Bedroom in the basement

I would first plan the bedroom upstairs and use the basement as a development reserve

Who knows if you will have 3 children
How old the oldest is until the youngest needs their own room

Thank you very much for your contributions so far. It actually turned out to be a sloping plot, so a living area in the basement would probably be quite possible. However, we did not like the surroundings, as there are only multi-family houses there. In addition, the trees next door would probably completely shade the garden in summer. So keep looking or wait...