Cat flap

A cat flap was a small entrance built into a door. It allowed a cat to enter and exit a building easily. Rose Tyler's flat on the Powell Estate had a cat flap. Jackie Tyler bolted it down to stop stray cats from coming in. The Ninth Doctor followed a signal to the flat and inspected the flap. In the process he knocked the bolts from it. (TV: Rose)

Roger Davey's house had a cat flap. When he was taking the Adipose Pills, the Adipose that was created left his house via the cat flap. (TV: Partners in Crime)