Cat flap in a glass door: can it be done?
Yes, in almost every case. But not the way most people imagine, and not by anyone who offers to cut your existing glass. Here is what actually happens, from a glazier who makes the units.
The glass gets replaced, not modified
You cannot cut a hole in the door you already have. Glass in a door is safety glass, which in practice means it is toughened, and toughened glass cannot be cut, drilled or ground after manufacture. It is held in permanent tension, and breaching the surface releases all of it at once. The pane does not crack, it disintegrates.
So the job is not drilling. It is making a new pane the same size with the hole already in it, cut while the glass is still soft and annealed, then toughened afterwards with the aperture in place. Swap the old unit for the new one and fit the flap into the hole.
Is your glass toughened? Look for the stamp
Every piece of toughened glass carries a permanent mark, usually a small sandblasted or acid-etched stamp in one corner. It will name the manufacturer and cite a standard, normally BS EN 12150 or BS 6206. Get low and look at all four corners of the pane, inside and out.
If you find that stamp, the glass is toughened and it cannot be cut. If you find nothing, do not assume the opposite: laminated glass is not always marked, and an unmarked pane in a door is more likely to be a compliance problem than an opportunity.
What the job looks like for you
| Your door | What happens | Rough difficulty |
|---|---|---|
| Fully glazed door, double glazed | New sealed unit made with the aperture, old unit out, new one in | Standard job, two visits |
| uPVC door with a glass panel | Same as above. The uPVC frame itself is untouched. | Standard job |
| uPVC door with a solid infill panel | The flap goes straight through the panel. No glass involved. | Simplest and cheapest |
| Sliding patio or bifold | Flap normally goes in a fixed pane, not a moving one, so the running gear is untouched | Standard, needs the right pane chosen |
| Single glazed, annealed | Occasionally cuttable in place, but often better replaced for compliance | Depends, we advise at survey |
| Composite or solid timber | Cut through the door slab. This is joinery work, not glazing. | A joiner, not us |
Get this number right before anything is ordered
The hole size is dictated by the cat flap, not by the glazier. A basic flap needs a modest opening. A microchip flap carries a reader housing and needs a larger one, commonly a circle around 212mm across, and full pet doors are larger again, nearer 265mm.
Those are indicative figures and they vary by model. Buy the flap first, or at least decide on the exact model, before the glass is ordered. A sealed unit made to the wrong aperture is not adjustable. It is scrap, and you pay for it twice.
Where in the door it should go
Low and to the hinge side, as a rule. Low because cats do not jump through flaps, they walk through them, and a flap set too high becomes an obstacle for an older or arthritic animal. Hinge side because the aperture weakens the pane slightly and you want it away from the lock edge where the door takes its stresses.
There is also a security consideration people forget. A cat flap in a door is a hole an arm can reach through. Keep it well away from the internal handle and any thumbturn, and if the door is on an accessible ground floor, a microchip flap that stays locked to unknown animals is worth the extra.
Why nobody quotes a flat price honestly
You are buying a made to measure toughened sealed unit, so the price follows the pane size, the glass specification and the aperture. A small side panel and a large patio pane are not the same job and cannot honestly carry the same price.
What you can compare between quotes: whether the unit is made with the hole cut before toughening (correct) or someone proposes cutting your existing glass (wrong, and they will be replacing it anyway), and whether the replacement is the same safety specification as the original. Our cat flap fitting page sets out the process, and a survey is free.