Cat flaps in uPVC, composite & glass doors: what’s possible (and what isn’t)

A complete guide to installing cat flaps in double glazing, uPVC, and composite doors.

1. Quick Guide: Where Can a Cat Flap Go?

2. uPVC Doors: The Straightforward Option

For most standard uPVC front or back doors, installing a cat flap is a simple procedure. We fit the flap into the lower solid panel, ensuring we keep well clear of any internal reinforcement and the locking mechanisms.

Expect a neat finish with draught seals fully intact. If your existing lower panel is highly decorative, moulded, or simply too thin to securely clamp the cat flap tunnel, we will swap it out for a new, matching reinforced uPVC panel that is pre-cut for the flap.

Good to know: Choose a mounting height that allows your cat to walk through level with their belly. Most manufacturers advise fitting it “as low as practical” depending on the design features of your door panel.

3. Glass & Patio Doors: Don’t Cut, Replace

Because of this fundamental rule of glass manufacturing, the hole for the cat flap must be created before the glass goes through the toughening furnace. That is why our process is:

1. Survey and measure your existing glass.
2. Order a brand new toughened Insulated Glass Unit (IGU) featuring a factory-sealed circular cut-out.
3. Remove your old glass and fit the new unit.
4. Install the cat flap using a proper circular glass-mounting adaptor.

Cat flaps in glass require a brand new, factory-cut toughened sealed unit with sufficient edge clearance.

Sliding Patio Doors

For sliding patios, we generally install the new glass pane and flap into the fixed pane (the side that does not slide). We use specific slimline flaps (often recommended by specialists) to ensure the sliding leaf still operates flawlessly without hitting the flap’s exterior tunnel.

4. Composite Doors: Proceed Carefully

Composite door slabs are complex, layered structures (typically a timber or foam core wrapped in a GRP skin). Cutting a hole through them post-installation presents several risks:

• It reduces the door’s thermal insulation.
• It can introduce points for water ingress into the timber core.
• Crucially, it will invalidate the door warranty with almost all major UK brands.

If you want a cat flap in a composite door, the safest route is to order the door factory-prepared from new, where the manufacturer routes the hole and seals the core during production. If you already have a composite door installed, we strongly advise using a glass installation (if you have a glass side-panel) or a through-the-wall installation instead.

5. Security, FENSA & Which Flap to Choose?

Security & Insurance

Any aperture in a door is a potential weak point. “Key-fishing” is a real threat; UK police guidance warns that thieves can use tools to fish car or house keys through openings like letterboxes and cat flaps. Always keep keys well away from the door, out of sight, and consider internal letterbox/flap guards where appropriate.

Pair the flap with good locking practice: always fully engage your multi-point lock (lift the handle and turn the key), and consider a split-spindle (auto-locking) setup so the external handle cannot be operated if an intruder reaches through the flap.

Which Cat Flap?

• Microchip Flaps (Recommended): These read your pet’s veterinary microchip to unlock, keeping out strays and local foxes. Brands like Sure Petcare are the industry standard for this and require specific circular adaptors for glass fitting.
• Manual 4-Way Flaps (Budget): Simple flaps with In/Out/Locked settings. Good for basic draught control, but offer zero selective entry.

FENSA & Fire Doors

Glass-only swaps do not require a FENSA certificate. FENSA covers the installation of replacement windows and doors including the frames. Replacing a glazed unit within an existing frame is classed as a repair, so FENSA scope does not apply.

Fire Doors: If your door is fire-rated (such as a flat entrance door or an internal door leading to an attached garage), do not install a flap. Manufacturer manuals explicitly prohibit fitting flaps to fire doors, as it immediately destroys the door’s legal fire-certification integrity.

6. Our Process in Hampshire

7. Frequently Asked Questions