Your original principal balance would be the amount you have drawn as of the acquisition date - if that was "0", and the program won't accept it, then round up to "$1".
With a construction loan, the lender has committed to lending you a certain sum, but you don't owe that amount until you have drawn it completely down in the process of building your home. You will only pay interest on the amounts you have drawn, not what they have committed to lend you over the course of construction.
And, you are not obligated to eventually draw the entire amount. If you decide to stop construction (illness, divorce, found a better property), you can stop - you'll be out the costs of getting the loan (points, application fees, appraisal, etc.) but you'll only owe the bank whatever they have actually lent you for the project.
