New builds are zero rated, so the builder does not need to add VAT to his invoices in this instance. They just need evidence that it’s a new build, such as planning applications etc.

Regarding the self build element and VAT for OP, I’m not sure. But it may depend on whether you are doing it as price work, or materials + labour + markup.

I personally do price work, i.e., I give client one lump sum price and handle everything else myself. In the case of new build, I don’t add VAT, but still pay VAT on materials myself, then claim it back.

So I imagine that it depends on who is technically buying the mats, because both of them can’t claim the VAT.