robgraham wrote:

Make sure that the flap you buy will actually fit a wall - most are
designed for doors and have fittings for that; quite thin doors at
that.

Rob

I've done this in my kitchen as I didn't want to hack holes in the
kitchen door. This goes through a double skin wall. This lets them out
into the utility area then there is another cat flap in the outer door,
having taken out one pane of glass.
The cat flap in the kitchen is quite a simple one and the more
elaborate one to keep the stray moggies out is the outer cat flap.

Kevin