Partition wall thickness for duplex house. Structural engineer plans 17.5 cm

We are currently leaning towards the 17.5 to be able to use about 7 cm more internal width.

That will be sufficient, but the premise seems unsuitable to me as a priority.

Yes, exactly, our neighbor is known and we have already exchanged information. He will also build with a basement. However, he is still in the phase of "collecting offers." We want to start in March (depending on the weather).

If it remains the case for you to start without a basement and for him as a latecomer with a basement, then pass on my urgent advice to him that he should start his basement at the same time as you. There is not that much margin in the bids up to this height, so he cannot afford to lose much, but the securing of your building during his excavation is also not free.

We are now wondering whether the additional 7 cm offer significantly better sound insulation or if we would rather enjoy the 7 cm of extra interior width.

In multi-layer partition walls, the thickness of the separating joint is more important than the thickness of the individual wall panels. At least 50, preferably 70 mm. The whole thing should be executed cleanly (= free of structure-borne sound bridges).

If it remains that Ohnekeller is for you as beginners and Mitkeller for him as a latecomer, then please give him my urgent advice that he should start his basement simultaneously with you. There isn’t that much variation in the allocations up to this level, so he cannot really lose much by doing so, but the securing of your building during his excavation is not free either.

We will also build a basement. Yes, that’s true. We will bring up this point anyway. We are already somewhat concerned about the shoring. Does it work well? That is, without later cracks or similar in our house?

For multi-leaf partition walls, the thickness of the separation joint is more important than the thickness of the individual wall panels. At least 50, preferably 70 mm. The whole thing executed cleanly (= free of structure-borne sound bridges).

Thank you for the hint! Does that mean that we practically do not build exactly on the property boundary, but offset by 3.5 cm (with a 7 cm separation joint)?

Does that mean that we practically don't build exactly on the property boundary, but rather offset by 3.5 cm (with a 7 cm joint)?

No idea how you build. Your architect has to tell you that.
With two semi-detached houses planned and built independently, it gets interesting what your neighbor plans and how it is implemented. You can argue all you want, but if they botch it, it's all for nothing.

We will also build a basement.

Then it is only half as bad. Only with no basement on your side would he have to put in the effort to prevent a leaning tower of Pisa from appearing at your place. Nevertheless, I stick to the tip - though not quite so much as an urgent advice - to practice synchronized swimming up to the basement ceiling. He still has plenty of opportunity to save a tenner on every trade.

Does that mean that we practically do not build exactly on the property boundary, but offset by 3.5 cm (with a 7 cm separation joint)?

You can lie down however you want, if he botches it, it’s all for nothing.

I can only agree with that: for safety’s sake, plan to complete the entire separation joint on your own property.

Important the sentence: according to recognized state of the art rules, should be included in the contract regarding sound insulation. Pay special attention to structure-borne sound bridges in the area of the ring beam, floor slab/screed, and the exterior wall/plaster. A planning target of increased sound insulation of 67 dB should be aimed for. Ask your architect if he can guarantee this. Referencing only DIN 4109 is not sufficient; that would only be 57 dB—this minimum requirement is outdated. Be cautious also with a continuous floor slab; this should not be done. Definitely fix the sentence from above in writing! If the houses are already built, it is too late and you will have trouble with the neighbors. Be sure to involve the neighbors regarding sound insulation. Good luck with the house construction