Spacious single-family house planned over 200 sqm

If I insulate the interior walls of the cool pantry in DIY so that the heat from the other basement rooms does not come in?

The guest room is important in the sense that we need one. But it should not be a beautiful, light-filled guest room where guests can spend the day and look out at the garden from the balcony, but rather a room that is not cluttered where you have a permanent bed. So you don’t have to pull out a couch or anything when parents or friends visit.

You have to store your wood somewhere and if you only use the fireplace for cozy heating (that is, if you want to have a fire in the fireplace in the evening), then you don’t need that much. Then it’s okay if we have to carry the few buckets of logs back up every few days.

What roof pitch do you want? With a high knee wall it becomes difficult to get light into the dressing room and the children's room

Yes, these are the problems that we cannot yet adequately imagine. We want to have them visualized by the construction company. Also whether to choose a gable roof or a hip roof. The issue with the rather suboptimal windows at the knee wall is the biggest point against building without two stories.

The design is a spacious marvel
A great, great deal of square meters is planned that are not necessary. This happens when one simply hasn’t had the experience of what a meter is and where it actually is. It can be small, it can just as well be large It always depends on where the meter is planned. By the way, it also works the other way around, meaning that one plans too small
In your case, the guest toilet is barely usable, whereas your hallway boasts the size of a small apartment.
Yet the kitchen was too big for you, so the stove just remains free behind it?
The right approach would be to have everything properly adjusted by a professional (or here) and also to consider what the cellar should be good for, instead of immediately shooting the loan amount into a value that takes away your quality of life.

Exactly for that reason, I wanted to hear your opinion before I completely make a fool of myself with my trusted construction company. So would you suggest maybe only 9 m instead of 10 m in width? I just don’t know if the living room wouldn’t become too shallow if, for example, a bookshelf + TV were placed on the north wall of the living room. And then such a nice big corner sofa? Hmm yeah sure, I lack the imagination for that. Maybe the architect from the construction company will create that when he virtualizes it in 3D?

Is one's own income and equity also aligned with that of the cousin? I don't see lighting problems with a high knee wall; windows can be installed there as well. The only disadvantageous knee walls are those where the bend between wall and roof is at a height where one would prefer a clear view. "Architects" from construction companies are to be read in quotation marks. To grant a visualization significant influence over the building form (what does the development plan say about that?), the budget must be very fluidly scalable. If the cousin's house already exists, I would, in your place, take photos of details with a high want-to-have priority and introduce them here into the discussion.

I think the idea is quite good. In the implementation, there might be some issues with the staircase and the wall thicknesses. But the architect or draftsman can perfect that later. Important for such a plan is first to know the exact plot. Where are the access roads, parking spaces, distances, neighboring houses, noisy streets, etc.