New Build Windows and Doorsets performance is determined by the Building Designer but the U Value (the thermal performance) of windows and doorsets is typically in the region of 1.4 to 1.6W/m2k – Higher levels of building airtightness can also be designed to improve whole building performance.
Replacement doorsets with greater than 60% glass area must have a max U value of 1.8W/m2k.
Replacement doors i.e. doors fitted into original installed frames (unchanged) are outside the scope of the changes but in any event should not be worse than the doors being replaced.
To understand more about U Values, please refer to Approved Document L Ventilation “Conservation of Fuel and Power” of the UK building regulations