Many thanks for all the answers. In fact, the building area was still relatively "affordable" 1-2 years ago. About 400 euros/sqm, now we are at double that because there simply is no more supply here. The seller bought the plot back then in the city's bidding procedure, you could bid starting from 900k at that time, how much he ultimately paid, we do not know.
There is already a building permit for a house with 600sqm living space (see picture) which would not be suitable for us. We would plan more "humanely," of course, we are still aware that it will be an expensive matter. The house was planned 22x9m, but it stands very close to the oaks. The oaks are protected, they should not be cut down, which would also be a pity. Only, for example, the oak processionary moth comes to mind, branches falling down due to storms (we have children and would also like them to play around the house carefree).
The crowns are about 12m wide, so these are really huge trees.