How to Make a Basement Window Bigger

Obtain a permit from your city or county building department. Familiarize yourself with the specific building codes for the project, including placement, size and window well requirements.

Dig down from the existing window to create a hole large enough to accommodate the new window, plus an additional 6 inches. Dig the hole at least 36 inches away from the wall to establish the footprint of the window well. Mark the cutout area on the inside and outside of the wall for the perimeter of the new window, adding 3 1/2 inches to the width of the rough opening and 1 3/4 inches to the height to accommodate the frame.

Cut along the lines with a concrete demolition saw, which is available to rent at most tool or home improvement stores, while wearing eye protection and a dust mask. Cut a line 1/4 inch deep initially to get an outline that is straight and accurate. Cut a deeper line on the next pass. Cut the window area both inside and outside to aid in removing the concrete blocks.

Break away the concrete blocks with a hammer once the cuts are made. Support the existing window as you remove the blocks underneath until it is loose enough to remove from the hole. Chisel any remaining border or pieces of concrete to clean out the opening for the new window.

Stuff newspaper into the holes of the exposed concrete blocks as a barrier. Fill the holes with concrete and place plastic sheeting over the wet concrete.

Cut a 2-by-10-inch board to fit the thickness of the wall for the window sill. Screw 3-inch deck screws into the wood sill. Position the window sill at the base of the opening so that the screws go into the wet cement in the bricks to hold it in place.

Use the existing header as the top portion of the frame, or replace it with new wood if desired. Cut side boards so they fit snugly between the sill and header. Use a hammer to pound the side pieces into position. Screw the side pieces to the header and sill once you ensure that the frame is level and plumb.

Predrill holes along the side of the frame going into the concrete blocks with a masonry bit. Attach the board with 3 1/4-inch concrete screws.

Fill in the gap between the foundation and the wood frame using exterior caulk.

Position the new window into the opening. Use shims to level the window inside the frame. Nail the shims into place and take the window back out of the opening.

Caulk around the wood frame to help create a seal for the window's nailing fin. Reposition the window in place, checking to ensure it is still level. Screw or nail the window in place through the nailing fin according to the manufacturer's directions.

Create a bevel on the edge of the exterior ledge so water runs away from the window. Place the mortar on the ledge and use a trowel to create the beveled edge.