In the same boat as you with regards the site in thats its free. We went for a BOI mortgage and after getting approval in August to go ahead they have retracted the offer until further criteria is reached. I wish we started building then!
Anyway to move forward I have to value the land, get the house valued and finally value both the house and land. Once I have valued it then the bank will value it separately (crazy or what!). The bank will then lend 80% of the combined value of the site plus house. Hopefully we will get the second approval in the next month so we can finally start the build.
On a separate note I have a few friends who are struggling to get their final stage payment as the home they have built has moved into negative equity so that is something to be wary of.