Own floor plan design 135 sqm opinions and experiences

No, I do not have any specialist planner knowledge. I am only concerned about where downpipes could go, and if they can possibly be used for the bathroom above. The same goes for the ventilation ducts on the upper floor. Where could they go, can I supply two rooms simultaneously, and so on. My concern is to think about this to avoid too many surprises when planning the floor layout with a general contractor. From what I have found out, some contractors do not provide much support during the floor plan planning.

I gladly refer to post #4. Particularly important for me are the zoning plan, budget, and whether there is a slope.

A cross-section is also quite nice, it clearly shows that you hit your head getting in and out of the bathtub. Why draw the 2 m line if you then ignore it?

The stairs get no natural light at all. For me, that's a no-go.

Before I forget, I think it's good that you are already thinking about this in advance.

After you answer the questions, there will surely be more helpful advice.

It's about thinking about it when planning a floor plan

That is always good and correct.

doesn't provide much support in floor plan planning.

And that is also true.
Nevertheless, you are leaning a bit too far a) out of the window when you think about where and how technology will be installed b) still a very long time away, to talk about technology and its implementation.
Because first of all, you want to discuss your ideas, which are not fixed yet, I hope. And I also patiently refer to #4 so that the floor plan planning can be addressed. It may be that your initial post makes sense to you. But to discuss, you start at the beginning, and therefore some information is missing.

No, I have no specialist planner knowledge at all. I’m just concerned about where downpipes could go, whether they can possibly be used for the bathroom above. The same goes for the ventilation ducts on the upper floor. Where could they go, can I supply 2 rooms at the same time, etc. It’s about thinking this through so as not to have too many surprises when planning a floor plan with a general contractor.

Place the bathroom above the kitchen or the utility room, which in a single-family house will usually also be your HAR. You will always have surprises in construction with a GC without architectural planning. The GC’s heating engineer and the GC’s draftsman don’t need you as a third player for skat. The air outlets will be linear and decentralized.

From what I’ve found out, some contractors don’t provide much support during floor plan planning.

That is generally not their task. If you’re envisioning building with a GC without an architect, then review their construction proposals to see how you would need to adapt them, instead of coming up with your own new creation. GCs are construction, not planning contractors.