A basic cat flap is just a hole you can place in your door to let your cat go and go whenever they want. This design isn't particularly secure and allows burglars easy access. You may also look into upgrading to a lockable or automated model with enhanced security features. https://cat-flap-in-upvc-door-pan85418.ltfblog.com/30019905/using-a-cat-flap-in-bifold-doors