Through wall cat flap.

My son had one put into his double glazed patio door.
Got a guy from a double glazing company to do, it not an unusual request.
Might worth investigating, it might be less invasive and disrupting than a hole in a wall?

Son in law installed one in their previous house - nowhere else to put it. He’s a whizz at any DIY, I have no idea of the brand, sorry, but it worked very well.

I would imagine they're not that different to installing an extractor fan. Just a bigger hole needed and probably no electrics.

We had one of these and it was very good:

Cat Mate 4 way locking catflap with tunnel.

Squiffy beat me to it.

Cat flap for double glazed door.

Cat flap expert of all sorts here!
We had a builder make an opening in a wall for the cats. You're right, it was like a tunnel about a foot deep. They took to it overnight. Infact they used to sit on it, (to get away from the dogs!). The builder lined it properly, the only issue was getting inside it to clean it when the cats dragged their vermin catches through it.

I spent the afternoon doing a month's worth of ironing. I know you're going to tell me you don't iron..I do, sometimes, when I've got laundry that I want to look good. OK?
SO! To distract myself I put the TV on.
I must ask...SMELLS?! All the adverts...smelly homes, smelly washing machines, smelly bedding, smelly clothes, smelly luggage, smelly lavatories, smelly carpets...the list goes on. Does anyone ever think of basic cleaning, doing simple laundry with soap and water? Who are all these people whose lives are plagued by stink which has to be masked by synthetic chemicals and 'purifiers'. AIBU?

Silly me, not a foot deep, but a foot horizontally through the hole! Although a drop chute might have been fun for them! 🐈 🐈

I have one in a double glazed door. The panel is replaced with one that the flap fits into.

A flap can be added to a double glazed window, but it will require a new sealed unit which could be expensive depending on the size.

Problem is I'd need a huge glass panel replaced, then I'd have to have another cat flap for the door. I have a full glass conservatory which encloses the entrance door.

We put one through the wall some years ago. A lot of drilling! I don't remember putting any sort of tube in, just a square hole in the wall and a cat flap on the outside.

We have a normal cat flap separated into tow parts with a plastic tube connecting the gap in the wall, when we redid the kitchen at our last house and bricked a door.