12x9.6m 2 full floors, basement, attic, 4 children's rooms

.. since the basement and attic are not to be developed and the rest strongly depends on the materials.

What do you mean by developed? That it is not a living room? But you still have electricity there, insulation against moisture, plaster on the walls, and a floor possibly with covering. Small change, small change...

450,000 is definitely not enough for the house and the wishes listed at the entrance. I see a 5 in front even for simple equipment. With 4 children, there is not much room for own contribution. And what do you save if you use your annual vacation to lay the floor coverings yourself. Built-up space costs.

I would separate building services and housekeeping (laundry). When I think about the humidity in the room with us when the washing machine and dryer are running hard on kitchen laundry, I can't imagine that this is good for the technology in the long run. When a service technician, chimney sweep, etc. comes, it also looks tidier right away. With 6 people, quite a lot of laundry piles up.

Technology and laundry are connected to the exhaust air of the controlled residential ventilation. Therefore, there should actually be no moisture problem (despite possibly shared room), even if the laundry is hung there to dry, right?

Are connected. Still, it doesn't get rid of the exhaust air as long as the dryer and washing machine are running. It just takes a little longer. Maybe in a large room, it is not as noticeable. You have space, so why not separate cleanly?