A: In England, the requirements are set by Approved Document L (Part L) of the Building Regulations:

  • New build dwellings: limiting U-value for windows and doors is typically 1.6 W/m²K or better — with many designers targeting ~1.2 W/m²K for compliance in energy models

  • Replacement windows and doors (existing dwellings): often expected to be 1.4 W/m²K or better, or a suitable Window Energy Rating (WER) alternative

These values are whole element values (including frame, glass, spacer, etc.) and are used to demonstrate compliance with Part L energy targets.