Received an offer for my shell construction

Are only laymen involved in your construction?
You should change that.

There are many technically skilled people in the family, but not exactly the most experienced in construction bureaucracy... Maybe also because this is our first time building and we hadn’t dealt much with the subject before

and things are not going smoothly with the architect either, hard to reach, too little time, too many projects... and always the unnecessary blah blah blah

What you lack is a scope of services. This will affect every trade. Hopefully, your family will have paid enough tuition fees by then.

Since you apparently have no experience and knowledge in this area, it will not really work. Unless you have a contractor from the neighborhood who closes the whole thing with a handshake and stands by his word. A phrasing like "all concrete work" etc. will not be signed by anyone for you. Exact volume, shape, quality, etc. must be defined. Just like the amount of steel.... To avoid additional costs: - Find a competent partner - Exact definition of the trades - Obtain offers and have them checked by the partner - Negotiate and then conclude the contract Any changes after contract conclusion will incur additional costs. Some uncertainties are the client's risk. You cannot exclude them.

So the acquaintances who have also built in recent years have experienced that the bill of quantities is usually not complete... Offer based on bill of quantities + then about 15% more...

Can that be true??

What you are missing is a bill of quantities. This will affect every trade. Hopefully, you have learned a lesson in the family by now.

There are always things that are the responsibility of the client. You must always plan sufficient buffers. A flat rate of 15% can be enough but does not have to be. The more precisely everything is defined, the less unforeseen things will affect you. For example, the ground could be very rocky. Then 15% is not enough. Or loaded... Also important are the interfaces between the trades. Both the flow of information and during construction itself. A blanket statement simply cannot be made. If you already have an architect, why doesn’t he handle all the tenders, etc.? You yourself are simply not technically capable of assessing offers. That will come back to haunt you later.

Your price seems rather inexpensive.
Surprising is the supposed fixed price without [Statik].
[Statik] determines the quantities of concrete and reinforcing steel.
These are significant, five-figure amounts for a shell construction.