According to the floor area ratio, only 138 m² is possible - 2 floors - Help :)

So it is 0.15 after all - I didn't misread it after all

We are still waiting to hear what our chief says, who doesn't seem to be online right now.

Ok, that would be interesting ;)

Then it will probably be the small house after all ;) good for the wallet ..

... again differently, the site coverage ratio, site coverage ratio, for the property marked with a red cross is 0.15. The floor area ratio, floor area ratio, is not specified.

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once again differently, the floor area ratio, floor area ratio, for the property marked with a red cross is 0.15. The plot ratio, plot ratio, is not specified.

Who says? The plan directly states Geschossflächenzahl. In my opinion, there is no room for interpretation at one's own discretion.
However, I could imagine that the building authority, after more than 50 years, might be receptive to one or the other exceptional regulation. Perhaps they only shy away from the costs of a completely new plan - because - that would be pointless for the already developed properties.

... an architect I once asked!

I received this further feedback:

I just did some basic research again. In the development plan, the floor area ratio is set at 0.15, meaning 15%, so you actually only have 138.50 m² available combined on both floors, which corresponds, depending on the construction, to a net living area of about 110 m² for the house.

Amazing ;) how much can be interpreted from that ..