Hi,
There is no procedure to claim back the reduced rate of VAT charged.
If the property has been empty for 2 years or more and a contractor completes renovation work for you then they would automatically charge 5% on the work, both for the materials and the labour. However, if you have completed the work yourself and have purchased materials yourself, then unfortunately there is no claim procedure for this.
There would only be a claim procedure if the house had not been lived in for 10 or more years. You can see guidance here:
Claim a VAT refund for a conversion if you're a DIY housebuilder
Reduced rating the renovation or alteration of empty residential premises
Thank you.