Building Regulations for Windows: Repair vs Replace

If the box frame stays in the wall, almost nothing you have done to a sash window counts as building work. That one fact settles most of the regulatory questions homeowners arrive with: whether a certificate is needed, whether a U-value applies, whether trickle vents have to be cut into the head of the frame. Quotes are routinely written as though the line sits somewhere else, so it is worth knowing exactly where it does sit.

Which side of the line your job falls on

Everything in the top five rows can be done without notifying anybody, without an inspection and without a certificate, because there is no certificate to be had. The bottom two rows bring in Parts L, F, B and K at once, and somebody has to sign the work off.

Why the box frame is what decides it

The Building Regulations 2010 define building work to include the provision of a controlled fitting, and a controlled fitting is a window, roof window, rooflight or door. Approved Document L then spells out what the unit means, and this is the sentence worth keeping:

Set that against the way a box sash is built. The unit is the box: outer and inner linings, pulley stiles, head, cill, and the weights hanging in the boxes either side. The sashes run inside it. Take them out, make new ones to match and hang them on new cords in the original box, and you have not provided a controlled fitting. Neither have you by re-glazing a pane, splicing new timber into a decayed cill, replacing staff and parting beads, or draught-stripping the pair.

Pull the box out of the reveal and fit a new window into the opening, and you have. Everything below applies to that job and to almost none of the others.

The certificate nobody can give you for a repair

This is where money changes hands over nothing. A firm cannot issue a FENSA or Certass certificate for a repair. The competent person schemes exist so that a registered installer can self-certify the installation of replacement windows and doors, notify the council afterwards and hand you the paperwork. There is no equivalent for new cords or a spliced cill, because that work is not notifiable in the first place. If a repair quote leans on being "fully FENSA registered", it is advertising a qualification that has nothing to do with what you are buying. Ask instead for a written specification: timber species, cord size, what is being spliced and what is being replaced, the paint system, and the length of the guarantee.

For genuine replacement work there are two routes. Either the installer is scheme-registered and self-certifies, or you make a building notice or full plans application to local authority building control, or to a registered building control approver, and the work is inspected. The first is cheaper and quicker, which is why nearly every mainstream replacement window firm is registered, and why very few specialist joinery workshops are. Their work is mostly repair and made-to-match sashes in existing boxes, and there is nothing to certify.

It comes up again when you sell. A buyer's solicitor will ask for a FENSA or Certass certificate for any window replaced since April 2002. If your windows were repaired rather than replaced, the accurate answer on the property information form is that no replacement took place, and the joiner's invoice and specification are your evidence for it.

Part L: the number that applies if the box comes out

For a genuine replacement window in an existing dwelling, Approved Document L Volume 1 sets a limiting standard of 1.4 W/(m²·K), with a Window Energy Rating of Band B as the alternative route. Two things about that figure get misreported constantly.

It is a whole-window U-value, not a centre-pane one. Glass, spacer, frame and sash are all in it, area-weighted. A slim sealed unit with a respectable centre-pane number can still miss 1.4 once it is set into a traditional timber sash with a lot of frame in the elevation. If the difference between those two numbers is not obvious, our guide to what a window U-value actually measures is the place to start.

And the old timber allowance has gone. Until 14 June 2023, timber windows could be fitted at 1.6 W/(m²·K) or Band C while manufacturers caught up. From 15 June 2023 the full 1.4 applies to timber the same as anything else.

There is a second test sitting on top of the table: a replacement element must be no worse than the one it replaces. A single-glazed sash sits somewhere around 4.5 to 5 W/(m²·K), so any modern replacement clears that hurdle easily. It bites in the other direction, when somebody proposes swapping a sound double-glazed unit for something cheaper.

A 2026 edition of Approved Document L has been published, and it takes effect on 24 March 2027 for building work that is not connected with higher-risk building work. It keeps the same 1.4 W/(m²·K) limit for windows in existing dwellings, so nothing specified now is about to be overtaken.

Listed buildings, conservation areas and the character test

Approved Document L says that work to a dwelling which is listed, in a designated conservation area or a scheduled monument does not need to comply fully with the energy efficiency requirements where doing so would unacceptably alter the character or appearance of the building. It then says the work should comply where that would not unacceptably alter character, to the extent that is reasonably practicable.

That is a proportionality test applied case by case, not a blanket exemption, and the guidance is explicit that the building control body should consider the advice of the local authority's conservation officer. Two fallbacks are offered where windows genuinely cannot reach 1.4 without harming character: a centre-pane U-value no worse than 1.2 W/(m²·K), or single glazing supplemented with low-emissivity secondary glazing. The second is the one conservation officers reach for most often, because it leaves the original joinery and the original glass untouched and can be taken out again, which is why secondary glazing is the usual answer in listed buildings.

Note where that exemption sits. It is an energy efficiency exemption. It does not switch off Part B, Part F or Part K, and it says nothing at all about whether you need listed building consent, which is a separate application to a separate part of the council.

What else a replacement drags in

Part F: putting the ventilation back

Replacing windows makes a house tighter, and Approved Document F sets out what has to happen in exchange. If the existing windows have background ventilators, the replacements should have them too, no smaller than the originals and controllable by the occupant.

If the existing windows have none, the ventilation provision in the dwelling should be no worse than before. One way of demonstrating that is to fit ventilators in the replacements with a minimum equivalent area of 8,000mm² in habitable rooms, 8,000mm² in kitchens and 4,000mm² in bathrooms. Those are equivalent areas rather than free areas, and the ventilator should be marked with its figure. Other routes are allowed if you can satisfy building control that the dwelling is adequately ventilated, which is where continuous mechanical extract comes in.

Two details that save arguments on site. Replacing 30% or fewer of the window and door units in a dwelling counts as a minor energy efficiency measure in Approved Document F's own categorisation rather than a major one, which changes what else is expected of you. And the guidance notes that draught-proofing measures "might not, on their own, constitute building work"; they get caught when carried out as part of other building work. That is the regulatory reason draught-proofing a sash window so rarely brings any of this into play.

Part B: the sash you might have to climb out of

Fire safety does not care what you called the job. Approved Document B sets out what an emergency escape window has to offer:

• an unobstructed openable area of at least 0.33m²
• at least 450mm in height and 450mm in width, with the route out allowed to be at an angle
• the bottom of the openable area no more than 1100mm above floor level
• the ability to stay open without being held

Where an existing window would be an escape window in a new dwellinghouse and is big enough to serve as one, the replacement should offer at least the same potential for escape, reduced no further than those minimums.

A London sash usually clears the area and the height without trouble, because the openable area is roughly half the window. What catches people out is the 1100mm figure on a raised ground floor with a deep cill, and restrictors on a first-floor bedroom sash. A restrictor with a child-resistant release catch is acceptable. A screw driven through the stile into the pulley stile, which is the cheap security fix, is not.

Part K: safety glass, and where a repair sits

Approved Document K draws the repair line in almost the same place. Its own application paragraph treats glazing installed where there was none before, and the replacement of a whole unit meaning frame and glazing together, as building work. Replacing the glazing while retaining the existing frame, as a repair, is not building work, though the supply of the glass may still be caught by consumer protection legislation. A cracked pane bedded back into the original sash is therefore not a building control matter, whatever a glazier tells you.

Where it does apply, on a new window or a new opening, the question is whether the glass sits in a critical location: for a window, glazing below 800mm from floor level; beside a door, glazing within 300mm of the door edge and up to 1500mm above the floor. In a London house that means basement and lower-ground sashes, garden door sidelights, and anything glazed down to a low cill. There are three ways to satisfy the requirement, and only the first is widely known.

Even on work that is not caught, the specification is worth a thought. If a lower-ground sash gets broken every couple of years, or small children use the room, laminated glass is a sensible thing to ask for whether or not anything obliges you to use it. It also holds the pane together in the rebate when it is broken from outside, which is why it is the usual answer where safety and security overlap.

Five things to get in writing

1. Is the existing box frame staying? If it is, this is repair, and no certificate exists or applies.
2. If the box is being replaced, what whole-window U-value is being supplied, and is there a document that states it?
3. Is the property listed or in a conservation area? Consent runs on its own timeline and needs starting early, and our note on when planning permission is needed for windows covers where that bites in London.
4. Do the existing windows have trickle vents? Photograph them before anybody starts, because the replacements have to match or better them.
5. Is any window in the house a first-floor bedroom escape window, and will the openable area change?

If the quote is ambiguous

Read the wording rather than the letterhead. "Supply and fit new box frames", or "new windows to existing openings", means controlled fittings, U-values, ventilators and a certificate. "New sashes to existing boxes", "splice and repair", "re-cord and re-bead" means none of that applies, and any certificate offered alongside it is decoration.

All of the above is England. Wales, Scotland and Northern Ireland run their own regulations and the figures are not always the same, so check locally if you are outside England.

Where the box is sound and only the sashes, cill and cords need attention, the regulations largely step out of the way, which is usually the cheapest outcome as well as the best one for the building. That is the shape of most of the sash window repair work we take on across London, set out in more detail under restoration, and if the box really has gone, what replacement genuinely costs is the next thing to read. </content> </invoke>