No - The cat flap/catflap must be glass fitting and circular as it is very expensive to cut a square hole, although not impossible.
Yes you can, if it is single or double glazed, then the glass will need to be replaced by an expoerianced glazier or glass company who can offer you a price for the replacement cost.
Cat flaps come with a kit that includes a paper template that guides you exactly where to drill. With the help of markings, drill a hole in to each corner of the template. Once you have done with drilling the holes, you can get the shape of your cat flap accordingly. These are great for wooden, UPVC/plastic and metal doors, but would recommend a professionaal for anything to do with glass.
It varies, but usually takes around 1 hour to install a cat flap in wood, UPVC, metal or a composite door, if it is a cat flap in glass or a double glazed unit, the glass will be toughened safety glass and usually take 3-5 working days to remanufacture and about 30 minuites to install again subject to type and style of door or frame.
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