Is it that you don't want to damage the doors by putting cat flaps in? I've had cat flaps in solid oak doors so weight/thickness of door should not be an issue. Equally you can put cat flap through a wall next to a door
Don't know if anyone has this problem but we have 2 indoor cats and large heavy wooden doors that we are not able to put cat flaps in. They do all their business in the basement (we live on 4 floors of a large house) which means they always need to have access to it - which in turn means we have to leave the kitchen door open and the basement door open ALL THE TIME. The basement is more or less Arctic! This is sooooo annoying because it means the house is freezing all the frickin time and never heats up! I have tried little door stoppers that close the door to a small gap - but it doesnt help. Does anyone have an alternative to cat flaps in doors? Anyone else have this problem? Am i just doomed to a cold house and ridiculous heating bills? I am also reposting this in DIY! just in case!!