13 sqm larger house 30,000€ additional price

With 540 euros per m2 living space, you would have done a good Diel.

A Diel is something for laymen *ROTFL*
The coffee is ready, I'm "briefly" away now ... (it's hiking weather outside).

The question is what you get for the 30 T€. If it's only square meters, that seems like a lot. If it significantly affects the usability of the house, that's little (because otherwise the entire rest is possibly badly invested). You are buying a house and not living space. That is a difference.

I would like to point out that you have the children in the house for at most half the time (in D at least). The house quickly becomes too big as you get older. Especially if you want to spend your entire life in it.

For me, it was just under 14,000 EUR for 12 sqm. The price is excessive unless it were one more bathroom.

since ideally you only build once and now make the decision for the next 40-60 years.

So we had a 300sqm house until about 3 years ago and when the kids moved out.... it was too big for 2 people. Then we sold the house and built ourselves a bungalow suited to our age ;) so never say ... you only build once ;)

Some of the rooms are not even half a square meter smaller

The floor plan obviously needs to be adjusted. It is simply insane to just expand the exterior walls without moving interior walls and redesigning the rooms.