Abstract: This disclosure pertains to a pet door apparatus, and in particular, a pet door apparatus integrated within sliding glass door frames of apartment units and other enclosed building structures. A pet door apparatus consistent with the present disclosure comprises a plurality of panels which features a flap within one of the panels. The plurality of panels are coupled to each other by at least one attachment mechanism. The coupling between the first panel and the second panel and the coupling between the second panel and the third panel forms an assembled unit. The point of coupling between at least two of the panels is adjustable to extend the length of the assembled unit. An end of one of the panels is fitted within the first track of the slidable door frame. An end of one of the other panels is fitted within the second track of the slidable door frame.