You charge the builder 0% if it's new build, there are various other rates ie 5% on energy saving work ewi renders for instance there are different rates on churches etc too. It Tallys up in your books as if your claiming more than your charging you get refunded the difference. I've always found being vat registered works for us due to me having that many overheads to claim against and not doing to many domestic jobs and even then in the render game most are vat registered, so it's like for like.