Building at the in-laws' place

Is that a technical term?

Well, the legislative branch doesn't use it, the executive branch very often, the judiciary rarely.

which apparently is very old

Age is relative. Funnily enough, it is just as old as you.

Thank you very much for your answers. Even though it has taken away some of my hope. I have read the development plan again and have another question because I have new hope from one paragraph.

Specifically the part about the building boundary. It says if the floor area ratio does not oppose it, does that mean the building boundary does not apply if the floor area ratio is actually larger? Or does it mean you are only allowed to build as much as the floor area ratio allows even if the building window is larger? I hope you understand what I mean.

Building boundaries apply even if the areas enclosed by them do not exhaust the floor space index - just as conversely the floor space index also applies if the building window extends "further". The building window does not specify the (maximum) outlines of the building, but rather the location within which buildings can be positioned. Deviations from both limitations (the amount of the floor space index on one hand and the specific location of the floor areas on the other) can be requested; without deviation requests - which would result in exemption procedures having to not be applied and switching to classic building applications - the "smaller" limitation effectively applies "practically".

In your case, I would grab a copy of the property including all distances and building plots, place the house on it - (best to get advice on where the best chances for a positive response would be) and then make an appointment at the building authority. Have a good reason why you want to build exactly there (5 children planned, grandmother in need of care, wife is a beekeeper... etc.) and then politely ask if it is even worth submitting a preliminary request.