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A cat flap allows you to give your pet more freedom without the headache of having to let them out and in. It will also help your pet to be less anxious and will stop them from coming into the home when they aren't allowed to do so. To install a cat flap take a measurement of the distance from the bottom of the cat's stomach down to the floor. Draw a line across the door's frame. Check that the line is level with the spirit level. 1. The type of door It is recommended to consult a professional if you want to install a cat flap in an uPVC or double glazing