We have now agreed on the following plan, also based on your suggestions at this point.
The floor plan will only be changed in details - the L-shaped staircase remains but will be clad with a wall on the straight section, which will serve as a handrail. We will leave the access to the living area without a door.
What (standard) width would be recommended here in order to possibly install a door frame and door afterwards?
We are thinking of a wide format. In any case, wider than a normal interior door...
We have inquired about the cost of opening up the hallway on the upper floor all the way to the ridge and installing a roof window there. I have no idea whether that will be elaborate/expensive and am therefore very curious about the builder's answer. Anyone have an assessment on this?
For Child 1, we might also add a roof window as proposed here.
If we can afford the bay window, we will include it. With almost floor-to-ceiling windows, and the dining table would partially extend into it (instead of an expensive custom-made bench corner). That should make it bright enough - also in the hallway.
Otherwise, we will widen the window surfaces in the living room by 1m if the bay window turns out to be too costly, in order to let in some more light.