Thank you very much for the many answers and advice. We have meanwhile had a measurement of the ceiling done. The result is, of course, just within the norm. But this norm is only for sounds above 100 Hertz anyway, and our problem is the muffled low tones below 100 Hertz that cannot be measured precisely and thus are not included in the new norm. Our acoustician was of the opinion anyway that it is not the ceiling but the sanitary installation shafts, since we hear the most around the shafts. The problem, however, is that this cannot be measured and it is extremely difficult to determine where this sound is coming from. So despite the measurement, we are still at the same point and the architect is still not any more willing to compromise. By the way, for him it is normal to hear people walking around on the ground floor when living in the attic.... Now we are left with only two alternatives soon: to initiate legal action or to sell again.....