Essentially, new build construction is zero-rated for VAT purposes (or it can be reclaimed). However, work to existing buildings is generally standard-rated (VAT is payable at the full rate of 20 per cent).
So the current position is that all work to heritage and other existing buildings carries VAT at the highest rate of 20 per cent, unless you can find a way of paying a reduced or zero rate or of not having to pay VAT at all.
The CLA website has a useful checklist of possible options for reducing the VAT on works.
This can be quite a complex issue and you are advised to contact a VAT specialist for detailed advice regarding your particular circumstamces.