"Construction project with a 'special' initial situation"

If the expenses are right, why not.

You don't have to finance 350-400, but only 250-300 - if you sell the other house. With 4,600 that should be doable! If you want to finance the entire amount, it will be more difficult, especially since the other house has not been paid off yet.

If you want to finance the whole summer, it will be more difficult since the other house has not been paid off yet.

But then rental income would also come in (at least €850 cold rent, if the location is that good, significantly more).

Then rental income would also come into play (at least €850 cold rent, if the location is that good, even significantly more).

And then again reserves for the house + potential risk from rent dodgers, etc. It can work, but doesn't have to :)

we received the tip today (from someone from the Bauherren-Schutzbund...) that it is wise to first buy a plot of land and finance it separately, and then build, and also not necessarily with a construction company but with an independent architect

what do you think about that?

It is also a normal procedure, but a construction company can still act as the general contractor. Otherwise, you (or the architect) would have to award the trades separately. We did it that way as well.