Rough construction flat-rate offer - forgotten necessary items

Unfortunately, the home builders know their power. It just doesn't go any further...

so is it a "can" thing, because it can also be solved differently?

"Isokorb" refers to a whole range of products from the company Schöck, whose common feature is to create a thermal break between interior and exterior components to avoid thermal bridges.

If you build without them, the house does not consume an oil tanker more per year because of it.

In my eyes, this is systematic cheating of the builders. [...]
An offer was made here and a decision was made based on the price, now suddenly it no longer fits and the builder is supposed to pay.
Since these Isokorbs are necessary according to the shell builder, he should have included them in his calculation.

If products are a "de facto standard," I also find it dishonest to leave them out in the calculation. In my opinion, this belongs to due diligence; if the layman accidentally configures the car without wheels, you have to point that out to him. Adding the Isokorbs without consultation afterwards and only "informing" the customer on the invoice squeaks, creaks, and cracks rather strongly in my opinion.

On the other hand, however, I think the layman's privilege of lacking expertise should not be overstretched into naivety "as a competitive sport." Whoever thinks that tenders are a good field for "own performance" must either do it thoroughly or pay their dues.

You cannot buy a 150 sqm house as "simply" as 150 g of mixed cold cuts. "Texting" tenders is not for "singing housewives."

well, if you read it like that, he has pretty good chances that he doesn’t have to pay the surcharge. the question is whether there are still works pending at the companies, whether the matter was carried out free of defects, and what amount it is about in the first place. this also has nothing to do with a tender. if I request a shell construction in condition x according to DIN, he can’t just charge me more afterwards without asking. if you agreed on a fixed price, he’s probably out of luck. if it’s a normal offer, he can add another 10-20%, but he has to justify it. wrong calculation or similar are not reasons! additional costs would be justified if he now needs these baskets because something on the property was not clearly recognizable before (e.g. high groundwater). this will hardly come into play here...

Additional costs would be justified if he now needs these baskets because something on the property was not clearly recognizable before (e.g. high groundwater). This will hardly apply here...

Correct, this does not apply here: "Isokörbe" have nothing to do with insulation against groundwater pressure, but are used, for example, on roller shutter boxes or balcony or canopy cantilevers of floor slabs. They thermally insulate the internal and external sides of an otherwise continuous component. So you will never be surprised to have to use them (and you don’t have to, but it is advisable).

I assume that they are patented as a system / compact product, so while replaceable, it is not by individual products but rather entire catalogs of measures.

we had a lump sum offer made by a shell construction company based on our floor plan. Unfortunately, several insulation baskets were forgotten here, without which the construction would have defects (numerous thermal bridges).

In my opinion, you cannot forget detailed items in a lump sum offer; the offer is lump sum -- "one piece of shell construction." If the construction would actually be defective without insulation baskets as per the offer, then in my opinion you have the right to execution with insulation baskets, that is, freedom from defects, at the agreed lump sum price.

In mechanical engineering, this topic comes up all the time: the machine needs a fence for safety. The fence was not included in the sale. The customer says, make the machine safe according to the machinery directive, because you are obligated to do so. Who pays for the fence? The manufacturer of the machine...

In construction, however, it's strangely not like that. Here the customer is not a multimillion-dollar company that can sue the manufacturer for the last pair of underwear.