I put a doggie door in the wall from the dining room out to the back patio, one of those double-door units. It wasn't that hard, just make sure there's nothing in the wall between the studs like an HVAC duct or wiring. I damn near cut into a space with a duct in it that feeds our guest room. But our beagles can come and go as they please, they use the hell out of it. Almost time for a new one.