First calculation - realistic?

... at least on paper. The rude awakening comes later. (There are also good prefab house providers, even near Cologne.)

First of all, thank you very much for the many replies.

Of course, this is still very naive at the moment, since we neither have experience nor can we really rely on people who have done this. Only a family of friends who have been living on a construction site for years. For example, the staircase to the front door was only recently finished; before that, it was pallets. So not really suitable advisors, at most a bad example in this context.

Maybe a bit more precisely about the finances. We made some mistakes there and are now paying them off dearly for several more years. Now the living situation has become very bad (previously a very well-maintained multi-family house, then a change of owner, now very noisy neighbors moving in, for example) and we have to change that. Renting with the kids (and dogs) is very, very difficult. We would have to put at least €300 more on the table than now. But to possibly experience the same crap again as with each of our previous rental apartments? No, that topic is done. If the situation were different, we would first repay the loan, maybe save a bit more, and then start.

Anyway, I find it very hard to ask the right questions at the beginning. Building/buying alone without parents is out of the question, we agree on that; they have to be involved.

Does it make sense to consult a financial advisor at this point and first clarify the budget? Or should we first visit various house-building companies and see what is possible with a rough budget?