Floor plans of a single-family house on 640m² with low eaves height

For me, this is a plot of land between existing houses and not in a new development area. It may be that the term is wrong or inappropriate.

Oh, I see. Then it is called an infill lot, but I thought something else could be meant: a plot for a house for a temporary phase à la "the children have already left the house, but we can only move to Spain when my wife also retires"

So far, I plan to grow old here. Gap in development ... I just couldn't come up with it.

Exploitation of the floor space index

That gives me 448 m². I find that quite enormous. Do you really need that?

Floor area ratio: 0.7

0.7 * 640 does result in the above number, doesn't it?

Oh, somehow I messed up every possible value in the opening...

Grundflächenzahl: 0.7
Geschossflächenzahl: 0.4
Traufhöhe, talseitig: 6m
Traufhöhe, hangseitig: 3m

The second architect thinks it might also work without a basement. It was also a nicer, more open conversation than with the first one.
Now first to sleep on it for a few nights.

2. Architect thinks it might also be possible without a basement.

Given the height difference in the building plot, I don't see it working without one, however, I would then rather call this basement a lower ground floor – in that respect, he would technically be quasi correct.