Living area in the basement - south slope location

I want to keep all options open.

If you have that much money to cover all options and eventualities with extensions and conversions, and the building plot allows everything (size, floor area ratio, etc.), then go ahead. Almost all floor plan discussions fail because too much is built into the house for "future eventualities," to the detriment of current needs.

So a bathroom on the upper floor and considering where later a separate entrance door should go should still be within the budget

The basement and the ground floor appear to be full floors,

For the basement, I see it depending exactly on the reference height – over the old site it may possibly protrude less than on average one point forty.

For the basement floor, I see it as dependent on the reference height - relative to the old site, it might protrude less than an average of one point forty.

I see it differently. But whatever... this is less about the question of full floors (any architect can calculate that so it fits), but rather about whether one is willing to manage three stories including the converted attic. It makes a difference to have to pay for 160 sqm of developed living space or 240 sqm.

The question is also always whether you want to have a stranger in the house later who tramples all over your head.
[Außentreppe und Eingang im UG] might also be a budget question, since the indoor stairs cannot be omitted because of that.