160m2 detached house in timber frame construction on the north slope with basement

But that's not nice towards the children, putting them in the north and by the street.

Somewhere we will have to make compromises to reduce costs. As a teenager, I always thought it was very cool to have my own area in the basement. Apart from that, the children are only in planning. Who knows, in the end we might not be able to get any at all....

But that is not nice towards the children, to put them in the north and by the road.

What's the point of crying for long? Better a northern children's room than none at all and better solidly built than over-indebted and unable to maintain your palace.

A new plan must be made.

Slope and budget do not make squaring the circle any easier. Compromises have to be made. Children's rooms on the north side, street side, do not mean a stone desert and dark holes. The rooms can be bright, friendly, with a view of a nice front garden.

At the moment we are considering changing everything fundamentally by putting the children's rooms and children's bathroom in the basement,

Somewhere we will have to make compromises to reduce costs

I am shocked.
Are your children getting everything you thought was cool? A banana bike.... but you probably aren’t that old yet.... Give them a Gameboy and they have to think it’s cool like you?!
At 15 years old do you have the same demands as at 1 or 9?

However, you are right: if there is no money, you give up the basement (here the third floor) and a garage that is built into the house and provides living space.
If you don’t want to build without a basement because of the slope, make a basement with the parents’ area, living on the ground floor, and leave the attic unfinished. If children then are expected, you can still expand.

The draft is therefore for the round file.

Ypg like this is better.

No matter what, some rooms have to go down

If you don’t want it because of the slope without a basement, build a basement with a parents' area, living on the ground floor, and leave the attic unfinished. If children then arrive, you can expand.

Just expanding the attic with 2 children's rooms and a loan on your neck - it’s not impossible, but I would be cautious. In 5 years, it certainly won’t cost less than it does now and the loan probably won’t be paid off by then. So where should the money suddenly come from?