Who Gets the Hole? Fitting Pet Flaps and Letterboxes Into Bristol Glazing the Right Way

Every week or so someone asks us the same question, usually while standing in a kitchen with a cat staring at the back door. Can you put a flap in that? The honest answer is yes, but not by cutting a hole in the glass you've already got.

This is one of those jobs where the difference between a tidy result and an expensive mess comes down entirely to how the unit is made, not how it's fitted. So let's go through it properly.

Why you can't just cut a hole in a sealed unit

A double-glazed unit is a sandwich: two panes, a spacer bar, a bead of sealant and either argon or air in the gap. That seal is the whole point of the thing. Break it and moisture gets in, the desiccant saturates, and within a season or two you've got fogging between the panes and a unit that has to be replaced anyway.

There's a second problem. Safety glazing rules mean glass in a door, or within 800mm of the floor, has to be a safety glass — normally toughened. Toughened glass is made by heating and rapidly cooling the pane, and the stresses that give it its strength are set for life. You cannot drill, cut or saw it afterwards. Try, and it doesn't crack politely; it shatters into thousands of small pieces across your kitchen floor.

The upshot: the hole has to exist before the glass is toughened and sealed. That means a factory.

Option one: a glazed-in flap unit

This is the standard, tidy answer. Your glazier orders a new sealed unit with an aperture cut to the flap manufacturer's template, the aperture edges are finished, and the flap is fitted into it. It's one unit, one delivery, one fit.

Pros: neat, no draughts around the flap body, keeps the existing door and frame, works with most flap brands including microchip models.

Cons: you're buying a whole new sealed unit, and the flap has to be sized to suit the space available — a big dog flap needs a big panel.

Rough cost in 2026: £150 to £300 for a made-to-measure unit with the aperture, plus the flap itself (£30 to £150 depending on whether you want microchip recognition), plus fitting.

Option two: a replacement panel or door

If the glazed panel is small, or the door is tired anyway, replacing the whole glazed panel or the door leaf can be more straightforward. Composite doors with a factory-fitted pet flap are widely available, and for a rear door that's seen better days, it's often the better long-term spend.

Pros: you can choose the flap position properly, improve the door's thermal performance at the same time, and reset any tired hardware.

Cons: noticeably more money, and for a period property in somewhere like Bishopston or St Andrews you may find a modern composite door looks out of place. Check whether you're in a conservation area before ordering.

Rough cost: £300 to £700 for a replacement glazed panel and fitting; £700 to £1,800 for a full door.

Option three: don't use the glass at all

Often the smartest route. A wall-mounted pet flap through a cavity wall, or a flap fitted into a solid timber door, avoids the glazing question entirely.

Pros: no sealed unit to replace, generally better security and insulation, and a wall sleeve can be positioned at a sensible height for an older cat or a small dog.

Cons: you need a suitable wall, you're cutting a hole in the building rather than the glass, and a cavity sleeve needs proper lining and draught sealing.

Rough cost: £200 to £400 for a wall liner kit plus installation, more if the wall is solid stone or the position needs making good.

Letterboxes: the awkward cousin

Same rules apply. A letterbox through a sealed unit needs a factory-cut aperture, and because letterboxes are wide and low down, they often fall into the safety glazing zone and need toughened glass — which brings the whole factory-cut issue straight back.

In practice, most people are better off with a letterbox in a solid door panel or a wall-mounted external box. If you're in a flat with a communal entrance in, say, Bedminster or Easton, the box by the door is almost always the pragmatic answer.

Costs at a glance

Treat those as ballpark figures — access, glass specification and whether the door needs rehanging all move the number.

The bits people forget

Which pane gets the hole. On a sealed unit with a low-E coating, the coating sits on a specific face inside the cavity. Get the orientation wrong and you lose performance. This is a factory instruction, not a site decision.

Flap quality matters more than the glass. A cheap flap with a poor seal will leak more draught than the unit it's set into. Magnetic or brush-sealed flaps, and microchip versions that only open for your pet, are worth the extra.

Security. A pet flap is a hole in your house. Choose a flap with a locking slide or a four-way lock, and don't fit one in a door that's out of sight of the street or a neighbour.

Tenure. Leaseholders and rented tenants should get written permission before anyone touches the door. In Clifton or Redland, a flap in an original front door of a listed building may need consent — and honestly, put it in the back door instead.

Draughts and weather. A south-west facing Bristol door takes a lot of rain. The flap needs to sit above the sill and drain properly, and the surrounding frame needs to be sound before anything goes in.

Please don't do it yourself

We understand the appeal. A new flap is forty quid, the door is already there, and the internet is full of videos. But drilling or cutting a sealed unit voids the unit's warranty, destroys the seal, and in the case of toughened glass can end with a pane exploding across the floor. It's also the sort of thing that quietly invalidates a door warranty you might need later.

The good news is that the proper route isn't dramatic or especially expensive. Measure the panel, pick the flap, and get a unit made with the hole already in it. The cat won't know the difference. Your glazing will.