6 Star report specifies the exact requirements for your windows.

Minimum Deemed To Satisfy (DtS) energy performance requirements for a building window U-value & SHGC are specified in NCC Vol 2.0, Part 3.12.2 for residential dwellings. This means if you want to know the acceptable range of U-values for your building, you need to follow Deemed to Satisfy (DtS) provisions. But what if you need to go beyond rigid DtS provisions? say you need to have the window U-values increased and thus, lower your construction costs? In this case, you need to undertake a Performance Solution instead of following NCC’s Performance Requirements. For instance VURB modelling is a Performance Solution for residential dwellings.