Cat Flap in Double Glazing: Why the Glass Is Replaced, Not Cut, and What It Costs

A cat flap in a double glazed door looks like a hole in some glass, so a quote that prices a whole new piece of glass can come as a surprise. It is not upselling. The glass in a modern door is toughened and sealed, and neither can be cut once it is made. This guide sets out what the flap makers and glaziers actually say, the numbers you need before you order anything, and why the published prices for the job sit so far apart.

Can you put a cat flap in double glazing?

Yes, but not into the unit that is already there. The flap makers are blunt about it. PetSafe says it is "not possible to cut holes in tempered or double-pane glass, except at the time of manufacture". Cat Mate's glass fitting instructions say that "for existing toughened/double/triple glazing, a new glass panel will be required". Sure Petcare, which makes SureFlap, says that "in most cases, the existing glass panel will need to be replaced with a new pane that has been pre-cut to the correct diameter", and that any cutting must be done by a qualified glazier. Pet-Tek says the same of its own glass flaps.

There are two reasons. Toughened glass cannot be cut after it has been toughened: CatFlaps.co.uk warns that trying "can cause the entire pane to shatter". And a sealed unit is a factory-sealed cassette; the moment you cut through it the seal has gone, and a unit with a broken seal is a blown double glazed unit waiting to happen. One flap maker's blog describes core-drilling an existing unit in a single pass. The makers' own instructions, and every glazier we read, replace the unit instead, and so should you.

What the glazier actually does

The glazier measures the existing unit and makes a new one to the same size. Two sheets of glass are cut to size, a round hole is cut in both, and only then are they toughened; the toughened panes are assembled into a new sealed unit with the holes lined up. The new unit goes into your existing frame, and the flap is fitted through it on site, which is how SureFlap, PetSafe and Cat Mate all describe the installation. So the job has four parts: a new unit, the hole, the toughening and the fitting. The flap itself is usually priced on top.

Four details are worth giving the glazier in writing:

• The flap model and its hole size. CatFlaps.co.uk puts it plainly: choose the flap before ordering the glass, because the diameter "cannot easily be changed after the new toughened glass has been manufactured".
• A non-metallic spacer around the hole if the flap reads microchips. Sure Petcare says a metal spacer there can interfere with the reader, and its glass manual says not to use metal in any seal between the panes around the hole.
• A round hole. Glass flaps are made for circular holes. One fitter notes a square hole is possible but very expensive, and CatFlaps.co.uk says many dog doors need a rectangular cut-out, which rules them out of normal toughened glass.
• The mounting adaptor.Sure Petcare says one is required to fit any SureFlap product into glass, and PetSafe's instructions drop it only for single glass. It is a separate part, £13.50 and £18.99 respectively on their UK stores.

Keep the old unit if you have somewhere to store it. One fitter advises it because, if you move or no longer need the flap, putting the original glass back is cheaper than ordering a new plain unit.

Timing: the unit is made to measure. Cornwall Glass quotes seven to ten working days for a double glazed unit, Aaran Glass about two weeks from measuring, and MyBuilder (August 2026) one to two weeks from order to installation. That matches the seven to ten working days our blown double glazing guide gives for any toughened unit. The visit itself is short: Checkatrade says most cat flap installations take around two hours.

Hole size and glass thickness, by flap

Two numbers decide whether a flap works in your door: the diameter of the hole and the thickness of glass it will clamp. Most UK sealed units in uPVC frames are 24 mm or 28 mm thick, so a flap that stops at 26 mm will not go into a 28 mm unit. Measure your existing unit before you order, or read the build off the spacer bar. The figures below are the makers' own unless marked.

Source: Sure Petcare UK support and help pages (2025); PetSafe GB support and adaptor pages; Cat Mate instructions (Pet Mate, 2018) and Closer Pets UK; Pet-Tek product page; CatFlaps.co.uk for the Cat Mate 357. Research in lib/research/cat-flap-double-glazing-2026.md.

Two of the four makers leave you guessing on thickness. Neither Sure Petcare nor PetSafe prints a maximum glass thickness on the pages and manuals we read; SureFlap's manual says only that a double glazed panel "may require the mid-sized screw". Ask the maker, or the glazier, to confirm the flap fits your unit before the glass is ordered. PetSafe's hole size is the other trap. Its support article says 260 mm with the adaptor, and 215 for single glass without one (printed as "215 cm"); its adaptor page says openings of 212 mm to 285 mm. Use the figure printed with the exact kit in your hand.

Where the flap goes

The makers agree on height: the bottom of the hole at about the height of your cat's belly, measured from the floor. Sure Petcare adds that in practice the door usually decides, and that it is usual to mount the flap as low as possible. Cornwall Glass asks for at least 70 mm between the edge of the hole and the frame, because less may compromise the glass; that is one glazier's figure, but it is the only one published. Two layout cases change the answer. On a sliding door, PetSafe says the flap goes in the inner panel. And on a narrow glass door, one fitter puts the limit at 360 mm wide, below which a uPVC panel is the better route.

The safety glass rule

In England, Approved Document K sets out where glass must be safety glass, meaning glass that breaks safely or resists impact. It calls these critical locations, and LABC's plain-English summary lists three: glass in a door wholly or partly within 1,500 mm of the floor; glass within 300 mm of the edge of a door and within 1,500 mm of the floor; and low-level glass wholly or partly within 800 mm of the floor. Safety glass in those places should carry its British Standard mark.

A cat flap sits low in the door by definition, so a door unit carrying one is always inside the 1,500 mm zone and the new unit must be safety glass. In practice it is toughened anyway, because that is how a hole gets into it: cut first, toughened after. The overstatement to ignore is one installer's claim that "all glass has to be toughened safety glass by law". A cat flap in a window away from a door is only in a critical location if the glass reaches within 800 mm of the floor. Two other limits: Cat Mate says its flaps are not fire resistant and must not go where fire spread is a risk, and one glazier will not cut into communal doors because they are usually fire doors. Wales has its own version of Part K; this section is England's.

No FENSA certificate for this job

A new unit in an existing frame is a glass-only repair. FENSA's own compliance guide says repair work, "for example, sash or glass only replacement", falls outside its scheme, so a glazier fitting a cat flap unit will not issue a FENSA certificate, and you do not need one. The certificate is for replacing the whole window or door, which our FENSA guide covers. The safety glass rule above still applies.

uPVC doors: cut the panel, or buy one pre-cut

Where a uPVC door has a flat solid panel in the lower half, that is the easy place for a flap: no glass, no glazier, no lead time. The steps below are SureFlap's, from its door manual; other makers work the same way with their own template.

1. Check the panel

It needs to be flat. Rydale Windows advises against raised decorative panels, which are not structurally suitable. Check too whether the door is uPVC-coated metal: Sure Petcare says these often look like ordinary uPVC and that a metal plate in the door can shorten the range of a microchip reader.

2. Mark it out

Measure your cat's belly height from the floor and hold the flap's template on the panel at that height; SureFlap lists Blu Tack for the job. Its cut-out for a door is 165 mm wide by 171 mm high. Mark the outline and the screw holes with a sharp pencil.

3. Drill and cut

SureFlap's manual says to drill a hole in each corner big enough for a jigsaw blade, then cut the opening with an electric jigsaw about 3 mm outside the marked line, and drill the screw holes at 6 mm. The tools are a drill, a jigsaw, a crosshead screwdriver, a pencil and a ruler.

4. Fit the flap

Fit the flap through the opening and screw it in as its instructions show, then seal the edges. Never cut the flap's tunnel to make it fit: SureFlap says not to cut or drill through it under any circumstances, and PetSafe that key components are inside it.

Whether you should do it yourself is where the sources split. Checkatrade (July 2026) says to hire someone for uPVC, composite or glass doors, because cutting them incorrectly "can crack the panel and invalidate the door's guarantee". MyBuilder (August 2026) calls DIY on uPVC "possible but less forgiving". Rydale says flat panels are usually the easiest option for homeowners. If the door is still under its installer's guarantee, ask before you cut; we found no installer's published terms that settle it either way.

The no-cutting option is a new panel. DNK Services sells a flat 24 or 28 mm panel with the hole already cut from £40 (£33.33 plus VAT), supply only, cut by hand with a jigsaw and about a week extra to make. Smashing Windows in Aylesbury fits a half panel with a pet flap for £100 and a full-length one for £160, fitting and VAT included, flap extra. And where the lower half of the door is glass, CatFlaps.co.uk and the Crime Prevention Website both suggest replacing that glass with a solid panel and putting the flap through it, which avoids a toughened unit altogether; Checkatrade prices the job below.

Composite doors

A composite door is the hard case. Astraframe, which makes them, says a cat flap is "not recommended" and that "most manufacturers, including Astraframe, will not honour the warranty on a composite door modified with a cat flap". If the door is new or being replaced, the better route is factory routing: a Solidor installer says Solidor will cut the opening at the factory for a fixed £35, and a sister site of the same company puts it at about £50, so treat those as one company's figures. The conditions are strict: a solid lower panel only, not a glazed or moulded one, and a square or rectangular flap supplied with a template before manufacture. On a door you already own, a retrofit is £160 for a small flap and £190 for a large one (Checkatrade, July 2026), £130 to £190 (MyJobQuote, September 2026), or £100 to £150 for the labour alone (MyBuilder, August 2026), set against whatever it does to the warranty.

What a cat flap in double glazing costs in 2026

The published prices fall into two groups that do not agree. The cost guides put a pet flap in a glass door at £150 to £250. The one specialist glazier we found publishing a price, and the homeowner quotes, sit above that. Checkatrade and MyJobQuote use the same row labels in their tables, so they are not two independent readings, and Checkatrade's own page gives three glass ranges: £160 to £200 in one paragraph, £180 to £250 in its FAQ, and £180 and £250 in its table. Its materials column, £80 for a small flap, is less than a microchip flap and a new toughened unit would cost together, so read it as excluding the flap or running low.

No source prices a cat flap unit by glass size; the cost guides tier by flap size. Research in lib/research/cat-flap-double-glazing-2026.md.

The £66 is the only published price for the hole on its own, and it rests on that one glazier, whose quote is its normal sealed unit price plus £66, so treat it as indicative. What it does show is that the rest of the bill is the unit. A plain replacement unit costs £74 fitted for a 500 by 700 mm pane and £117 for 800 by 1,000 mm in the table in our blown double glazing guide, before toughening and the flap. The cost guides' £160 to £250 sits above those plain figures, as it should.

For a uPVC door, the cost guides agree more closely. Cutting a flap into an existing uPVC panel is £100 for a small flap and £130 for a large one (Checkatrade), £80 to £120 (MyJobQuote), or £80 to £130 for labour alone (MyBuilder). Swapping a glass lower panel for a uPVC one with a flap in it is £180 small and £250 large (Checkatrade) or £150 to £220 (MyJobQuote).

Source: surepetcare.com, petsafe.com/gb, closerpets.co.uk and the Pet-Tek store, checked 19 September 2026.

If you are totting up a list of glass jobs at once, the repair cost calculator prices the standard ones, and where the whole door is due, the door cost calculator prices a new one, which is the moment to have the flap built in.

Buy the flap before you ring a glazier, and hand over the hole size printed with that exact kit. Toughened glass cannot be trimmed afterwards, and one maker publishes three different numbers across its own pages.

Double Glazing Cost Calculator Team

Heat, security and insurance

Heat.Pet-Tek, which sells glass flaps, says any opening in a glass panel reduces its thermal performance to some degree. Nobody independent has measured how much. The only figure we found comes from The Petflap, which sells a rival insulated pet door: it puts a typical acrylic flap at a U-value of about 6 to 6.2 W/m²K, worse than single glazing at about 4.8 to 5.8, though over a small area, about 0.05 m² for its own flap. Treat it as one interested party's number; our U-values guide explains the scale.

Security.Cat Mate says the locking system on its flaps "is not intended as a security device", and the Crime Prevention Website that a flap lock is designed to keep other cats out, not burglars. The same site says nearly all cat flap burglaries happen because a key was left in the door, and warns that a dog flap a child could fit through is a real risk. Take the key out.

Insurance. GoCompare (June 2026) says a pet flap can make a door less secure and that you should tell your insurer, or a burglary claim might be rejected. MyJobQuote says it will not usually change the premium but can matter if there is no sign of forced entry. Ask your own insurer before the flap goes in.

Guarantees.The new unit comes with the glazier's guarantee, which varies: Aaran Glass and Cornwall Glass give five years against misting, and Cornwall Glass says it does not guarantee the flap, which is the maker's job. Whether fitting a third-party unit affects your original installer's guarantee on the door, we could find no published answer. Check your paperwork, and ask.

If the door is due for replacing anyway

If the door is near the end of its life, a flap is a poor reason to spend on the old one. A new composite door can be ordered with the opening routed at the factory in a solid lower panel, and a uPVC door with a pre-cut panel, so nothing is cut after the door is made. If you want the door or glazing priced as a whole, tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode.

Price-checkedLast reviewed 19 September 2026

Reviewed byDouble Glazing Cost Calculator TeamCross-checked against Checkatrade, the FMB & GreenMatch