Apparently, the authority follows DIN 18920. A Mr. Brudi was so kind as to summarize some key statements from it. However, it is only available as a PDF.
The requirements during the construction phase are no joke. In the root area, digging is only allowed by hand. The distance of 1.5m in the root area is clear. But what about the upper floor? Is it allowed to build an overhang right up to the crown? After all, 30% of the root area may be covered with paths.
The question remains whether it is all worth it. But the original poster is determined. I have not seen any property where § 4 g) of the tree protection ordinance fits better than here. The tree has an expropriating effect and prevents use beyond a reasonable measure. Did the previous owner already sue unsuccessfully?
As a nature lover, I am also against cutting down the oak. But then the land must not be designated as building land. The building law usage has been confirmed by the original poster and there is even a positive preliminary building inquiry (although it is unclear to me for what). So in my opinion, the tree would have to go.