Location of city villa or single-family house on 500 m2 plot - rectangular

It must be considered that the neighbor also wants to build something with the floor area ratio. This must be carefully thought through so that both properties can be developed sensibly.

Ok so the answer from the house builder is:
1. The path does not reduce the plot ratio
2. we are not allowed to build 23% more for ancillary areas, but overall with 0.23 of 500m² (so 115m²). That means we need to use the strip fully, otherwise we won't reach the area. And we definitely must not be larger than 10*8.
80 house
12 terrace
15 parking space
8 shed

Phew.

Everything is getting quite tight again

I think the calculation goes more like this: 950 x 0.3 (floor area ratio 0.2 + 50% of the floor area ratio = 0.1) = 285 285 - 64 path = 221 221 / 950 = 0.23

That means: for both plots, a floor area ratio I+II of 0.23 remains. Since the path counts 100% towards floor area ratio II, the office turns a blind eye to the 0.03 or you still need a walkway to the front door somewhere anyway, so in truth it probably doesn’t know anything about its "generosity."

So the path definitely counts fully and reduces the buildable area overall. The seller just subtracts it right away so that both plots can be split and sold and the buyers don’t start coming up with any alternatives.

It would be worth considering whether to sell the path to the city instead.

I have illustrated it here.

Useless effort, because in this way one does not recognize any connection beyond the two land parcels; that is, there is no chance to possibly find a better proposal. I join the faction that recommends the path as a third parcel, which belongs to both neighbors jointly. : could a setback area be adopted on it without this excluding a (non-sealing) paving of the driveway?

It seems clear, but just for completeness. There are no additional roads adjoining the two properties?