Brainstorming / Exchange on the house for the "Weekend Patchwork" family

Are you building with a basement?
Then there is always a storage room there...

well, for 2 overnight stays within 14 days, siblings don't need two rooms in my opinion. Make one big room and have a carpenter craft a bed as a room divider. It doesn't cost a fortune and you get a bit more privacy.

Consider a larger "experience room" with sleeping accommodations for 2 people. A kind of sleeping cubicle that saves space and provides privacy is an option. Depending on the roof and the layout, a private mini-attic accessible from the larger room can also be installed for each child. Do the two older ones get along well? In a boy/girl configuration, it may need to be considered that during puberty a better separation is desired. Relationship and configuration are important.

I really like the idea of the two small rooms that will be combined later (e.g. a small living room for your shared child). Allesandro's idea can also be used as a proper wall. Saves floor space, but still two separate rooms.

Yes, I also like Alessandro's idea.

In my opinion, this cannot be answered only with space/area; there is an individual history behind it and how the children themselves can deal with it or feel about it. Do they talk about it honestly or are they rather reserved, etc. Of course, you don't have to/can’t hold a wish concert, but I did want my own children to also have the feeling of being just as welcome there as the other child. I think this should be taken into consideration. The little child should also notice that the others belong just as much. This is doable with one room or two shared/divisible ones. I would find, in principle, a good idea, for example, to build a "special" room for the two older kids, like this cool bed or with funny details, so to give up some area in favor of more exciting content. That can be a funny loft bed as well as the nook, and it should be their room, so not for general use. I think privacy is important and that maybe you as the father "manage" their room because maybe there are still insecurities towards the new partner, which wouldn't be unusual, or maybe some jealousy against the smaller one. Therefore, it should feel like THEIR room alone, where no "stranger" walks in except Dad… something like that I could imagine.