There is a simple rule of thumb for a new build:
a. any contractor who is vat registered MUST invoice you with zero vat for all materials, labour, hire fees, etc..
b. For any contractor who is NOT vat registered then you MUST purchase, be invoiced for and pay for all materials including paying the vat on those invoices.
At the end of the project you complete a vat return to claim back the vat element you have previously paid out but VAT on some charges is not recoverable, e..g on fees, hire charges, etc.
If you organise your contracts carefully most of the irrecoverable VAT can be avoided. You do this by arranging for a vat registered contractor to contract and pay for fees, hire, etc. You then get invoiced with zero vat.