Go for the maximum approval in principle saving you having to reapply if your initial thoughts on a price are less than what the actual cost will be.
Once you apply for a formal loan offer you will have to show costings from the builder/engineer/architect to support the amount you want to borrow.
Don't worry, on a self build project you will end up drawing down the whole amount!
In fact most lenders will require you to have savings to cover overruns.
Banks deal with the loan to value issue in different ways, some compare the mortgage amount to site + build costs (i.e loan to cost) resulting in a higher rate initially but once the house is completed they will then use loan to value that should result in a lower rate.
[broken link removed]