To qualify for reduced rated renovations and alterations on residential property is that the property must have been unoccupied for the previous 2 years. The contractor/supplier can only charge the reduced rate of VAT if you can prove that nobody has lived in the property for at least 2 years, otherwise they will charge you VAT at 20%.
The best evidence is a letter from the “Empty Property Officer” at the local council confirming that the property hasn’t been lived in during the 2 years concerned. HMRC says this is sufficient evidence to confirm that the property qualifies under the “2 year unoccupied” rule. You can look for other evidence from the local electoral and council tax records and other sources such as utility companies. HMRC usually accept a combination of such evidence.