Single-family house floor plan approx. 190m² - Optimizations

Yes, what is supposed to happen in the guest room? I think it can be turned into the children's room. At 13, when moving in at 14 or 15, detaching from the bedroom is much more valuable than square meters added. But the child could decide that for themselves, unless they have already chosen the room upstairs. If they want to stay there, then the office can be made downstairs.

Yes, what is supposed to take place in the guest room?

The OP explicitly said that it is supposed to become a bedroom later. Until then, it just remains an office. Whether that will really happen... who knows.

...gain sufficient space on the upper floor to achieve a practical walk-in closet (the trapped walk-in closets are not),

The OP explicitly does not want to access it through the bedroom and prefers it to be trapped. I think he already knows the pros and cons.

Here is another variant with a kitchen bay window and gallery. Bathroom in the east.

Is another child planned? If not, what should go in there?

I can combine office and guest room since I only have guests so rarely - that doesn't change the fact that I might later use this room as a bedroom. If I interpret it correctly, it's about having a sleeping room downstairs in old age - by then the child has long since moved out.

However, I would point out that people probably wouldn't want to give up the comfortable bathroom upstairs if there's only a small bathroom downstairs. But that is up to each person. I would always prefer to plan a staircase in a way that I can possibly install a stairlift later rather than engaging in my personal hygiene in a tiny shower bathroom in old age.

Enclosed dressing room or not - often discussed, so let's leave it at that now. And the OP didn't necessarily want an enclosed dressing room, but just not to go through the dressing room into the bedroom. And there are certainly more options than an enclosed dressing room - for example, by simply giving the bedroom an extra door to access it directly but also having the option to enter the bedroom through the dressing room. If the office moves downstairs, then there is space for it. Or a separate dressing room separate from the bedroom. It wouldn't be my solution, but it is one.

I can combine work and guest room since I only have guests so rarely – that doesn’t change the fact that I might later use this room as a bedroom. If I interpret this correctly, it’s about having a sleeping room downstairs in old age – by then the child will have long since moved out.

I really don’t understand the question about the guest room/office. Of course you can combine them – isn’t that what the OP is doing?

However, I want to point out that one probably wouldn’t want to give up the comfortable bathroom upstairs if there is only a small bathroom downstairs. But that’s up to each individual to decide. I would always prefer to plan a staircase in such a way that I could potentially install a stairlift rather than end up taking care of my personal hygiene in a tiny shower bathroom in old age.

I see it the same way. If you really can only crawl downstairs because of frailty then it’s a matter of a few years or months before you don’t need a bathroom at all but are washed…

there are more options than just a trapped walk-in closet – for example, by simply giving the bedroom an extra door where you can enter directly into the bedroom but also have the option to come into the bedroom via the walk-in closet.

I think the house is basically not yet very tailored to the OP. Many rooms still lack a function, while other areas are problematic. As it is now, it’s easy to sell – two children with a bedroom – that’s what most people want. But do you build like that if you only have one child?

The TE does not have two children's rooms upstairs, but rather an office and a children's room – which makes sense if you only have one child. However, I find the layout on the upper floor unsuitable – with only one child, I can afford more spaciousness. I have the impression that the planner simply took a standard floor plan, which naturally includes two children's rooms, renamed the second children's room as "office" and left it at that. I would remove that and use the gained square meters in such a way that I have a walk-in closet solution that I want, and give the rest to the child, either for an extra bedroom or an extra bathroom – or a somewhat more separated, smaller sleeping area and still a bathroom. You just have to see. But now "guest" is downstairs and, according to the TE, is only used 3-5 times a year, and upstairs there is a very luxurious office, which is a waste of space.