Pet door inserts for window frames and sliding doors

A pet door insert for a window comprising:

• a base panel system including:
• a first base panel defining a pet door opening;
• a second base panel coupled to the first base panel along a first lateral side of the second base panel;
• a pet door moveably mounted to the first base panel within the pet door opening and configured to move between a pet door covered position covering the pet door opening and a pet door open position allowing a pet to pass through the pet door opening;
• a track extending longitudinally along a medial portion of the second base panel;
• and
• an adjustment panel system moveably mounted on the track and adapted to translate relative to the base panel system to adjust the effective length of the pet door insert, the adjustment panel system overlapping the base panel system in a compact position and extending longitudinally beyond the base panel system in an extended position, the adjustment panel system including:
• a first adjustment panel defining a horizontal groove extending longitudinally along a medial portion of the first adjustment panel, the horizontal groove being complementarily configured with the track and selectively receiving the track to selectively mount the adjustment panel system to the base panel system.