Regulations · 7 min read · Updated 2026
Building Regulations for replacement windows and doors
Replacement windows and doors are controlled work. A competent installer handles compliance for you, but it pays to know what is being certified.
Thermal performance
In England, replacement windows must achieve a whole-window U-value of 1.4 W/m²K or better (or WER band B). Doors have their own limits. Scotland, Wales and Northern Ireland set their own standards, which differ in detail.
Ventilation
If the existing windows have trickle vents, the replacements generally must too — and recent guidance has increased the required ventilation area. Removing background ventilation can cause condensation and mould, so resist the temptation to leave vents out for looks.
Safety glazing and escape
Critical locations — low-level glass, glass beside doors and in doors — require toughened or laminated safety glass. Habitable rooms above ground floor generally need an escape window meeting minimum clear-opening dimensions.
Certification
Work should be self-certified by an installer registered with a competent person scheme such as FENSA or Certass, or signed off by building control. You should receive a certificate — keep it, because your conveyancer will ask for it when you sell.
Common questions
- Do I need planning permission for replacement windows?
- Usually not on an ordinary house, as it falls under permitted development. Listed buildings, conservation areas and some flats and new-build estates are the exceptions — always check before ordering.