German construction law places a duty on the contractor: where he has concerns about the intended method of execution, about materials supplied by the client, or about the work of other trades, he must notify the client immediately and in writing, and preferably before starting. The provision is § 4 Abs. 3 VOB/B.
Doing so releases him from liability for defects arising from those points. Failing to do so leaves him liable for consequential damage he did not cause.
The obligation runs the wrong way for a contractor’s own interest, and it is the law.