You would have to be careful how you would present the case to the lender. Usually the max mortgage available on a self-build is 80% of the site value + 100% of the cast to build. I doubt the bank would lend 80% of the purchase price if the site, the full cost to build and then an additional €27k, this would be 100% of the total cost of the house and lenders don't offer 100% mortgages on self builds.
You could borrow the 20% deposit for the site as Bahman recommended but if possible borrow it from a family member or a CU not in the ICB. Then you could easily add an extra 27K onto the cost of building the house to then repay this loan.