12×20 Portable Garages

In most areas, a 12×20 garage will require a permit to install. Many places require a permit for any shed or garage size above 120 square feet.  If you live in North Dakota, check out our article on shed permits in North Dakota. This article provides various resources for applying for shed and garage permits.

A 12×20 garage foundation generally requires 3.5 cubic yards of gravel. Since we recommend making your foundation a foot longer and wider than the garage on all sides, you will need a pad that is 13×21. We also recommend that you make it at least 4″ deep. These are the assumptions that we used to arrive at the above calculation for gravel needs for a 12×20 garage pad.

Please note that these calculations will only work if your site is level. If you are installing your garage on a slope, more gravel will be required. For more site prep information, check out our site prep page.

The total footprint of a 12×20 garage is 240 square feet. Keep in mind that a footprint of 240 square feet does not equate to 240 square feet of usable interior space. To get a calculation of usable interior space, you need to remove at least 5″ from all sides. This allows for 3.5″ for the wall studs, and up to 1.5″ of sheathing and siding on the outside of the building.

Thus, the usable interior space for a 12×20 garage should be calculated by multiplying 11 feet 7 inches by 19 feet 7 inches, resulting in approximately 216.5 square feet of usable floor space.