Building a single-family house with/without a basement on a small plot

2.5 floors should just about be enough for 4 people in my opinion. If another 2 children come instead of 1, it will get tight. Then the basement should definitely be added, I think. (You have to be able to afford 3 children. Here comes again the hammer, the space and children discussion.)

I see it differently. Originally, 9x9 meters were mentioned. That is 125 to 130 m2 in 2 floors, so about 60 m2 per floor. That is a very typical semi-detached house for a family with 2 children!

Ground floor: small utility room and large built-in wardrobes in the hallway, plus a small storage room under the stairs.

Upper floor: 3 children's rooms with bathroom and small storage room.

Then comes the joker attic floor, so another approx. 35 m2:

1 shower bathroom, 1 master bedroom (without hallway, directly adjacent to the stairs) and a utility/technical room. Next to the bedroom a three-meter deep terrace over 9 m wide.

As long as there are only two children or child 1 plus two additional small children, one of the children's rooms can be used as an office. Otherwise, you can also see if you can optionally separate a corner with sliding doors in the attic floor.

Basement yes or no also sounds so casually easy. How about the finances? If it’s not a problem anyway, I would always build it.

If it weren’t a problem, the original poster probably wouldn’t be asking, I naively suppose.

My opinion on basements is otherwise clear (for slab-on-grade plots it is a luxury to build one, for steep slope plots it is a luxury not to build one, in between it requires a case-by-case calculation), but apparently there is a concept behind the development of the building area here – whether that is a good one or not. I may be wrong, but when looking at the overall framework conditions, I read the basements stipulated above as a hint not to poke into the swamp or other ground loads. Land developers – whether property developers or municipalities – have (not mostly, but) regularly motives that stand roughly one hundred seventy degrees opposed to altruism, public welfare & co.

I see it differently.

Yes, a question that everyone perceives differently. When I see the drafts at Tolentino https://www.hausbau-forum.de/threads/Doppelhaushälfte-Möglichkeitsabschätzung-Ideen-hinweise.33444/ then he is also clearly struggling there with 10x6.50m for 3 children. 9x9 would be a bit more, but not much more. Besides, 9x9 is a very unfavorable dimension where many standards often do not fit and space is lost. With 5 people, 9x9 would be too tight for me – but there is still the option of 10.5 x 9 – that would probably be okay without a basement. But then there is again the question whether one would rather build with a basement and have a bigger terrace instead. Well – difficult. I think the OP first has to create a complete room concept – what he needs and whether it has to be sufficient for 2 or 3 children.

Just as an example of what would be possible on 9x9 with SG, based on the Villa Casa 131 by bgw (the windows for the garage still need to be relocated): 3 children's rooms, a technical room, a utility room on the ground floor, a separable work niche, a storage room on each floor, and the roof terrace can even be used without having to go through the bedroom, which then has a panoramic window to the outside, ideally with pleated blinds.

Besides, 9x9 is a very inconvenient dimension, [...]
With 5 people it would be too tight for me at 9x9 - but there is still the option of 10.5 x 9

Conversely, I see it as urgently advisable here not to use the option to voluntarily self-limit within this building envelope. Nine by nine is social villa construction – you already need 3D with Dolby Surround and doll furniture to disguise that. For four and with simple demands, that is enough, but already linked with "or," rather not. The stepped floor remains far from a penthouse at these dimensions, but you can still make it quite nice up there for correcting manuscripts.

Just as an example of what could be possible on 9x9 with SG, based on the Villa Casa 131 from bgw

Oh dear, I don't find that very desirable now, aside from the fact that I have my doubts about the exterior wall thicknesses. But let's be surprised about what the original poster has in mind.