Obtaining platform support - development plan

The grey-hatched area below you will be an underground car park or what kind of area is that? [TG]?

The hatched areas will be existing buildings, the marking TG the possible building envelope for underground garages beneath the multi-family house. It is only the draft, meaning it does not necessarily imply that there will still be existing structures on the TE's property when he buys it. I believe that was the actual question.

The plot has no disadvantages as far as I understand

I see it the same way.

and next to the semi-detached house there would be an office.
Now I have to find out if there is a slab there and if so whether it can be converted to passive house standard or should be completely removed

I don’t understand a word.

Can you please delete the post fully or partially with plot number [...] I will be careful with pictures

On this, fundamentally: Answers in public forums are given for "the whole world," one questioner is read by more than ten lurkers (who thankfully often read silently). Answers are given free of charge – that they remain after the questioner’s thirst for knowledge is satisfied is part of this free-of-charge deal. In this respect, it is unfair to want "your" thread deleted or to have it deleted. It is not called "thread owner" without reason but "thread creator." Moreover, no one who asks questions here needs to be worried about their plot number: even the most desperate property seekers would never think of combing forums for plots to snatch them away from other interested buyers – simply because it would crown an already peak level of inefficiency. The popular defamations of street names are nothing but childish.

The gray hatched area below you will be an underground parking garage or what kind of area is that? UG?

The gray hatched areas represent "future removed" existing buildings, i.e. buildings that still exist at the time of the plan preparation but usually are not to be preserved. The UG starts with the driveway (white, opposite the forecourt with parking spaces, gray at the edge) and extends up to the orange dashed line. It belongs to the adjacent multi-family house block, which here serves as part of the noise protection concept shielding the single-family houses. It is not part of the existing buildings and does not extend under the OP’s plot.

... even the most desperate property seekers wouldn't think of combing through forums for parcels they could snatch away from other interested buyers ...

In fact, I am also one of the anxious property buyers. As long as we are not yet registered as owners in the land register, I will be accompanied by the worry that someone could take it away from us again, somehow and for whatever reason. Even though the property has already been paid for and we are now waiting for the real estate transfer tax assessment notice from the tax office as the next step.