It's the missing buffer. None of the trades will cost what you have there, and none will be cheaper. With 20 trades, you have an average surcharge of 2000 each, then you have 40 more. 10 more as a buffer for unforeseen events.
It will happen that way, believe me
Strange. For us, almost every trade became cheaper, even if not by much.
For example, with the cladding, wall openings over 2.5 sqm are deducted, and bam, 1.5k less costs than in the offer. The roof truss required only 11.2 m3 KVH and not the offered 11.5 m3, and bam, several hundred euros saved again. Of course, it can also go the other way, but to say flat out that there will be approx. 40k additional costs compared to the offers is, in my opinion, extremely exaggerated. 40k can certainly mean a worse interest rate on the loan, which would not necessarily be required...