The living space is approximately 8 m² smaller in the application drawings compared to the draft (GU).

Right: 11.5 KS + cavity + 11.5 clinker = 26cm
Left: is basically the exterior wall of the house. Instead of the clinker, an 11.5 calcium silicate brick is simply used.

what would you have expected here?

Thanks, that's right. The air gap. I'm already too tired today.

I don't see why there should be any discussion about this. The basis of the contract are the drafts - the square meters are clearly visible there - by the way, the only measurement given. Wall thickness according to structural engineering - there is now no room for interpretation, in the end the square meters agreed upon must come out.

To the OP: no, that is not normal or usual, with us there were also deviations in square meter sizes from the contractually agreed plan during the detailed planning, but those were minor differences behind the decimal point... sometimes a bit more, sometimes a bit less, in the end almost fitting the total amount - something in that range would be normal...

However, difficult to judge whether in your case it is the company's intention or the stupidity of the draftsman. Find out, say that the square meters must be adhered to and see what happens.

Hehe... Stupidity to one's own advantage, now that's a creative assumption. The builder is charged for the thicker masonry as an extra, and the reductions in all trades that charge by m² are pocketed. Screed, underfloor heating, included floor coverings, drywall ceiling on the upper floor. That adds up when you calculate with your subs using a sharp pencil.

I want KSL because of the soundproofing, which we unfortunately need at our location.

Do you think you don’t need it here in this overall external wall construction, I’m only telling you to fool you? I have been in construction planning for four decades (almost exclusively residential construction, mainly single-family homes). Professionally. But I won’t tell you that again now, go ahead and indulge in that nonsense.

I wouldn't know why there should be any discussions. The basis of the contract are the drafts – the sqm are clearly visible there – by the way the only thing given as a measure.

No, the external dimensions are also specified.

"The contract states what this could involve only once: 'Ground floor, in the size as shown in the drawings.'"

Which can just as well mean that the external dimensions shown in the drawings must be adhered to. So plenty of room for interpretation.

As someone who has built with aerated concrete, I can tell you, take it. You still have insulation in front, and clinker brick and plaster inside. You won't hear anything through that. The sound is more likely to come through the window glass. I believe , he knows what he's talking about. By the way, aerated concrete also comes in different strength classes and is therefore either heavier and more soundproof or lighter and better insulating. Just go to a model home park and look at aerated concrete and sand-lime brick houses. If necessary, have the wife scream in front of the wall.