Ground floor approx. 100 sqm, upper floor expandable (planned bathroom, 2 children's rooms, 1 storage room)

I am 61 years old and have never had breakfast on the terrace and will not do so. Because

Unpleasant for you... but has little to do with the OP.

Unpleasant for you... but has little to do with the OP.

But it has to do with contrived examples that are practically irrelevant.

However
That doesn’t work either. You run into the slants.
(But if necessary, still doable with a different door position)

Yes, something like that:
Although here the staircase was simply moved a bit towards the entrance. So far, no one really knows exactly where it is and how long, etc. The window is also still a question, and whether you could get the drainage all the way to the utility room.

But it has to do with far-fetched examples that are practically irrelevant.

They are practically irrelevant because you, who is not even the subject here, haven't managed to have breakfast on the terrace in 61 years?

Having breakfast on a terrace is really nothing exotic.

Yes, something like that:
Although here the staircase was just shifted a bit towards the entrance. So far, no one really knows where exactly it is and how long etc. The window is also still a question, and whether the drainage could be extended up to the utility room.

Some of the proportions might not be quite right.

But as I said, I’m only concerned with the upper floor. There, we want 3 living rooms, 2 of them a bit more spacious than the 3rd room + bathroom and a small storage room.

Sorry, but you have to see it yourself... Nothing will be spacious, become spacious or look spacious. You will have a good floor area, but overall it will not really be suitable for the location. Why should there still be a bedroom and a bathroom in there? You have those downstairs. Make 2 children's rooms, storage room and shower toilet.

Exactly...because here in the whole neighborhood, despite an exceptionally good summer, nobody does anything like that. So marginal problem. Regarding the attempts to squeeze 4 rooms in up there. Leave it. The homeowner will learn, when the time comes, that 2-3 max.. will have to suffice.