cat flap

How to Fit a Cat Flap in a Door, Double Glazing or Wall

Gabriel

13 Dec 2025 — 3 min read

Fitting a cat flap is one of the most asked-about small DIY jobs, partly because there are several different substrate types that require completely different approaches. Cutting a hole in a wooden panel door is trivial; cutting through a brick wall requires specialist equipment. This guide covers all four common scenarios.

Choosing the Right Cat Flap

Before cutting anything, choose the cat flap correctly for your situation:

Standard magnetic or manual flap: Basic, inexpensive (£10–30), suitable for any substrate. Operates with a magnetic key worn on the cat's collar, or freely swings both ways.
Microchip-operated flap (SureFlap, PetSafe): Reads the cat's existing microchip ID. Prevents other cats entering. The market leader is SureFlap — around £80–120. Requires power (batteries or mains adapter).
4-way locking: Can be set to entry-only, exit-only, both ways or locked. Useful for keeping the cat in at night.

Measure your cat's width across the shoulders — the flap aperture should be at least 25mm wider than this and tall enough for the cat to pass through comfortably without crouching.

Fitting in a Wooden Door

The simplest installation. Most cat flaps come with a cardboard template — tape it to the door at the correct height (the bottom of the flap should be slightly above the door threshold to prevent water ingress; typically 150–200mm from the floor).

Drill a starter hole for the jigsaw blade at one corner of the template outline.
Cut around the template with a jigsaw. For hollow-core doors, cut slowly — the jigsaw can tear the skin on the exit side. Tape the cut area on the exit side to minimise tear-out, or score with a knife first.
Smooth the cut edges with sandpaper and seal with primer/paint or wood preservative to prevent moisture absorption.
Insert the inner frame through the hole, fit the outer frame on the other side, and tighten the connecting bolts.

Fitting in a uPVC or Composite Door

Technically possible but riskier than wood — uPVC panels are thin and the door may contain a steel reinforcing frame that could be damaged by cutting. Check the manufacturer's guidance. Use a jigsaw with a fine plastic-cutting blade. For composite doors, cutting through the foam-filled core is straightforward but the outer skin is hard and chips easily — score with a knife first.

Fitting a cat flap in a uPVC or composite door will likely void the door manufacturer's warranty and may affect the door's thermal and weather resistance. Consider whether a separate low-cost panel door might be a better solution for a utility room or side entry.

Fitting in a Double Glazed Unit

You cannot cut a hole in an existing double glazed unit without destroying it — the argon gas fill will escape and the unit will fog permanently. The only correct approach is to order a new double glazed unit with the cat flap aperture pre-cut and a tunnel fitted before sealing the unit. This is a job for a glazier, not a DIYer.

An alternative is to fit the cat flap in the door frame itself (if the frame has a solid panel section below the glass), or in an adjacent wall.

Fitting in a Brick or Block Wall

The most challenging installation but often the best solution — no door security compromise, and the aperture can be positioned anywhere convenient. You'll need:

An SDS drill with a core drill attachment (the correct diameter for your cat flap tunnel — most require a 175–200mm hole)
Cold chisels for clearing masonry
The cat flap's tunnel extension kit (for walls over 130mm thick — a standard 225mm brick wall needs a tunnel extension)

Mark the cut position and core-drill from inside. Work slowly and clear debris frequently.
Break out the masonry core — it rarely comes out in one piece. Continue drilling until the hole is through the full wall thickness.
Fit the tunnel through the wall, ensuring it's slightly angled downward toward the outside (water drainage). Pack with mortar or expanding foam around the tunnel.
Fit inner and outer frames and seal around them with exterior silicone.

Alternatively, hire a specialist for the core drilling if you don't have access to the equipment — this is a common job for builders' merchants that offer hire services. Core drill hire: £30–50/day.

Draught Sealing

Cat flaps are a notable source of draughts when the flap itself doesn't seal well. Look for flaps with magnetic seals around the frame aperture and a weighted or magnetised flap. For microchip flaps, the powered motor holds the flap more firmly closed. Consider fitting a draught-excluding surround or replacing a cheap flap with a higher-quality model if heat loss is a concern.