Its good that you add on,because walls are all different thickness mine happened to be 11".

Wanted an extension piece to fit a Petsafe cat flap to a single-skin (i.e one brick thick) wall. Whilst this is easy to fit to the existing flap, it is around 5-10mm too short (so you need 2 extension pieces). The supplied extension piece comes with no instructions (not that any are really needed) but more importantly, no longer screws to clamp the inside part to the outside part. The extension piece is a simple push-fit onto the existing "tunnel". You'd think it would screw into the existing part and have its own screw fixing to clamp the outside part to the extension piece - but no. Solution? Some strong "Gorilla" tape to fix the extension tunnel to the original part and some strong double-sided tape (aka 'Servo Tape') to fix some wooden blocks to the outside of the extension tunnel allowing the original screws to be used to fix the outside part to the extension tunnel. This is fine if you have these already, but an additional cost (to add to having to buy TWO extension tunnels) if you haven't. So whilst it is easy to fit and does the job, the above two 'problems' (too short and no way to clamp the inside and outside parts together without resoring to 'bodging' the job) means it only gets one star from me. Recommended (as there is no real alternative) but poor design.

Both the extender and the chip-enabled door itself are reliable, attractive, and well-designed. We have an aggressive cat and a teenage kitten, so the door allows the kitten access to her own safe room in a crisis. The extender was required bc we needed to install through a sheet rock wall, rather than a door. The fit was perfect, the installation not too difficult.