Dog and Cat Doors
Door Mounted pet doors were the original doggy doors. Today many doors are aluminum, steel, or fiberglass and the fact that the door isn't wooden is discouraging for people who want to install a dog door. Most of these newer doors have decorative recessed panels as well, making the situation worse. If you actually take a straight edge and lay it across the door, most of these newer doors are flat all the way across, with the exception of the recessed part that is meant to look like routered wood, and these can be filled with caulk.
So if you want to install a pet door in a fiberglass, steel, or aluminum door, it's really pretty simple to do. The doors come with a template that you trace out onto the surface of your door. Then you drill a hole at the corners and connect the holes with a jigsaw, using a metal blade instead of a wood blade. The pet door will have a frame for both sides of the door, and the frames screw together with bolts.
Another common issue is getting a pet door big enough for a giant breed dog like a Mastiff or Newfoundland. We have doors to fit the very largest breeds, but you won't find a 36" tall pet door for your 36" tall Great Dane. What you will find is a 27 ½" tall Giant Hale door or a 24" tall Plexidor or Ruff Weather. The way that works for a 36" tall door is to mount the door higher up off the ground. at 9" off the floor the top of the Giant Hale will be 36 ½ inches off the floor. The dog can easily step over the 9" threshold height. In general, smaller cats and dogs can push open the larger flaps without trouble, until you get down to around 8 lbs. If you have a big dog and a teacup chihuahua, you may need to install a small door like the Ideal Cat Flap in the same door as your giant one to accommodate both at the same time.