Ok, thank you very much for the explanations, it all makes sense to me.
I think it really depends a lot on the location and other conditions one brings along.
Because we practically got the plot gifted to us, there is already a big item taken care of that otherwise one would first have to be able to afford. And especially the plot is simply worth its weight in gold for us. Last week, a house was sold on the neighboring street, 160m² living space, 950m² garden, 700,000€. The house was from the early 60s and the only new thing was an oil central heating system from the year 2000. It is strongly suspected that the house will be torn down and something new will be built. If that is the case, about 735€ per m² of land was paid, compared to a benchmark of 230€.
We thought long about whether we even wanted to build directly next to my parents, but the security due to the location of the property and especially the great location then made us decide to do it, provided we create some physical distance from my parents. And since prices for a 50,000-inhabitant town are currently skyrocketing like this, building a house would not have been possible for us at least for now if we were looking for a plot ourselves. And regarding the location, we wouldn’t have been able to afford anything anyway. That would have been a plot 15 minutes from the center by car, no public transport connection, and so on.
Therefore, in our case, I actually expect that the property should be able to be sold at a profit or at least without loss at some point. But that is not the reason for us why we even want to build a house, but simply the fact that then you have your own domain, where no one annoys you, no neighbor stomps above you or turns the TV up too loud. That will probably be taken care of by the kids instead.