I think we have already had this topic. There are no legacy issues. Unless you count the moral reprehensibility of forced labor as such here. But that was long before our time.
I was not aware that building materials were rationed during the war. Only because it became a war-important operation could the building be remodeled (originally it is even older). Every brick and every nail (really!) was counted, requested, and approved.
What is a big problem, however, is groundwater. The thing is located in a depression next to a gigantic former clay pit, which feels like an underground lake. Groundwater is already being permanently diverted. But as soon as you stick a shovel into the ground, it immediately starts gushing again.
Therefore, rather without a basement....