No information from the building authority to the property owner

But it is not taken away from you just because there was a preliminary inquiry, or am I mistaken?

Maybe you might know more about the property

The plots are between 65 and 80 m deep. One has built quite far forward on the street with a large garden at the back, another more in the middle with gardens in the front and back, yet another has his house in the rear third with lots of garden facing the street. So I would assume the building area runs "from the foremost house" plot a to "the last house" plot b with the respective boundary distances. Mostly 1.5 stories, but also two houses with two full stories. Crosswise and lengthwise, the ridge direction thus rotated by up to 90°. So from-to. One semi-detached house, otherwise all single-family houses, some with lots of sheds.

But how exactly does the interested party want it?

One should just be careful who bears the costs in case of a rejection.

However, this refuses to provide information to either me (I have a power of attorney) or my father about the floor area ratio, site coverage ratio, DN, number of full floors, and the building envelope/building boundaries.

How often have there been cases where the described location is actually hundreds of meters away, and the property is not in the middle but no longer in the 34er area?

The sale is not about making a perfect preliminary inquiry so that someone would coincidentally build exactly like that.