Can I install a pet door under a window? You can install a pet door under a window like a regular wall installation if there is enough room. Small and medium size doors usually fit. Depending on pet door model, you may need more room than just to fit the door – make sure you have room to attach the security plate. This isn’t a problem with the PlexiDor since the security plate screws onto the frame, but many other types of doors have a panel that loads from the top. This can become a problem if there is a window sill in the way.