https://jsfiddle.net/optioncourt28/whzej4yf/ It is possible to insert an apron flap inside an upvc panel or double glazing sealed unit. However, the best alternative is to install an entirely new glass pane that has a hole that is already made. Read the instructions that should be included in your cat flap kit and follow them carefully. Make sure that the template drawing accurately represents the shape (circle or square). Safety A microchip activated pet door in your door made of upvc is a great option for those who have a dog or cat and would like to let them out into the garden, but do not want to open the door. It will allow only your pet to enter without preventing any other pets or insects from entering. It can also stop burglars from getting through your door by locking them out. This kind of security feature is essential when you have children, elderly, or live in an area that is prone to burglary. Installing a cat flap on an solid uPVC door is fairly simple and can be done by anyone who is comfortable using tools and has basic DIY skills. You can use a jigsaw or a saw to cut the hole that you require however it is essential that you make sure that the flap is wide enough to accommodate your pet and that it is placed at the right height from both the inside and outside of the house. If your door is constructed from a composite material it is recommended to talk with the manufacturer before making any changes in order to avoid invalidating the warranty. Certain composite doors have raised decorative features that are difficult to cut and special provisions may need to be created where these protrusions are found. It is also a good idea to apply some weatherproof sealant to the bare sections of timber that have been exposed after cutting, as this can keep rainwater and moisture from soaking into the wood, leading to it rotting. It is more difficult to install the cat flap in a double-glazed glass door because an entirely new glass will have to be put in place