What interest rate is realistic?

The house banks usually make the worse offer, it's worth comparing.

I don't want to leave it at that. I believe it strongly depends on the house bank and their current approach to construction financing loans. Our regional house bank was only 0.01% worse than the best offer from a direct bank.

There are no other loans, I do not know my Schufa score, but there has never been a problem with payments or loans. I will now get some offers from the common internet providers, as they use the same data (Schufa etc.). If the offer there is significantly better, then I know that they just wanted to rip me off. I will report on the terms under which I will get the loan.

I don't know my Schufa score

Then just request it from Schufa. It costs you nothing, you print it out online – send it by mail with a copy of your ID (I think it says so) and two weeks later you'll know if any unpleasant surprises come from that direction. You can request it once per (calendar) year free of charge (make sure you get the right form).

You just wrote that there have never been any problems with payments and loans. Maybe a paid-off loan was not reported as such. Personally, I would rather clarify such things to be safe (better to check beforehand than to be unpleasantly surprised later).

0 is not the risk, it is exactly 200k

The risk is neither 0 nor 200k€ but something in between. The maximum damage amount is of course 200k. But if the probability of occurrence is 1 (= The bank knows that the loan will not be repaid, damage will definitely occur), the bank will hardly offer the loan. How high the risk is for the bank is determined by it based on its guidelines by setting the probability of occurrence. In general, the risk is always calculated using the simple formula: damage amount x probability of occurrence.

The damage amount is regularly not the full loan amount but may be adjusted with discounts or surcharges.

Now I have obtained an offer from an independent financing broker with 1.89%. After I confronted my house bank with this, they were suddenly willing to offer the same conditions. This shows how you are initially being ripped off. So, comparing and negotiating is worthwhile!