Financing house construction / Land (self-financed) overpriced?

With a 1930s year of construction, one probably doesn't need to discuss that. ;)

You're right. My sentence also generally refers only to the observation of recent years. Unfortunately, the houses on the advertised (building) plots are getting younger and younger.

Are there already houses in the second row on the neighboring properties, or does everyone have exactly one house on a huge plot of land. demolished a similar house near Berlin and surely has current cost information available

Development in the 2nd row is generally not permitted in Brandenburg municipalities. However, there may be individual municipalities as exceptions where this is allowed. In this respect, the typical large plots are mostly what they have historically always been and what they were usually parceled into in the Berlin metropolitan area: namely garden land for Berlin summer vacationers.