These summaries apply to replacement windows in existing homes in England. New homes, extensions and other buildings have further requirements; see our design guide.
1. Who signs it off
Replacement windows are notifiable building work. There are two routes. An installer registered with a competent person scheme self-certifies the work and registers it, and you receive a certificate. Otherwise, you or your builder apply to the council’s building control before the work starts. Keep the certificate: buyers’ solicitors ask for it when you sell.
2. Part L: energy
A replacement window in an existing home normally needs a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B or better. The U-value is for the whole window, frame and glass together, not the glass alone. Ask for the figure for your actual window size and glazing.
Listed buildings, buildings in conservation areas and other traditional buildings can follow a different approach, so that energy work does not harm their character. See heritage and conservation.
3. Part F: ventilation
Old windows are leaky; new ones are not. The rule is simple: the room must not end up with less ventilation than it had. If the old windows had trickle vents, the new ones need them too. If they didn’t, adding them is usually the easiest way to stay safe from condensation and mould, especially in kitchens, bathrooms and bedrooms.
4. Part B: escape windows
In a house, habitable rooms on the first floor, and some on the ground floor, usually need an escape window: one you could climb out of in a fire. The openable part must be:
- at least 0.33 m² in area,
- at least 450 mm high and 450 mm wide,
- with its bottom edge no more than 1,100 mm above the floor.
A replacement must not make escape worse than the window it replaces. Casements can use easy-clean escape hinges; sashes and tilt & turn windows can be sized to meet the opening.
5. Part K: safety glass
Glass in a critical location must be safety glass, toughened or laminated. For windows that means glazing within 800 mm of the floor; for doors and the panels beside them, within 1,500 mm of the floor. Low sills, window seats and full-height glazing all fall into this. Upstairs windows that open low also need restrictors or guarding where there’s a fall.
Source: Approved Documents B, F, K and L, GOV.UK. Contains public sector information licensed under the Open Government Licence v3.0. Summary only, not legal advice.
6. Planning permission
For a house, replacing windows with ones of similar appearance is usually permitted development, so no planning application is needed. You do need permission or consent:
- for a listed building (listed building consent),
- where an Article 4 direction removes permitted development, common on front elevations in London conservation areas,
- often for flats, which have no householder permitted development rights, if the look changes.
Quotation checklist
- Whole-window U-value for your sizes, or the Window Energy Rating
- Trickle vents where needed, and where they will sit
- Escape openings marked on the first-floor rooms
- Safety glass in every critical location
- The certification route: competent person scheme or building control
- Planning or listed building consent checked, if it applies
Questions
What U-value do replacement windows need?
In an existing home in England, replacement windows normally need a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B or better. Historic buildings can follow a different approach agreed with building control.
Do replacement windows need trickle vents?
Replacement windows must not leave the room with less background ventilation than before. If the old windows had trickle vents, the new ones need them too, and adding them is usually the simplest way to keep ventilation where old windows were leaky.
What is an escape window?
A window you could climb out of in a fire. In houses, habitable rooms on the first floor usually need one: an openable area of at least 0.33 m², at least 450 mm high and wide, with the bottom of the opening no more than 1,100 mm above the floor. A replacement should be no worse than the window it replaces.
Do I need planning permission for replacement windows?
Usually not for a house, if the new windows look similar to the old ones. You do need it, or listed building consent, for listed buildings, where an Article 4 direction applies, and often for flats.