Construction company rejects change requests in construction service description and construction contract

the flow temperature probably goes too far, since it is only calculated later.

That is an input value in the calculation.

Sure, but it is not uncommon to receive the detailed design and MEP only after the contract has been signed...

So guaranteeing the flow temperature in the construction performance description certainly goes too far. I have never heard of that either. It should probably be enough to know which brand you are getting (or want for such and such reasons). The rest is done by the plumber.

Which construction company is it now?

The bank guarantee should normally not be a problem either. The only question is who bears the cost for this.

There is a clear legal regulation and it is stated in the Building Code § 632 a: "If the client is a consumer and the contract concerns the construction or conversion of a house or a comparable building, security for the timely completion of the work without significant defects in the amount of 5 percent of the remuneration claim must be provided to the client at the first partial payment."

Yes. In my opinion, the supply temperature should be specified for the calculation, something like: "Underfloor heating in all rooms with individual room control in all rooms. The calculation is based on a maximum supply temperature of xx °C. The bathroom additionally receives a towel radiator (1800 mm x 600 mm), which is connected to the underfloor heating circuit.".

For us, it only says that we will get a gas condensing boiler. It will be installed in the utility room. Collectors will be installed on the roof and all rooms will have underfloor heating. That's it :)

It could say "... diameter according to statics." And then it should be specified how the columns look in the end ("made of proven concrete according to statics and painted twice in white with a facade color."). Something like that ... The type and quality of the paint can be described better/more precisely.

Okay, I see that about the diameter of the columns. You're right about that.

I lack the experience to judge that. Maybe we've just been lucky so far. However, I also suspect a connection. To me, the construction site looks tidier than the rest of the area, there are safety notices everywhere, and the workers all wear helmets and work clothes with the developer's logo. On neighboring construction sites, it looks significantly more chaotic to my layman's eye.

Safety, order, cleanliness, and security give a first impression. However, the "final product house" is not necessarily the best just because the construction workers wear nice colorful helmets and stylish work pants ;-)