New construction project single-family house in the Bergisches Städtedreieck

Unfortunately, I haven't dared to tackle the upper floor yet. I don't know where the rooms should be best placed there. The children's rooms are definitely on the south side, but everything else - no idea. I have already read something about the downpipes,

It would be the other way around for me: I would have no idea what should go on the ground floor before the entire room program is sorted out. First, you should create the room program, "qualified" (= with approximate floor areas), then distribute (what goes on the ground floor and what goes on the upper floor) and then start with the upper floor. Not only because of the downpipes, but also because of the load-bearing walls (and because the upper floor is usually trickier due to being more segmented) starting from the top. Children in the south means shading during homework time - not ideal.

second entrance area

In my opinion, you don’t need a second entrance area, just a second front door. Maybe that can still be accommodated somewhere. However, I find the grandparents’ bedroom quite cramped, especially considering that older people might not be as mobile anymore and/or might need a walking aid.

- I feel differently about that. I don't have major problems with the ground floor, except for the allocation of square meters to the available space. When it comes to the upper floor, I'm lost. But I might take a look at it sometime. However, the children's rooms facing south have come up in several conversations with architects. I don't know why you don't see daylight in these rooms as an advantage. There are various ways to protect against too much sunlight. Personally, I would find having to turn on the lights every afternoon in the east or maybe even north more disturbing.

the grandparents' bedroom is quite cramped

Yes, I agree. This urgently needs to be improved. Because the walking aid is already in use.

You are not supposed to start with the [OG], that is step 2. Step 1 is the room list, with a table where the approximate square meters go directly into the columns [EG] or [OG]. There you can also see the sums and their ratios to each other: 66/34 for the "Anderthalbgeschosser", 50/50 for the "Stadtvilla". If you don’t have to reshuffle, you can get started. You should start with the [OG] as the more difficult floor, like a bull before a mountain. That is better than starting with the [EG], because otherwise the one before the [OG] turns into a rabbit before the snake. And then it becomes a real mess: that means the man takes over, and that regularly means too little closet space.
I have no problem with sunlight, only its overdose is undesirable. Children's rooms in the south practically mean permanently lowered "Raffstores".

Step 1 is the room list with a table showing the approximate square meters

I can probably do that if I can generate some time. But the rest will be difficult. I need a few ideas for that. I have already read some very good, constructive suggestions in other threads. Maybe someone with expertise can comment on this? The upper floor should have two children's rooms, a bedroom, a bathroom, a smaller office, a storage room with space for a washing machine, dryer, and if it still fits - a small fitness corner (wish of the master of the house ).