GUIDES · CERTIFICATION
Fire door modifications: what voids certification?
A guide to fire door modifications by Daniel Storr, Founder & Managing Director · Last reviewed 11 September 2026
A certified fire door only holds its rating if it stays within the configuration that was tested. Fire door modifications such as cutting in a cat flap, over-trimming the leaf, fitting a non-certified letterplate or lock, or painting over the intumescent seals can all take a door outside its test evidence. Once that happens, it can no longer be relied on as a certified fire door. The safe rule: change nothing that is not covered by the door’s certification and manufacturer instructions.
Why a modification can undo a fire door
A fire door earns its rating as a complete, tested assembly. The test proves that that door, with those components, fitted that way, performs for its rated period. Alter the leaf or add something the test never included and you are no longer looking at the tested product. You are looking at an unknown. That is why apparently minor changes can quietly turn a certified door into a nominal one, with no evidence behind it. The government’s fire door guidance lists alterations and damage among the things a routine check should look for.
Cat flaps: the fire door modification that is almost always a no
Cutting a hole in a fire door for a cat or dog flap is one of the clearest ways to void its certification. A fire door has to be a continuous barrier. An aperture with an ordinary flap in it is not. There is no routine “fire-rated cat flap” retrofit that keeps a standard certified fire door compliant. If a door needs a pet flap, that is a conversation about the door, not an accessory to add.
Trimming and planing
Doors sometimes need easing to fit an out-of-square frame, but only within tight limits. Every certified doorset has a maximum amount that can be taken off each edge, set by the manufacturer. Often it is just a few millimetres. Trim beyond that and you can expose or remove the parts the fire performance depends on. Over-planed doors are a common find, and they cannot simply be planed back.
Letterplates and vision panels in a fire door
You cannot cut a standard letterplate or glazed vision panel into a certified fire door and keep its rating. What you can do is fit a certified intumescent letterplate or a certified glazing kit. However, that is only allowed where the door’s own certification and field of application permit it, and strictly to the manufacturer’s instructions. The component and the installation both have to be within the door’s evidence, not just “a fire-rated part”.
Locks, closers and ironmongery
Hardware matters more than it looks. Additional locks, latches, closers, hinges and handles should be certified, UKCA or CE marked, and compatible with the doorset’s tested configuration. Drilling in a non-compliant lock, or adding ironmongery the door was never tested with, can compromise both the fire and the smoke performance. Our door closers and hardware page explains what we fit and why.
Painting and finishes
You can usually redecorate a fire door. However, you must not paint over or clog the intumescent strips and smoke seals. Heavy build-ups of paint or applied finishes can also affect how the door sits and performs. Keep the seals clean and functional, and do not treat a fire door like a feature to be layered up.
If a fire door modification has already happened
If a door in your building has been altered, do not panic, but do get it looked at. That includes an old cat flap, a non-standard letterbox, or a door that has been planed to fit. On a fire door inspection we record modifications and their effect on the door’s status, so you know exactly where you stand. Some situations are remediable. Where a door has been taken outside its evidence, the honest answer may be a certified replacement, and we will tell you plainly which it is. The decision, as always, rests with the responsible person, informed by the facts.
Think a door in your building has been altered? Book a fire door inspection in Bournemouth or Poole and we will record exactly where it stands. We inspect from £20 +VAT per door and give you a photographed, door-by-door report. See our fire door inspections page, or go straight to fire door inspections in Bournemouth or fire door inspections in Poole. Elsewhere on the south coast? See the areas we cover or call 01202 399399.
Common questions
Can I fit a spyhole or door viewer?
Only a certified fire-rated viewer, fitted where the door’s certification allows it and to the manufacturer’s instructions. A standard DIY viewer drilled through the leaf breaches the barrier.
Can I change the handle or lock on my flat entrance door?
Yes, if the replacement is certified for use on fire doors and compatible with the doorset. Check with the building’s responsible person first, because many leases and the 2022 Regulations ask residents not to alter flat entrance doors.
Does a modification always mean the door must be replaced?
No. Some changes can be reversed or made good with certified components. Others, such as a cat flap, cannot. An inspection tells you which, and our guide to repair or replace explains how we decide.