why are then 295k for a local BU without additional costs, without special requests or other items (so pure house price just brick-clad) unrealistic? Are we talking past each other?
Thinking from the other side: 1. Building a house financing on a calculation that assumes the optimum at every step is shaky. No problem if you can add more money, bad if you run out of financial breathing space during construction. Here I see a considerable risk. 2. Pay attention to the fine print in the fixed price. Especially for efforts regarding earthworks up to the base plate, which are hard to estimate, there are clauses that shift partial risks to the buyer. This quickly amounts to five-figure sums. 3. Whoever calculates very tightly must be very disciplined down to the smallest detail. For floor coverings, every €/sqm counts, and then you sit there thinking "wow, that looks awful, that one is so much nicer and we want to live with it for a long time..." Suddenly the floor covering costs four digits more. And don't believe that this question only comes up once. 4. Without outdoor facilities – we did the same with our terraced house in 2001, only a paved 3x1 pathway to the front door helped keep the worst dirt out (move-in in September, a two-year-old and a newborn = stroller). The joy about the house could compensate for the dirt problem for a while, but then... I am not a financing professional, I have the experience that you can go bankrupt (twice) and also how to get out of it again. Failure is no shame, but dealing with the consequences requires much effort and deprivation. I would not be afraid to approach the project like you. I would never advise you to do so, that would be irresponsible.