Replacement window regulations

Replacement window regulations in England turn on one question: are you replacing the whole window, or only the glass? If you fit a complete new window with its frame, it has to meet the energy limit in Approved Document L, and someone has to certify it. If you only change the glass in the existing frame, the energy rules do not apply. The detail below is from Volume 1 of that document, which covers homes.

Replacement window regulations: the whole-window test

The document calls the entire unit of a window, including its frame, a “controlled fitting”. Replacing glazing, or a window in its existing frame, does not provide a controlled fitting. That work does not need to meet the energy efficiency requirements. So a new sealed unit in your old frame is outside the energy rules, and a new window in a new frame is inside them.

The energy limit in the replacement window regulations

A whole replacement window must reach a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B or better. The same paragraph says the unit should be draught-proofed, with insulated cavity closers where appropriate. Building control may accept a Window Energy Rating from a certification scheme that checks performance from calculation through to installation.

Old houses and the character exception

Some windows cannot reach that figure without changing the look of a building. For those, the document allows a centre pane U-value of no more than 1.2 W/(m²K). It also allows single glazing with low-emissivity secondary glazing added. The route is meant for buildings whose character depends on their windows, so agree it with building control or your council first.

Making a window bigger or adding one

If you enlarge a window or create a new one, the document also limits how much glass the house can have. Windows, roof windows, rooflights and doors together should not exceed 25% of the floor area, unless you make up for it elsewhere in the house. A like-for-like replacement is not affected.

Who certifies replacement window regulations work

The document says that where the work consists only of fittings such as replacement windows, and the owner employs an installer registered with a competent person scheme, a building control body does not need to be told. FENSA is the scheme most window fitters use, and it describes itself as working across England and Wales. You get a certificate when the job is done, and your buyer’s solicitor may ask for it when you sell.

If your fitter is not registered, FENSA says you are responsible for the building regulations, and you would need to pay for an inspection by building control. Make sure the fitter agrees in the quote to put right anything the inspector finds. You can check a fitter on FENSA’s site before you order.

What changes with the next edition

A new edition of Approved Document L has been published. It takes effect in England on 24 March 2027, and a circular letter sets out the transitional arrangements. Its table of limits for windows keeps the figures above, so the headline rule is the same. Which edition applies to your job depends on when the work starts, so check with your fitter or building control.

Other nations and other rules

These rules are England’s. Scotland has its own building standards, covered in replacing windows in Scotland, and Wales and Northern Ireland have their own building regulations too. Glass in low windows and doors also has its own rule, explained in BS 6262 safety glass. Your planning position is separate again, so check it if your home is listed or in a conservation area.

Next steps

Ask each fitter how they will certify the job, and look for the FENSA certificate or equivalent in the quote. Compare quotes, see what replacement windows cost, and get free quotes.