Living area in the basement - south slope location

I claim that with clever planning it is absolutely no problem.

Wardrobe and stairs should not be between the bedroom and the children's room.
If the budget allows, maybe plan with a gallery.

How many meters of height difference in the building plot are we even talking about?

How about three floors: basement living, kitchen, dining; ground floor children's room, wardrobe, utility room; top floor master bedroom, dressing room, master bathroom? One would possibly only need an air conditioning unit on the top floor

How about three floors: basement living, kitchen, dining; ground floor children's room, wardrobe, utility room; upper floor parents' bedroom, dressing room, master bath? Then you might only need an air conditioning unit on the upper floor

I always have to think of the floor plan of Gussek Haus "Haus Malaga"... I would take that as a basis and adapt it to my measurements. Great floor plan.

I always have to think of the floor plan of Gussek Haus "Haus Malaga"...

This is a semi-detached house (in the sense of an end terraced house), and also classic for flat plots (?)

This is a semi-detached house (in the sense of an end terrace), and also typical for flat plots (?)

I am also interested in the basic layout for three-story living

Such a variant is often recommended - whether it will actually be built, we do not know.
Do it!

Similarly, the question is whether we should then build 2 or 3 stories.

2 floors

Likewise, the question is how it will be when we are old, the children have left the house, and we can no longer walk stairs so well.

Think about now and the next 30 years. When old age arrives, you will be 80.
P.S. You are not immediately frail when the children leave the house.