Offer(s) for a new building

As I said, I only looked a little into the first two pages. If everywhere only the unit price is given, but no quantities have been calculated, the low price is obvious. It will definitely be higher if you use real numbers instead of quantity "1".

And sorry to say this, but after seeing your other thread as well: YOU should put time and effort into your project and not the users here.
To pick up the example with 27 cm ceiling thickness: why would that be better? Or shouldn't the structural engineer rather say what is necessary – possibly after it is clear whether ventilation is installed in the concrete ceiling? AND THEN also a look into the development plan is taken, which might regulate the wall height, so that a 7 cm thicker structure could cost an exception from this height?
(all just fictitious examples that have to be adjusted to YOUR reality – which surely no one here in the forum will take off your hands)

€55,000 for the shell construction? What is that supposed to be? Certainly not a house, you're missing at least €100,000

What do you think about the offer, should I have anything changed?

Every provider often lists it differently, so you lose track or can't tell which would be better for me...

It's not just about the price, but also about improvement suggestions...for example, an acquaintance told me: instead of a 20 cm ceiling, better take a 27 cm one.....

It is not the first time that I read "too cheap" as a statement from you, whether it is the shell construction, garden, terrace roofing ... Either you simply have a completely different price level or you have a big heart for craftsmen. Because in my experience, these statements are consistently wrong. Just recently completed a flat roof construction, 2 floors, 195sqm living space for 75,000€ shell construction. Completely in sand-lime brick. In NRW. In addition, there are 25k€ for the facade (ETICS) and 10-15k€ for the roof. A neighbor spent 150k€ shell construction for 250sqm living space, integrated garage, full basement (130sqm), monolithic, perlite-filled Poroton, KFW 55. Just for comparison. But that is basically the highest level and hardly comparable to the usual 140-150sqm single-family houses, which are usually the subject here.

Unfortunately, all that can be said is, serves you right. Obviously, you are trying to assign the trades yourself and this is what comes out of it. A professional would have created a bill of quantities for you based on the house planning. Same positions, same quantities. On this basis, offers are obtained and compared side by side. That is now your task. The alternative would be to buy a "piece of shell construction," then the risk of having incorrect quantities lies with the contractor. But potential conflict also belongs to that. So better turnkey from the start.

Is there a reason for that? Is the acquaintance a structural engineer? Has he considered the effects on building/room heights, costs, and statics with his "good" tip? Probably not. Then he can do it like Dieter Nuhr ... and you should buy qualified expertise, otherwise this will get pretty funny for you.

Is there a reason for that? Is the acquaintance a structural engineer? Has he considered the impact on buildings/room heights, costs, and structural engineering with his "good" tip? Probably not. Then he can do it like Dieter Nuhr ... and you should get qualified expertise, otherwise this will get pretty funny for you.

Yep, he also does structural analysis......

And why does he recommend this then? Soundproofing?