Insufficient slope of the sewage pipe

There is a wiring diagram, but no heights are indicated there

There is a wiring plan, but no heights are indicated there

And how is the company supposed to know how to build then?

The plans were created by the general contractor or by the architect provided by the general contractor, so I cannot answer this question.

We were handed all plans or they had to be signed by us for the drainage building application. That didn’t happen with you? I would now check who might be responsible for that.

Do I understand correctly:
- The sewer connection was already in place and the height (theoretically) known
- First, the contractor built the base plate including the connection from the base plate
- At a later time, you yourselves (or a company commissioned by you) installed the shaft including the complete connection and the pipes from the base plate to the sewer connection
- Then it was found that it does not fit together or that the slope is not sufficient
- There was no pipe planning including heights and the exact reference point on the base plate was not defined in writing

I am wondering how the contractor decided at what height the connection is? Pure luck? And you never spoke to the contractor about it, since the pipes were installed by you, to ensure that it fits together?

That is correctly summarized. To my shame, I must admit that we relied here on the architect's statement that the heights are correct. In hindsight, one is always wiser...