How to correctly determine the loan amount when there is a lot of personal contribution?

Oh, I wasn't really aware of that. In that case, material and equipment rental (each with an invoice) would be the required proof. Oh, the motivation is completely missing in the initial post. Sorry about that. We have the following concern: We underestimate the costs in advance and then either a) have too little loan and have to refinance expensively or b) have too much loan and pay interest and commitment fees on money we actually don't need. The overarching goal of the own contribution is, of course, to save costs (who would have thought). The bank advisor told us in the first meeting a total price of 450 - 500k€ (1.5 floors, basement, double garage, KFW55) for a house in the village, where in my opinion, a significant loss of value must be expected upon sale (sale is not actively planned as of today). And despite equity, that would mean easily >30-40 years of loan term. The other (and almost even more important) point: I want to maintain a certain flexibility in execution during the construction and have the trades done by people whose craft I know (for years) and that it won't be botched. I hope that this doesn't drift into fundamental discussions at this point.