Changes to Help-to-Buy Scheme

For self-build applicants to the HTB scheme, what classifies as the Purchase Price of the house? The final valuation? the amount paid to contractor for builders finish? etc
I ask because of the 70% LTV rule which I am on the wrong side of at present, but wondering if there is a workaround.
 
Hi,

Can someone please advise how the help to buy scheme works?
I am starting from scratch (haven’t saved yet) so would love to know how to go about it.

Thanks