I know this topic is quite old and no longer of great relevance. However, I came across it during forum research and think that this information might still be useful to one or another person.
I assume that you misread something. Or was it a parody of his construction service description? No one disparages their house construction if they have to make a living from it. No company builds negligently below the minimum requirements, and no company calls a DIN that describes the minimum standard (and which is sufficient in most cases) inadequate. That is simply all illogical.
But yes, that is completely logical. Because only in this case may the developer truly build solely according to the minimum requirements of the standard. If he simply states that he builds according to 4109-1 without explaining to the private buyer that this is worse than the standard, then he owes better soundproofing. Even if he explicitly states that no better soundproofing than 4109-1 will be built (without the explanation), he still owes better soundproofing.
Furthermore, according to the construction description and thus according to the developer's written statement, the soundproofing according to 4109-1 is significantly behind a high level of soundproofing that would usually be expected with townhouses or semi-detached houses, and one can clearly hear noises from other terraced middle houses.
And precisely because of this case law, the explanation follows, which the developer would certainly have preferred to do without. Thus, the buyer is sufficiently informed, and only the minimum standard is owed.