DIY Add-Ons to a Cement Porch

Dig out a rectangular area adjacent to the concrete porch at the desired add-on location. Dig the area to a depth 2 to 3 inches deeper than the depth of the original concrete porch. Dig out three sides (minus the side where the existing concrete sits) at least 2 inches farther than the desired add-on size.

Clean the exposed edge of the existing concrete by scrubbing it with concrete cleaner. Rinse the concrete with fresh water.

Rake the bed of the dug area to level the soil, then compact the soil by tamping it repeatedly with a hand tamper.

Fill the bed with 2 to 3 inches of gravel. Tamp the gravel.

Surround the perimeter of the dug area, except for the side where the existing concrete exists, with form boards. Secure the form boards into the ground with nailed-on stakes. Position the tops of the boards level with the top of the existing concrete, propping them up with rocks if necessary. Position the form boards so they slope down slightly from the existing concrete, with the ends of the boards farthest from the existing concrete sitting 1/8 inch lower than the ends of the boards that sit against the existing concrete, which will allow water to run away from the existing concrete. Backfill behind the boards with soil.

Drill 6-inch-deep, 1/2-inch-diameter holes every 12 inches across the center of the exposed, existing concrete. Insert 12-inch-long, 1/2-inch-diameter pieces of rebar into each hole.

Place wire mesh in the bed of the dug area. Cut the wire mesh to fit with wire cutters. Elevate the mesh a few inches by setting it on larger rocks.

Fill the dug area with concrete. Smooth the surface of the concrete by lightly skating a metal float across the top of the concrete in slight arcing movements.

Slice the concrete where it meets the form boards with a knife with at least a 2-inch-long blade, to ensure the boards separate from the concrete when removed. Run a concrete edger along the entire perimeter of the concrete to round the edges.

Cover the wet concrete with plastic. Leave the plastic for at least two days until the concrete cures. Occasionally spray the concrete lightly with water during the curing period.

Remove the plastic. Pull out the form boards. Fill the gap around the concrete with soil.