Sound insulation VDI Guideline 4100 & DIN 4109 in prefabricated house construction

It is difficult for the builder to prove what is now "owed" if nothing concrete has been agreed upon.

If nothing concrete has been agreed upon, what applies is what is approved and introduced by building regulations, i.e., in sound insulation DIN 4109 (version depending on the federal state). Beyond that, what is contractually agreed applies; anything below the requirements of DIN 4109 (version depending on the federal state) is a defect.

The measurable TARGET often remains unclear in reality for laypeople. And that makes me so uneasy.

Basically: The target is what has been agreed upon. If nothing is agreed: state of the art, building authority introduction/approval.

that SST III means an extreme additional effort

Extreme was never mentioned. It means additional effort mainly regarding proper planning, execution, and choice of materials. Very few can/want to afford this "additional effort," which in very rare cases approaches "extreme."

1. 12.5mm diamond GKFi 2. Vapor barrier 3. 15mm OSB board 4. 110mm mineral wool insulation in a 110/60 mm timber stud (spacing <625mm) 5. 2x 18mm gypsum board GKFi 6. 50mm air layer up to the boundary of the semi-detached house

Unusual: timber stud wall as party wall. I have only experienced that in solid construction, usually 20cm or 24cm sand-lime brick RDK 2.0. That means to your neighbor you have in total from both party walls not even 130mm of gypsum board/wood fiberboard at a wall thickness of ~360mm? Ambitious. That it yields no more than the minimum requirement is somehow obvious.

Instead, mineral wool insulation everywhere where there would be air?

That can be done; usually the effectiveness is marginal. The "air layer" toward the other semi-detached house is supposed to prevent any structure-borne sound bridges. Which would be catastrophic with this wall construction; therefore, I would rather refrain from polystyrene just to be safe and choose something softer.

Which SST is achieved thereby remains unclear to me,

As long as nothing else is agreed or owed, I would say: minimum requirements, DIN 4109, valid version depending on the federal state.

Let's see what the GU says if I want a solid communal partition wall. They will surely have a lot of fun with me ... and my wallet even more. What the heck, if we're going for wood studs, better not completely walk into the trap regarding soundproofing to the neighbor...

if you already want a wooden stud frame, then rather don't completely run into an open knife regarding sound insulation to the neighbor...

I see sound insulation there ...

Construction in bullet points from inside to outside (on a semi-detached house): 1. 12.5mm diamond GKFi 2. vapor barrier 3. 15mm OSB board 4. 110mm mineral fiber insulation in a 110/60 mm wooden stud (spacing <625mm) 5. 2x 18mm plasterboard GKFi each 6. 50mm air gap up to the boundary of the semi-detached house

... none listed at all or only represented in the form of decoupling from the neighboring half by the 5 cm "nothing." And the 2x 18 mm plasterboard are supposed to represent the entire fire protection?

what the general contractor says if I want a massive party wall.

I suspect: that your house will then become narrower by these 20 cm of concrete or 24 cm of calcium silicate brick.

And here we are again Ks like 45 posts ago...

I don’t see any sound insulation listed there ... or only in the form of decoupling from the neighboring half by the 5 cm "nothing". And the 2x 18 mm gypsum board is supposed to represent all the fire protection?

I have to admit I wasn’t that shocked, I already suspected it.
Correct, point 1 F30B and point 5 F90B.

By the way, I have a growing suspicion that you and your house provider were really talking past each other. The described wall structure seems to me to be the usual construction of the exterior wall, on the shared side of course (assuming that both halves are joined along their full length) less U-value sensitive and without facade plaster. You speak of a "Kommunwand" which, as far as I believe, has not existed in row or semi-detached houses since about 1980. However, you do not seem to mean a fire wall - from which I conclude that you did not actually intend a legal semi-detached house, but rather a two-family house in the form of a semi-detached house with two residential units on one common, not legally divided plot. Then a "prefabricated house" builder will plan a multi-family house with apartment separation walls - here merely "spec'ed up" for their double execution for the purpose of separating the ceiling sections on the one hand and the 5 cm separation joint between the two on the other. I see it as preprogrammed that you will end up with something different than what you think you ordered.