Conversion of agricultural land to building plot, objection, building authority, building regulations

Exactly. I'm going to the building authorities. But if they tell me it "looks bad," I don't have to trust that in good faith, but if necessary, I have to sue it through if I am right. Then, however, with tricks or reasons, some of which I hoped to find here.

You cannot sue - that is the whole trick. You cannot sue to have a development plan drawn up, but only request from the office the establishment of a development plan or a building permit for an individual project in the outer area. If other plans oppose your request, it probably won't work.

I got the land use plan and cut out the important area. Since I am colorblind, I would be very grateful if you could explain what these "blue" bold lines mean and why my garden area is white dashed. Is this a "reserved area for special climate function" (white dashed) and "priority area federal" (blue bold line). I can't do much with the latter.

Is this ultimately a secure construction exclusion criterion, or could the construction still somehow be justified, since, for example, the fire department access and development are secured?

Your area is not designated for residential development. Go to the office and have them explain calmly to you how high the chances are that "your" grassland will become a residential area; no one here in the forum can reliably answer that for you.

Best regards
Dirk Grafe