How do I know what U values I need for my home?

How do I know what U values I need for my home?

Approved Document L Ventilation “Conservation of Fuel and Power” -

New Build Windows & 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 1.8.

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.