Thank you for the good advice!
An expandable attic is unfortunately not possible, otherwise we would have done it that way too. We just don’t want to give up on child 3 just because the house has one room too few. Of course, the spare office upstairs is possibly "left over" if K3 doesn’t come.
Larger plots were unfortunately "sold out." Although we would have preferred to buy a small one with a cellar option. But you take what you get.
Of course, the house is ultimately too big for the plot, but neither of us wants a garden that is too large. Perhaps the acute tightness in the current 74m2 apartment with four people is the trigger for us to now completely overdo it in the other direction...