Here's a pet door that doesn't require a people door.

Imagine a pet door installed behind a couch or in a utility room. Our Wall Entry Door can be installed in a number of exterior wall materials, including siding, stucco and even brick.

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Features
Double-flap system insulates for energy efficiency.
Easy do-it-yourself installation kit.
Aluminum frame with durable plastic tunnel extension.
Telescoping tunnel allows for installation into walls.
Extension kit sold separately to extend beyond 7 1/4".
Fits walls 4-3/4" to 7-1/4" thick.
Dimensions - 12-3/4" x 17-1/2".
Great for pets up to 40lbs.