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Challenges and Costs of Building a Detached Garage On Your Existing Slab

Updated: Mar 18

Site-built detached garages can be a great addition to any home. They allow for more storage and safekeeping of your valuables and toys or can be used as a workshop/getaway. In some cases, a person might have an existing slab on their property and think they want to use it for their new detached garage to save some money. This article will explore why that is typically not the case and can cost you more in the long run. Everyone knows when it comes to your home, if you don’t have a good foundation, it can cause many problems. The same is true for any site-built garage or other accessory building.

Size of the slab

Metal buildings typically go in 3′ increments, while wood buildings go in 4′ increments. You will want to make sure whichever type of detached garage you are building that the slab has the correct dimensions. This is so important because the outside of the building must line up with the outside of the slab. Unless it has a threshold, this is to ensure the detached garage is properly sealed and any warranties can still be honored. You also run the risk of damaging the interior and any belongings in the building if you do not have your detached garage adequately sealed.


If you do not have the ideal slab size for a detached garage, you can have it customized to fit your slab depending on the builder, but this will be more costly to make it work because you are not using a standard size. Therefore more labor and cutting down of materials will happen.


Square and even dimensions

It is essential that a slab is built correctly, square, and is high enough for the grade of the land. The grade of the land can be fixed if that is your only issue, but it would add to your overall cost.


When we talk about ensuring the slab is square, we are not necessarily talking about the shape. Let’s say you have an existing slab and its dimensions are 16 feet wide by 20 feet long. You will want to measure corner to corner on opposite sides and ensure the measurements are the same. If the measurements are not the same, the slab is not square, which can happen with non-engineered slabs. If the slab is not square, it can cause a significant price increase on the labor side when constructing the roof.


You will want to ensure the slab is even from side to side and end to end to ensure a properly constructed detached garage that will not give you issues or require additional time and resources to correct in the future. An unlevel slab can significantly affect the structural integrity of any structure you are building; therefore this is a leading factor of post-purchase / build costs for ownership of a detached garage.

In summary

While it may seem like a cost-effective way to go about getting your dream detached garage, utilizing a slab that you already have on your property can add up over a long time. Once you add up additional labor and time correcting problems, it takes away some of those savings, if not all of the money saved. The other option would be to either tear up the old slab and get an engineered slab built just for your detached garage or get that same slab built elsewhere on your property.


If the purpose of your garage is something more than storage, for example, you will want it finished out. Going with a metal building will provide some challenges. Since the R-value is not there naturally, you will have to frame the interior to insulate the walls. While this is possible, ultimately, it’s going to take away from your usable interior space. This will need to be planned for when planning your garage and which route you choose to take.

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