Hello,
District LB
I have now also found a construction contractor who demolishes and also builds houses. Possibly something else could be put there or instead of demolishing the barn, leave a 5m distance and put up a prefabricated house; alternatively, it might be possible to remove the barn and attach a new one basically onto the other house. Some kind of two-family house (or semi-detached house) or something like that.
First, gather information now and compare compare compare!