Cat-flap solutions

Bad idea.
Double glazing is designed and manufactured as a sealed unit in order to maintain it's insulation properties.
The glass is often toughened glass, especially if it's in a door.
Cutting a hole risks;
1) shattering the whole thing as it's toughened glass and not designed to be cut
2) ruining the seal and leaving your glass door permanently clouded with condensation
3) the window will lose all it's insulation properties.
Your only real option is going to a double glazing supplier and ordering a custom made door with a cat flap designed and built in.

It's kinder never to let the cat out of the house in the first place. One local cat recently succummed to rat poision, and another was the victim of a tragic road accident, and we have a lot more birds in the garden this spring.

So save your money and keep Kitty happy and safe indoors.

If it is a double glazed panel you can take it out and put in a new panel in insulated pvc the same as you see in most new doors these can be cut into and fitted with a cat flap, the better quality ones close efficiently so as to keep the heat in.

also I work in this area so I can be sure that " Your only real option is going to a double glazing supplier and ordering a custom made door with a cat flap designed and built in." is not true.