Draft at the corners of the windows?

What does your purchase contract say about freedom from defects and liability for defects?

The house still has a warranty, I have already sent an inquiry to the builder. The purchase contract only stated that all rights transfer to the buyer.

The problem is the Building Energy Act as well – we have openings as visible in the photos and a few cracks, all inside. There is nothing unusual on the outside of the windows... There was already an opinion that if everything is sealed on the outside (or rather, I don't see how one can be sure about that without taking the window apart), it is not a problem, because air moves inside the timber frame walls behind the vapor barrier, which is considered normal. So the position then is to seal on the inside, install the windowsill, use silicone and construction foam = done. I am waiting to see what the developer says, but if he says more or less the same, I probably have bad odds and can only hope that nothing rots away in five years...

I bought a 2-year-old house, a prefab house in timber frame construction. It was not finished by the builders in all the small details, for example, there are no interior window sills. [/] The house is still under warranty, I have already sent an inquiry to the developer.

What kind of "developer" is that, and why "warranty"? That "prefab" houses are used in developer projects is rare but occasionally happens – but I have never heard of it in the form of a shell house. Warranties can always only cover the contractual object; with a shell house, the owner’s own work or subcontracted trades are therefore not the responsibility of the house manufacturer. You can only assert claims here against the seller (presumably the owner doing the work, as the botched work in the pictures looks like it was done by an amateur or weekend handyman) or against their craftsmen. The manufacturer or any possible reseller developer have nothing to do with it.

What kind of "developer," and why "guarantee"?
That "prefabricated" houses are used in developer projects happens rarely, but occasionally – but I have never heard of the shell house form in this context. Guarantees can always only cover the contractual object; in the case of a shell house, the own work or subcontracted trades are thus not the concern of the house manufacturer. You can therefore only have claims here against the seller (as the presumed own worker, the botch in the pictures looks like an amateur or Myhammer) or against their craftsmen. The manufacturer or a possible resale developer has nothing to do with it.

Sorry, I might not always have the terminology exactly right, maybe the word developer was wrong.
The house comes from a well-known prefab house manufacturer Massa Haus, whose services also included installing the windows (which I understand was still done at the factory when manufacturing the house walls).

And installing window sills was then done by oneself, which had never been carried out?

I wouldn't know that anything except [Frnsterbänken] is missing, yes