Should we build on our own?

Since you only need a relatively small loan, I assume that you don’t have to watch every last cent. In that case, I definitely wouldn’t put myself through that, especially since you don’t have the time yourself and the father-in-law’s knowledge is probably outdated as well. I am currently planning to modernize only 2 bathrooms, and even then I leave the planning and coordination of the trades to a craftsman because I have realized that I simply don’t have the time during the day to deal with it sufficiently. My craftsman has already given me many useful tips during the planning and is (hopefully) also making sure that the work is done roughly within the given budget. He knows affordable and good suppliers that I would otherwise have to search for laboriously. For a shower partition, you can pay 200 EUR, but it can also cost over 1000 EUR, and it’s only conditionally better. One is custom-made directly on site by the glazier, the other comes from the bathroom equipment manufacturer. It might get a bit more expensive, but I have a good feeling about it. You should consider that too, no matter what the father-in-law thinks. Maybe you can give him a meaningful task (supervising the craftsmen), but not so much that his presence drives the craftsmen away. And if you finish earlier than if you do it yourself, you might save the double living costs, which also offsets the extra cost a bit. My brother is also starting to build his house soon, and he is not doing anything himself either. Those who are professionally very busy usually have no other option.

Thank you for the many comments. That's exactly how I thought it too - then I was already on the right track.

Thank you for the many comments. That’s exactly what I thought too - so I was already on the right track.

you just have to negotiate with the father-in-law

I think we can manage that - after all, he still says the sentence "you have to know what you want - I won't interfere."