House costs, first reality check

The basement of the old building is of course not deep enough. I estimate we have to go 1m deeper. But area-wise it fits quite well. The building lines and boundaries dictate quite a bit. I’d say the old building is 9x9m + extension. The new building should then be 8x10 or 8x11m. Garage old building at the same place as garage new building. The extension of the old building 5x9m would then be roughly the area where the terrace is supposed to go.

An almost location-identical new building of 80 or 88 sqm floor area replacing a building of 81+45 sqm floor area sounds like at least a medium-sized blunder, and a raised ground floor instead of a "floor-to-ceiling" ground floor is also not a catastrophe. For a utility basement, a headroom of 2.0 or if you like also 1.8 meters is sufficient. I am happy to advise you individually on this, as you know.

Considering the construction costs, selling the property is also an option for us. (Since it is quite large after all)

I consider that very worthwhile to examine benevolently before unnecessarily driving up your construction cost by a blunder. We are apparently still talking about

Due to the given assumption, I agree with you. Basically, I would have preferred a renovation initially rather than demolition + new construction. However, we have since had two appraisers come who took a look at it. Quote: "Anyone who wants to renovate must have sadistic tendencies." Therefore, renovation is unfortunately no longer an option for us. On the topic of sale - alternative purchase, I am already somewhat involved. Why use a plot that is too large when one with 2/3 of the area will do. And someone else (project developer) might be able to create more living space for the common good than we would. The risk then is that you might not achieve the price you expect (need). And a suitable alternative property would also have to be found. And there are additional incidental costs. These easily amount to six figures and could possibly negate the advantage again.