Single-family house floor plan 175 m2 - improvement suggestions?

As often as the OP emphasized in the opening post that divisibility is important, it is pointless to criticize this point.

Unfortunately, the fact is that this is not true! It is divisible with a caveat, but not convertible!

What do you particularly like? Why? Flexible and convertible floor plan, you could later make two units out of it

you cannot do without: Floor plan should remain convertible

It is now split into 2 residential units. That is not a possibility to convert.

It is important to us that the single-family house can be divided into two residential units

What do you not like? Why? Too much living space wasted on hall/corridor/stairs

Because division is the top priority and therefore creates 3 corridors.

Can the hall/corridor/stairs be reduced?

Yes, for example with a more open and generous single-family house concept.

The caveat is something like this, for example:
Who wants to use something like this as the main bathroom “in old age” or later after division, after having used over 10 sqm?

Or this here: who can reasonably furnish the bedroom with a wardrobe and double bed (“age-appropriate”) or at all with a width of 2.85 m, if they previously had a dressing room?

-> The width of the bedroom would basically already be the problem here for both bedrooms.
Of course, one could remain on the upper floor and make the ground floor including the garden available to tenants. … but is that what you want?
I can only speak conditionally for myself: I don’t want any of that, not today and not in 20 years. I wouldn’t spend money on that. Not even rent.
And after having read a lot here in the forum, the first mistake in house planning is to put this as a later priority focus only.
I advise planning for your first 20 years. What happens then, you don’t know.
By the way, I find the justification for the “convertibility” to be missing. Two small children are already there anyway, so one can expect to use the house as one family for about 20 years.

I strongly recommend drawing furniture to scale and considering the movement space. The table by the bay window – aside from the fact that benches are impractical, the dining area extends into the walkway between the dining and living areas.

A wardrobe for 4-5 people is missing. The utility room for technology, laundry, and storage is far too small. The ground floor feels cramped due to the desired division and conservative floor plan.

Upper floor: presumably room 2.3 is the master bedroom. The bed in front of the window makes cleaning difficult. The room is very narrow. Depending on the bed frame, you might only have about 50 cm between the bed and the wall. The dressing room door, as it stands now, can almost only open against one wall where a wardrobe is, and you desperately need the storage room. I would attach the dressing room to the bedroom to get a functional, generous space. I would convert the WC into a storage room or add it to the bathroom. The bathroom lacks space for a second sink – not a luxury for 4-5 people – as well as space for cabinets.

The room program is ambitious for the size. I would forgo a division to visually create more space in the house. Also, I would consider whether to reserve 2 rooms for 1 guest per month and possibly another child.

I totally agree with you. Most people who plan like this hope that one of the children will move in there. You want to have space here and now, but not face empty rooms in old age. I would rather plan for the here and now. And also expect the children to have small rooms. :D

The children can still move in later. The separation does not have to be as strict with one's own children as with renting to others.

The children can still move in later. The separation does not have to be as strict with one's own children as with renting to outsiders.

and then they live four people in 80 sqm? Or one child stays single and takes care? These are scenarios that basically never work out. An apartment for a caregiver? It will not become cheaper in the future to be cared for at home. So the classic tenant after all? Changing every few years and having to deal with every dripping faucet as a landlord? Or getting annoyed about this and that the tenant does with the nice house? ;)

Well, rental apartments are rare with us. Often a child with a partner stays in the parents' house. Usually, a small apartment is set up under the roof. Provided that the rooms fulfill their function, it would be sufficient here. For a third generation, extensions and renovations are necessary. Then almost everything is useless anyway. The ground floor is also not suitable for physical disabilities with the appropriate aids.

Why not build for now and adapt accordingly later? Everything that is new and modern now will be outdated by then.