Building Regulations for Replacement Windows and Doors (2026 Guide)

Replacing windows or external doors is “controlled work” under the Building Regulations, which means it has to meet minimum standards and be certified. Here’s what actually applies to a like-for-like replacement in an existing home in England.

Energy efficiency (Part L)

A replacement window must achieve a whole-window U-value of 1.4 W/m²K or better (equivalently, a Window Energy Rating of Band B or above). Replacement external doors must reach 1.4 W/m²K as well. See the Approved Document L for the detail.

Certification: FENSA or Building Control

You have two routes to prove compliance: use an installer registered with a Competent Person Scheme such as FENSA (who self-certify and send you a certificate), or make a Building Control application to your council. Using a registered installer is simpler and is what nearly everyone does — and it’s the first thing to check when you compare installers.

Safety glass, ventilation and escape

• Safety glazing (Part K) — toughened or laminated glass is required in “critical locations”: within 800mm of the floor in windows and 1500mm in doors/side panels.
• Trickle vents (Part F) — if your old windows had background ventilators, the replacements must provide at least equivalent ventilation.
• Emergency escape (Part B) — habitable-room windows may need a minimum openable area for escape; a fixed or heavily-mullioned design can fail this even if the opening is large.

Scotland, Wales and Northern Ireland have their own (broadly similar) regulations, so confirm the figure for your nation if you’re outside England.

Frequently asked questions

Do I legally need a FENSA certificate?

You need proof of Building Regs compliance. A FENSA (or equivalent) certificate is the usual evidence; without it, selling the home later can be complicated.

What U-value do I need for new windows?

1.4 W/m²K or lower for a replacement window in England; many modern windows comfortably beat this.

I lost my FENSA certificate — what now?

You can request a copy from FENSA, or a local authority can issue a ‘regularisation’ certificate for past work.