build something brand new and completely different, say a much larger ADU that occupies the garage’s original space, then you will have to start from scratch anyway.

How Do I Bring my Garage Up to Code for Converting it to an ADU?

Since garages are not built with the purpose of being lived in, you will likely have to make some upgrades to the infrastructure of your garage to keep it compliant with building codes. Below is a list of some of the most common upgrades needed to properly make a garage habitable:

  • Ceiling joists to support the installation of new drywall
  • A moisture barrier between the slab concrete garage floor and the flooring installed on top
  • Insulation in the walls and roof of the garage
  • A fully functioning electrical system with sufficient outlets
  • A functioning plumbing system capable of bringing in clean water and removing waste water
  • Replacing old glass windows with modern energy efficient windows. You may also have to replace any old doors with energy efficient variants
  • Depending on the zoning laws in your area, you may need to make upgrades or improvements to the exterior of he building

How Will an ADU Affect my Property Taxes?

The treatment of additional dwelling units in tax laws varies between jurisdictions, however generally the value of your property is based on an assessment of the value of your home.

After building an ADU, the value will be added to the total value of your home. The value of the ADU is based on the original evaluation of your primary house, rather than the cost of building the structure.

If you are converting your garage, you may end up paying less in property tax. Since the original assessment of your house should have included the existing garage, the difference realized by adding an ADU will be smaller than if you had built the ADU separately.

Other Considerations

Once you’ve figured out your vision for your ADU, planned your budget, and made your design, you still have a few final decisions to make about your new property.

Separate Utilities

When installing the electrical and plumbing systems, you will have the option of tying it into your existing home or having separate meters for the ADU’s utilities. If you are renting the property, the latter may be the cheaper option. If you are using the property yourself, keeping things bundled is more likely to be the cheapest.

Separate Address

In some jurisdictions your ADU will automatically be given a separate address from your primary residence. In other jurisdictions it may be optional. The choice to have a separate address or keep the same as your primary residence is up to you.


In this context, a setback refers to the distance of separation between a structure on your property  and your property line. If you are building a new structure, the required setback will likely be a few feet. Likewise, any additions made to your ADU have to follow the same setback rules.

If you are converting a garage, you may be granted more leniency. Even if the garage does not comply with setback rules, you do not have to move or rebuild it when converting. As stated above, however, any extensions or additions to the garage will need to be properly setback.

Should I DIY or Hire a Professional?

Many people prefer the idea of converting their garage themselves to save money and to feel more connected to the project. The choice is an entirely personal one, and no one knows your skillset better than you. However, consider the below list of skills that will be required to complete the project on your own:

  • Electrical engineering: You will need to fully install an electrical grid in your garage, within the walls. It will need to be fire-safe and up to code.
  • Plumbing: You will need to install plumbing in the walls of your garage. It will need to connect to both a source of water and a drain. There may also be codes for ensuring the water quality.
  • Carpentry: You will need to know how to work with wood to build the frame, cut and install drywall, and finish many other aspects of the project.
  • Physics: Although it may sound unnecessary, it is essential that you have an understanding of the physics of your build. You need to know the weight of your ADU, the distribution of that weight on the top of your garage, and how to build the ADU to withstand heavy snow, rain, winds, earthquakes, and all other natural disturbances it will be subject to.

A professional contracting service can ensure that all of these challenges are overcome safely. They will also bring along more than one worker, so the work can go faster.

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