Floor plan planning shortly before submitting the building application

I consider the savings to be completely exaggerated. Besides the stone, it saves on the facade (plaster, paint, possibly [WDVS]) and interior plaster, paint, wall covering. 5K€ would be good. Maybe a bit more if the windows also become smaller.

A bit clearer again:

How are you supposed to save 600€/m³ when the average construction costs are around 350 - 400 €/m³?

There is definitely something seriously wrong there.

I also find it strange to still be working with formulas at this stage of planning. Fingers crossed.

I also find it strange to still work with formulas at this stage of planning. Knock on wood.

Perhaps does not speak well for the construction advisor.

The construction consultant also got in touch regarding the expected construction cost reduction. He assumes about €600 / m³. With about 41 m³, not bad.

I consider the "allocation" of saved cubic meters to the cubic meter price of the remaining cubic meters to be a senseless philosophical calculation – apart from the minor detail that a decimal point was probably misplaced.

According to the absolute numbers you mentioned, about 2.75% of the cubic meters are eliminated. If these were of average cost, the savings would correspond to exactly those 2.75%. But they are not, since only wall height meters are removed here – not ceilings, not roof structures, not pipes, etc. So proportionally relatively unvaluable cubic meters.

I’d say, "Seaman, stop dreaming," and assume 1.6 to 1.7% of the construction costs, which will effectively be reflected.

Doesn't speak in favor of hiring the construction consultant for construction supervision.