Cat Flap in Double Glazing Installation Guide | Window Fitters

Cat Flap Installation in Double Glazing: Can It Be Done?

For pet owners, giving cats or dogs independent access to the garden is a priority. However, if your back entrance features a double-glazed glass door or a low patio window, you might wonder if it is possible to fit a pet door. The short answer is yes, it can be done — but it requires a specialized process. Attempting to cut a hole in your existing double glazing will break the glass and destroy the window. This guide explains how to install a cat flap in double glazing safely and professionally.

Expert Reviewed by Marian Gheorghe, Head Window Fitter — Last reviewed: July 2026

The Critical Rule: You Cannot Cut Existing Double Glazing

Many homeowners ask if a glazier can visit and simply cut a circular hole in their existing double-glazed door. This is physically impossible for two reasons:

1. Toughened Safety Glass: By law (Approved Document K), all low-level glass in doors and adjacent side panels must be toughened safety glass. Toughened glass is heated and rapidly cooled during manufacture to increase its strength. If you attempt to cut, drill, or even deeply scratch toughened glass, the built-in tension will cause the entire pane to shatter instantly into thousands of tiny, harmless pieces.
2. The Hermetic Seal: A double-glazed unit is a sealed environment containing dry air or Argon gas. If you break the seal to cut a hole, the insulating gas escapes, moisture-laden air enters, and the unit will immediately mist up internally and lose its thermal insulation properties.

The Professional Solution: Custom Glass Swapping

To install a cat flap in a double-glazed door, the entire glass panel must be replaced with a new, custom-manufactured glass unit. Here is how the process works:

1. Step 1 — Site Measurement & Survey: Our surveyor visits your property to take precise measurements of your existing door glass (width, height, and unit thickness, usually 24mm or 28mm). We also note the brand and model of the pet flap you wish to install.
2. Step 2 — Factory Manufacture: We order a new, bespoke double-glazed unit from our glass manufacturer. During production, the manufacturer cuts a circular hole in both panes of glass *before* the glass is toughened. The glass is then toughened in a kiln, and the two panes are sealed together around an internal plastic spacer ring that surrounds the hole. This preserves the hermetic seal and the Argon gas fill.
3. Step 3 — Installation Day: Once the new unit is manufactured (typically taking 2 to 3 weeks), our fitters return to your home. We remove the glazing beads, lift out your old glass panel (which we can leave with you to reinstall if you move house), fit the new custom glass panel, install your cat flap through the pre-cut hole, and seal the unit. The entire fitting process takes roughly 1 to 2 hours.

Safety Glass Regulations and Compliance

Because the installation is in a door, the new double-glazed unit must be fitted with toughened safety glass conforming to BS EN 12150 standards. The glass will carry a permanent etched mark in the corner. If you hire an unvetted contractor who fits standard float glass, it violates UK Building Regulations, presents a severe safety hazard to your family, and will fail home buyer surveys.

Choosing the Right Pet Flap

When selecting a cat flap or dog door for a glass installation, ensure it is specifically rated for glass mounting. Glass-mounted flaps feature circular tunnels and weather gaskets that seal against the circular hole in the glass unit. Popular options include:

• Microchip Cat Flaps (e.g., SureFlap): Read your pet's existing microchip to prevent stray animals from entering your home. These require a specialized circular mounting adaptor for glass.
• Manual 4-Way Locking Flaps: Allow you to set the door to \"in-only,\" \"out-only,\" \"open,\" or \"locked.\"

At Window Fitters Ltd, we supply and fit custom pet-flap-ready double-glazed units for composite, uPVC, and aluminium back doors. We measure the glass, order the pre-cut toughened units, and fit your chosen pet door, ensuring a draftproof, secure seal.

About the Author: Marian Gheorghe

Head Window Fitter & Active Director, Window Fitters Ltd

Marian Gheorghe is the Head Window Fitter and Director of Window Fitters Ltd. He has spent over 15 years carrying out glass swaps, window repairs, and custom glazing alterations across London, ensuring all work complies with BS safety standards.

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Pet Flap Glazing FAQs

How much does it cost to put a cat flap in a double-glazed door?

The cost varies based on the size of the glass panel. For a standard uPVC back door half-glazed panel, the custom-manufactured toughened double-glazed unit with pre-cut hole averages £250 to £350, plus the cost of the cat flap itself. Large sliding patio doors or floor-to-ceiling panels can cost more due to glass weight.

Can I put a dog door in a double-glazed panel?

Yes. The process is identical. However, because dog doors are larger, we must check that the width of your door frame can support the size of the required hole while maintaining structural glass margins.

What happens to my old glass?

We recommend keeping your original glass panel in your garage or loft. If you ever move home or decide you no longer want the cat flap, we can return and swap the original glass back in, restoring the door to its original state.