uPVC panel doors: usually straightforward
A uPVC door with a foam filled solid panel (as opposed to full glass) can normally take a cat flap cut into that panel. The panel is reinforced but not to the same degree as a composite slab, so a competent glazier or pet door specialist can cut, box in and finish the aperture without compromising the door's strength.
It won't touch the door's weathertightness if it's done properly, with the flap unit sealed all the way round and screwed through the panel skins rather than just the outer face.
Composite doors: more often a no
Composite doors are built around a solid timber or insulated foam core faced with GRP or similar skins, and that core is exactly what gives the door its strength and its security rating. Cutting a hole through it removes material the manufacturer designed the door around, which is why several composite door manufacturers explicitly exclude cat flap cutting from their guarantee, or refuse to warrant the door at all once it's been altered.
We won't cut a composite slab on your behalf and then leave you holding a voided guarantee. If your composite door still has cover left, it's worth checking with the manufacturer or original supplier before anyone drills into it, ourselves included.
If the door has a glazed unit
You cannot cut a hole in a sealed double glazed unit and reglaze it around a cat flap; the unit will simply fail. The proper way to add a flap where the aperture would fall on glass is to have a new sealed unit made with the pet door already built into it, supplied by a glass fabricator, and refitted into the same frame.
This is quick to order but it isn't free, so it's worth factoring into the decision if your door is mostly glazed.
Alternatives worth considering
If your composite door can't be touched, a few alternatives keep the door intact. A cat flap fitted into a fixed side panel or top light (where one exists) avoids the main door slab entirely. Fitting the flap through an adjacent wall, rather than the door, is another option many pet door fitters offer. Or, if the door is due for replacement anyway, choosing a uPVC panel door with the flap specified from the outset avoids the whole problem.
- Cat flap in a uPVC panel: usually possible, low risk to guarantee
- Cat flap in a composite slab: often excluded by the manufacturer
- Cat flap in a glazed unit: needs a new sealed unit made to order
- Cat flap in a side panel or wall: avoids cutting the main door
Not sure if your door can take a cat flap?
Tell us the door type and age and we'll give you a straight answer, including whether replacement makes more sense than cutting it.
How the aperture is actually cut and finished
On a suitable uPVC panel, the flap position is marked out first, allowing clearance from the door edge, any internal reinforcement bar and the lock keeps, before a pilot hole and jigsaw are used to open up the aperture. The cut edges are then boxed in with a purpose made surround, usually supplied with the pet door itself, which is screwed through both skins of the panel rather than just glued or clipped to the surface.
A tidy job leaves no bare foam or exposed cut edge visible from either side, and the flap should sit flush enough that it doesn't catch the door seal when the door closes against the frame.
Where the flap sits matters for security too
Position isn't only about the cat. A flap placed too close to the main lock can, in theory, give an intruder something to reach through toward the handle or keeps, so it's worth positioning it away from the lock side of the door and low enough that reaching up to the handle from outside isn't straightforward. Lockable cat flaps, which can be shut and secured from inside overnight, are worth considering on any door where this is a concern.
What this means for a like for like replacement
If you're eventually replacing the door rather than altering the one you've got, specifying the cat flap position at the ordering stage is far tidier than cutting a brand new panel afterward. It also means the manufacturer's guarantee applies to the door as supplied, flap included, rather than being voided by an alteration made after delivery. This is one of the more common reasons homeowners bring cat flaps up with us in the first place, once they realise their existing composite door is likely to say no.
What we'd advise before you commit
We don't currently list cat flap fitting as a standalone service, so if this is all you need doing, a dedicated pet door fitter or your original door supplier may be the quicker route. Where it overlaps with a wider door job, replacement, repair or a new composite door supplied with the flap already speced in, we're happy to talk it through honestly on 0116 442 0332 rather than guess over the phone what your existing door will allow.