Land acquisition - Initial ideas in early phase

I believe the lady of the house especially liked the French door in the original floor plan.

You certainly have much more experience. As a complete novice, I don’t see any particular problem with the one slanted wall, for example.

The problem in the original floor plan is the pantry. It can really only go where I scribbled in red here. And that possibly at the expense of the office, where I still need at least one wall of 2.5m+ (without a window).

In addition, the shower then somehow still has to fit into the guest toilet downstairs.

I think the lady of the house especially liked the double door in the original floor plan.
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The problem in the original floor plan is the pantry. It can actually only go where I scribbled in red here. And possibly at the expense of the office, where I need at least one wall with 2.5m+ (without a window).

Also, the shower then somehow has to fit into the guest toilet downstairs.

The original floor plan is in your first post.
If someone says that it is too long, then it must be there ;)

A double door also works in a straight wall, but many say that it is impractical.

On the other hand, it is of course much more expensive to build 2 full stories.

To put it like the Bremen Town Musicians: "we can find something better than a knee wall cramped into 87 cm everywhere."

What should be more expensive about the walls that rise straight up? – and using a "setback story," you can still build a one-and-a-half-story house if you can use less area on top than below.

When looking for inspirational floor plans, you have to remember to look one size smaller if you vary a straight full story on top.

I don't find slanted walls bad, but somehow they feel very '90s ;-)

What is supposed to be more expensive about the just emerging walls? - and with a "setback floor" you can still build a one-and-a-half-story building if you can use less area at the top than at the bottom.

I may be mistaken, but I do think that a setback floor is considerably more expensive than a simple gable roof. After all, you immediately have several roof surfaces of some kind that need to be planned, calculated, and constructed.

[Double door possible. However, I have marked the version I consider better with a large glass surface + glass door.]

but I do think that a mansard story is significantly more expensive than a simple gable roof. After all, you immediately have several kinds of roof surfaces,

I agree with that as well. I just wanted to point out that straight-wall stories and upper non-full stories are not mutually exclusive. I also see the ideal solution in the two-story house variant with the area distribution "upper floor like ground floor," classic with congruent exterior walls. Then, of course, with a smaller floor area than in the one-and-a-half-story house.

Double door possible.

Your floor plan suggestion makes me smile – it looks a bit like a better foundation for – and compared to Polydeikes’ first approach (and considering his plot), I find it a bit too tame.