I'll summarize: - Zoning plan from the 60s - Floor space index of 0.15 We are now in 2019: Nowadays, the opposite applies - land should not be "wasted" but used well. I hardly think that a floor space index from the 60s can still be "enforced" in such a fundamental way today. Either there is an adapted zoning plan (after all, over 50 years have passed) or it is steered into the present era informally via the "Hilfsgrösse Ausnahme". I am optimistic about this - and I would have long ago made a preliminary inquiry with the building authority - either informally through the architect or directly.