Furthermore, a ground-level access to the house and the garden is not possible.
It is possible. My entrance is on ground level and then you stand in a 2m² hallway / stair landing.
If the window hadn’t been possible, the entire floor plan would have been rubbish.
Of course the window is definitely possible. But do you want that? Depending on the rain conditions, water splashes from the garage roof directly onto the pane and then it is always dirty.
Very unpleasant to clean the window at this height. You can just keep the stool in the guest WC. If the window couldn’t even be opened, you would always have to climb onto the garage first.
How can the entire floor plan depend on the guest WC window? That is one of the least important rooms at all. You only stay there briefly. You don’t need an outside window there. I don’t have one either, and it doesn’t bother me. You can’t really look outside through the window anyway.