As thick as you are willing to buy extenders for — a SureFlap tunnel is 70 mm as supplied and each extension adds 50 mm, so a 300 mm masonry wall needs about five of them.
The flap is designed for a door panel. Everything thicker is an accessory purchase.
The arithmetic
Wall thickness, minus the 70 mm (2.76 in) the flap provides, divided by 50. Do it before buying the flap, not after cutting the hole — extenders are proprietary and they cost real money.
Fitting matters as much as parts
The tunnel has to be square to the wall and sealed at the outer face. Filler packed round a crooked tunnel is what makes a flap drafty and stiff, and on an exterior wall it lets rain into the cavity.
What the cat thinks
A long tunnel is a corridor to commit to. Cats that walk through a flap in a door often hesitate at 300 mm (11.8 in) of wall for days. Prop it open at first.
Glass
A glazier cuts the hole, the size comes from the manufacturer’s template, and it is permanent. Measure twice.
This does not replace your device’s manual. Blade temperatures, locking mechanisms and cleaning procedures differ from model to model, and the manufacturer’s instructions take precedence over anything written here. Unplug a device before dismantling it, check a hot blade or an air nozzle against your own hand before it goes anywhere near an animal, and keep a mechanism that closes or rotates under supervision until you know how it behaves.
Last reviewed 2026-08-17