Floor plan planning of an old building from the 1930s

Actually, yes. :D

Unlike other forum members, I agree with you, as a non-expert and completely subjective (this refers to me, not to you). :-) It is exactly the kind of house that I, based on my previous experiences with our/a new build, would buy. I can't say anything about the location etc., but these pretty details—front door, roof, bay window, staircase, etc.—are what make a house charming/beautiful to me, and which we can no longer realize in our new build. The alternative you described of the standard gable roof house I find less attractive, but that is not decisive.

And with such plots, you can also convert 2/3 into meadow and mow twice a year.

Or maybe lease it out if there are nice people who would like to have a few fruit trees or beds there? I have also thought about letting a few sheep graze there from time to time, but then there are the residues. :-( Is it entirely ruled out that it could once be developed into building land?

What does the Geoportal say? Is it designated as [Bauerwartungsland] or [Grünland]?

I like these old houses, it was originally my dream to redesign and refurbish something like this. I think it would require someone with a lot of expertise who can assess 1) what is worth preserving and can actually be preserved, 2) what effort (also financially) is reasonable - many things ARE POSSIBLE, but can sometimes be a money pit, and 3) can also visualize it.

Judging by the pictures, I wonder if it would be something for you to completely tear down the extension and replace it with a modern addition, thus deliberately creating a contrast between classic and modern.

Otherwise, I cannot judge your floor plan well because it is not clear to me how you live or would like to live. The door labyrinth on the lower right side would be a no-go for me, as the rooms there seem to consist only of passageways.

Judging by the pictures, I wonder if it would be an option for you to completely demolish the extension and replace it with a modern addition, thereby deliberately creating a contrast between classic and modern.

My thought went in exactly the opposite direction, to replace the [Schlaginsgesichtanbau] with a harmonious extension.

I would have to be listed in the ad now, but I think it was all building land. Although then, of course, there could be problems with access again, it looks relatively steep. And the question is whether in a relatively structurally weak area you get anything significant for having someone moving in on you.