Well, he obviously didn’t know, the civil engineer just installed the drainage. Therefore, I don’t want to open that can of worms yet. I will call the building authority once the civil engineer is back from vacation.
Then you won't have any trouble with each other, but at most you and your neighbor will have trouble with the civil engineer. I would inform the neighbor (after all, he is the "client" towards the civil engineer!). Together with him and the civil engineer, you can then find a solution.
That is exactly the plan. I also never said that we are in trouble. But as I said, I just want to know exactly what the situation is before I wake sleeping dogs.