We are currently having a similar problem with this application. The developer does not want the application to be filled out by the engineer before we have signed the construction contract. However, we only wanted to sign it after we received the loan commitment from KfWBank. I didn’t know that it was supposed to cost 300 euros. We had offered the developer to pay something for the application and thought that it could be offset later. But he was not really keen on that. He wants to mediate the loan through his own financial services company, but we are well advised by another financial person and want to do it with them. Ultimately, the loan is applied for at the same bank (Ing-Diba and KfW).
Actually, the 300 euros should be offset against the total price with you, since the calculations are part of the construction plans, etc. You probably haven’t signed the construction contract yet either, have you?
For us, there is also an extra surcharge of about 3000 euros for "Effizienzhaus 70", for more insulation, etc. Then something extra comes for triple glazing as well.