Creating your own cat door in an interior hollow-core basement stairway door is not such a difficult task, if you exercise patience and focus. You might want to enlist a helper for some of these steps, depending upon your level of strength and experience.
- Remove the basement door from its hinges. Using a flat-bladed screwdriver and hammer, gently tap the hinge pins out of the hinges, holding onto the door to ensure it does not fall over when released.
- Layout and cut cat door opening. Mark a square or rectangle on the door. The opening should be approximately two to three inches above the bottom of the door, and it needs to be about two inches larger than your largest cat, all around. For example, if your cat measures around six inches across, your opening needs to be about eight inches across. Likewise for the height of the opening.
Drill a 3/8" starter hole through the door in a corner of the proposed opening. Using a jigsaw, cut out the opening as marked. Remove the piece from the opening, and sand the edges of the opening smooth with medium grit sandpaper.
- Make a simple frame for the cat door opening. Measure the inside dimensions of the cat door opening. Using 1x2 furring material, construct an "L" shaped frame to fit the inside dimensions of the opening. In most cases, a 1x2 piece of furring will end up flush with the other side of the door thickness. Using 1x2 furring material, construct a flat frame to fit over the other side of the door and flat against the "L" frame. Screw the flat frame to the "L" frame with 1 1/2 inch deck or drywall screws.
- Make and install a simple mini-curtain. From an old piece of cloth, such as an old towel, curtain, sheet, etc., cut a piece of material that is about two inches larger than the cat door opening, all around. Using a sewing machine, hem and stitch all four sides, leaving the top side open at either end of the hem to act as a "rod-pocket".
Cut a piece of 3/8" dowel to about the length of your cat doorframe width, outside dimension. Drill a clearance hole for deck or drywall screw diameter near each end of the dowel, and in line with each other. Place the mini-curtain onto the dowel using the "rod-pocket." Using two small nuts as spacers, place screws through the dowel; place nuts on the screws on back side of the dowels, one on each screw. Screw this assembly into the cat doorframe on the stairwell side of the basement door.
- Reinstall the basement door. Place the basement door back into its frame and hinges. Put hinge-pins back into hinges. Test door operation, swing and latching.
Now it is up to your cats to find the new cat door and learn how to use it. It usually does not take long for them to get it, especially if there is something in the basement that they want to get to; like perhaps their litter boxes or food?