Are the ancillary construction costs calculated correctly?

There is a very long list of things that you are not aware of and that are not mentioned to you by the general contractor either because they are so-called "construction-side", meaning your problem. There are entire threads here where this has been compiled. Highly recommended reading.

I don’t know if our construction company can/could also be called the general contractor!? But they also have architects (whom we have mainly dealt with so far). They also handle planning tasks such as buying and selling the property for us. As I said, everything that is possible is done by them. Except finances, of course. However, we will still commission an independent building surveyor.

Then watch the contract carefully. It is not standard that everything really comes from one source or is included in the final price.

Thank you for the hint! I was at another meeting at the construction company for several hours yesterday with my brother and asked all possible questions again. The rough outline has been discussed for now. Where which connections for the kitchen will go, whether we take different tiles, just paint or paint with wallpaper and so on... these little details will come later or will then be discussed again with the craftsmen's companies. So, in other words, the special requests! We will receive the contract in the next 2-3 weeks. I will certainly have some questions here then.... or maybe not.

Please make sure to read the mentioned thread!! I can't say more about it. We have warned you anyway.