Building window on the corridor map - approval

Yes, okay. I will speak to the office again. However, it is the case that they do not want to give me any commitment, or rather, they do not want to. I would only get a firm commitment with a preliminary building permit, which has a long duration.

Regarding my initial post. I acknowledge that it was not particularly advantageous. However, I can no longer edit it now.

... or as already described several times, to build a house that fits the plot, or else to look for another plot for the shell around the home theater.

Yes, that would be the solution that would then be the only one left, that's right.

Are they allowed to do that? If yes, why?

The neighbor's house, which you can also see on the plot map, is already very old. I estimate about 50 years. The building regulations are probably newer, and they did not go back to him and tell him that he would now have to tear down 3 meters of his house. But I can't say for sure. All the other houses have been built with 3 meters to the rear boundary, and I am supposed to keep a 6 meter distance. Hence my possibly inappropriate choice of words regarding "unfair" and whether the fact that others are allowed to do this also grants me such a right.

In my opinion, the image you posted is a montage of the development plan and the current land registry map. The existing buildings in the development plan are shown hatched. Even though the quality, especially due to the lighting conditions, is not very good, it seems that the existing building on 10/3 still fit within the building envelope at the time the development plan was drawn up. So the question would be when the extension took place and whether it was approved? Is there an exemption for this? Can the justification for this also be used for your project? The other houses fit and still fit within the building envelope. The altered setback was explicitly desired from an urban planning perspective.

Nevertheless, one could try to put oneself more into the shoes of people who are building for the first time, have no construction experience yet, don’t know the pitfalls, are looking for plots of land, only find junk, and then also come across slow administrative apparatuses that are as flexible as a crowbar and send you like the Captain of Köpenick from Pontius to Pilate.

We do!
At least I have given you the advice not to fixate on the house or the general contractor.
Basically— as you can see very clearly from your example— the plot of land comes first, and the house is then designed based on that. Period.
You expect a flexible administrative process, you expect someone to put themselves in your position.

I have done that: I showed you a comparable general contractor with a comparable house with which it would work. Without tricks!

Building a house is always a compromise— it doesn’t help to speak badly about everything and not even start by changing yourself.
You haven’t started building the house yet— how do you want to face other craftsmen who offer you good alternatives if what you want doesn’t work?

Building regulations that date back to times when people still found kermit-green bathroom tiles combined with piggy-pink sanitary objects nice.

But everyone has to comply with building regulations - whether they are old or revised. You are not the only one affected by this property.
And honestly: even you, at the age of 40 (if that’s true), grew up with these colors – that wasn’t that long ago to call that era outdated.