If you live in a small house, you can increase your floor space by removing a stairway that goes to a seldom-used area, such as a crawl space or a basement. By replacing a stairway with a trap door, you can maintain access to the area, but gain more useful space in your home. A properly installed trap door is nearly invisible, but there to be used when necessary.
Cut a 12-by-12-inch hole through the floor at the center of the location where you are planning to put the trap door.
Using this hole as an access point, define the direction and location of the floor joists underneath the floor. Size and orient the trap door hole so that its sides are cut over the centers of the joists.
Set the blade of a circular saw to a depth that matches the thickness of the flooring. Cut out the hole with the circular saw.
Measure the distance between the inside faces of the joists. Cut two pieces of 2-by-6-inch planks to this length. Install these two pieces between the joists, so that their centers are directly under the edge of the hole. Go into the basement or crawl space and drive two 3-inch-long screws through the joists and into each end of these 2-by-6 planks. You now have a hole in the floor with a supporting frame underneath it on all four sides.
Cut a piece of 3/4-inch-thick plywood to a size that is 1/8 inch smaller than the width and length of the hole in the floor. Test fit the piece of plywood in the hole.
Cut two notches for hinges on one side of the plywood and at the corresponding locations on the edges of the hole in the floor.
Screw hinges onto the piece of plywood and onto the edge of the hole in the floor.
Install a recessed pull into the end of the trap door opposite the hinges. Drill a hole the diameter of the pull fitting, fit the pull into it and screw it on. A recessed pull includes a hinged ring that sits in a fitting that is attached to the wood. When not in use, it is flush with the surface of the floor. When you pull on the ring, it extends from the floor and you can lift the trap door.