Floor plan design - Constructive criticism welcome

Your property is not suitable for a residential basement. You will have enormous costs for earthworks, light wells, and entrance. The light wells could affect your outdoor area. That alone would be a no-go for me. I build so that I can live without restrictions. That will also be the reason why there are hardly any granny flats in the settlement. Whether the children will ever move in with you is also questionable.

I would try to grant the kids at least one south-facing window each. They use the rooms upstairs the most. If the roof terrace next to it is also supposed to face southwest, I would rather look for rectangular floor plans than squares.
Something like this:

The width of the property is 19m. The length of the short part is 24m. We are already working on a new floor plan. We found a floor plan on the internet that might fit. It now has to be adapted to our property.

Aren't 19m too narrow for a double garage? It would then take up almost a third of the property, has to be integrated into the house, and will be correspondingly expensive.

Yes, it would be semi-integrated, see page 4, post 20.

Aren't 19m too narrow for a double garage?

19m - 3m building setback on both sides = 13m house width, with 3m of that projecting as one half of a garage.