How to Dig a Foundation for an Addition

Meet with a building inspector for your area to determine the necessary depth of your foundation. The required depth will vary according to your location, as you’ll need to dig both the hole for the slab deep enough to hold the slab and the gravel drainage layer beneath the slab, as well as the deeper footer around the edges of the foundation that provides support for your walls and roof. The footer must extend beneath the frost layer for your area to avoid cold snaps causing heaves in your concrete slab.

Call the Common Ground Alliance by dialing 811 on your phone to find out if there are any utility wires or pipes running beneath your proposed addition location. If there are, you’ll need to find another location for your addition.

Mark the location of your new foundation on the ground using orange landscaping paint. Stake out the marked lines to help you keep them straight. Use a laser level to mark the line, and then position a wooden stake every 5 feet along the lines. Drive the stakes a few inches into the ground with a mallet. Tie a string around the tops of the stakes to create a straight line that follows the laser of the level on each side of your foundation.

Use a backhoe to dig your foundation. Start the hole near the existing foundation and work your way outward toward the marked edges. Dig the slab space deep enough to contain the concrete and the gravel layer, and then dig the footer along the edges of the hole down to the depth provided by the building inspector. Your footer should be at least twice the width as the wall you intend to place onto it, and positioned so that the center of the footer sits at the center of the wall line.

Level out the base of the foundation with the spade to remove any teeth marks left by the teeth of the backhoe bucket. Remove any soil remaining against the wall of the existing foundation left by the backhoe. Double-check the depth of the foundation hole for the slab and the footer to make certain each is at your desired depth. Make any small adjustments needed with the spade, or use the backhoe if more than an inch or two in adjustment is required.

Lightly dampen the base of the foundation hole using a garden hose. Run a vibratory plate compactor over the floor of the foundation hole to compress the soil. This will create a solid surface ready for filling with gravel and concrete.