Hi Variable8 Sze,
Please see the guidance below:
8. Reduced rating the renovation or alteration of empty residential premises
The builder will need to be satisfied that the property has been empty for 2 years or more in order to charge the reduced rate of VAT.
We do not specify the evidence which must be kept by the builder but it does need to be reliable evidence.
Please see the guidance as above, specifically 8.3.2.
Thank you.