owed soundproofing terraced house new build

It is a WEG divided terraced house with corresponding special ownership of the house and land.

So you effectively have a multi-family house with maisonette apartments. I don’t know if the required sound insulation is different in this case, but it could be. In particular, the single-layer partition wall should be sufficient for apartments.

And also consider the issue of the WEG’s approval for structural changes.

According to the DIN you initially quoted, for MFH(!)

Example (1989), separating floor multi-family house:
normal sound insulation: R‘w ≥ 54dB L‘n,w ≤ 53dB (2)
increased sound insulation: R‘w ≥ 55dB L‘n,w ≤ 46dB (1)

Example SUA 181 (2006), requirements between two living rooms:
minimum requirement: Di ≥ 52dB L‘ ≤ 53dB
increased requirement: Di ≥ 55dB L‘ ≤ 50dB

where do your repeatedly mentioned minimum attenuation values of 6x dB come from?

The contract with the developer states the following: "The minimum requirements according to DIN 4109:2018 for building acoustics (sound insulation in interior spaces) are met." Furthermore, it also states: "The seller undertakes to deliver the contract object in accordance with the building description including building plans and the recognized rules of building technology, turnkey and functional for the agreed residential use." According to my research, however, the minimum requirements do not correspond to the recognized rules of technology, raising the question of whether 62dB or 67dB sound insulation is required. Could you help me here?

With your research, you are treading on very thin ice. Among experts, it is disputed (just as with the 2020 revision) whether it corresponds to the aRdT or not. There is also no relevant case law on this.

So you effectively have a multi-family house with maisonette apartments.

That is correct, but I currently also live in condominium ownership, divided under the WEG, and know the possible problems well. In my opinion, these have already been addressed quite well due to the changes in WEG law.

Since the purchase contract, sales prospectus, building description, sample catalog, etc. almost exclusively use the term "terraced house," I do not think it can be claimed here that it is a multi-family house.

Where did you get your frequently mentioned minimum damping values of 6x dB from?

Accordingly, I see the following jurisdiction as appropriate: BGH, 20.12.2012 - VII ZR 209/11 (I better leave out the link)

"The court of appeal is of the opinion that the defendant's planning is objectively deficient because he only provided single-leaf party walls with correspondingly insufficient sound insulation values for the terraced houses to be constructed. At the time of planning in 2000, terraced single-family houses were already supposed to be constructed with double-leaf party walls according to recognized rules of technology. The defendant was required, according to the content of the architect’s contract, to provide a corresponding sound insulation level of at least 62 dB. The defendant, who was not supposed to plan apartments in multi-family houses, developed the concept of "vertically divided residential units," thereby abandoning the double-leaf construction of party walls and accepting losses in sound insulation. In fact, however, terraced houses were still to be planned. It does not matter in this respect whether these are terraced houses on separate land parcels or condominium ownership on undivided land in terms of property law. What is decisive is the technical constructional appearance of the object. The defendant was therefore required to provide a level of sound insulation 7 that, at the time of the planning, corresponded to the generally accepted rules of technology for terraced houses."

This is a legal dispute between developer and architect, so the developer is assigned partial blame due to his professional expertise. This is not expected from a private buyer according to other judgments.

Since in the purchase contract, in the sales brochure, building description, sampling catalog, etc., almost exclusively the term "townhouse" is used, I do not think that one can rely on the position that this is a multifamily house.

Why not? After all, several families live in a row of townhouses. Division according to WEG also rather speaks for a multifamily house than independent houses. What does the building description say about soundproofing?

This is a legal dispute between the developer and architects, so the developer is given partial fault due to his professional expertise. According to other rulings, this is not expected from a private buyer.

You are playing a risky game here if you sign a construction contract that you consider invalid and now try to push things through on that basis. The consequence now is that you get a pretext, lose space in your house section, and still do not have the desired soundproofing. And that is not even the worst possible outcome; if you are unlucky, you may find a judge who considers this as a multifamily house with vertically divided residential units.

Since the purchase contract, sales brochure, construction description, sample catalog, etc. almost exclusively use the term "terraced house," I do not think that one can rely on the position that it is a multi-family house.

Then first explain to us where and how a "terraced house" is legally clearly defined. But: In your purchase contract(!) it also does not say "terraced house" but as you yourself said: "apartment (with terraced housecharacter)" You must have signed a community agreement and a declaration of division at the notary, right?! What exactly was unclear to you here that you did not buy a "house" in the usual sense? By the way, the ruling you quoted concerns the internal relationship between the appointed architect and the developer The defendant was required according to the content of the architect's contract to provide a corresponding sound insulation level of at least 62 dB. The defendant, who was not supposed to plan apartments in multi-family houses, developed the concept of "vertically divided residential units," whereby the two-shell design of the party walls was abandoned and compromises in sound insulation were accepted. In other words, it was a planning error by the architect, the developer wanted at least 62 dB but did not get that delivered. According to the contract, you have a right according to DIN (to at least 52 or 55 dB). Why and with what right should your developer suddenly deliver 67 dB to you?! Be glad if you get the 62 dB. Do you already live in the house and is the sound insulation subjectively bad?