Once you are ready to proceed we go back to the principle agreement and select it for a full application. Then when it's opened all of the data is still there and we then fill in the new relevant sections of the full application. Whilst a mortgage in principle does not 'turn into' a mortgage, it does give you 'pre-approval' if your financial circumstances remain the same, helps you understand how much you can borrow, and gives you more power to make a serious offer to purchase.