According to Swiss Law you have to react immediately (i.e. within days) if a builder (in German Handwerker) has made an error or if the quality of the work is not sufficient. You should lodge a complaint immediately after discovering the defect (this is called a Mängelrüge in Geman). It is best to do this by sending the builder a registered letter (eingeschriebener Brief). In your complaint you can request the builder to either fix the defect or give you a price reduction. If you and the builder disagree then it is best to get an expert to assess the defect. IMPORTANT: The onus of proof lies with you and therefore you should collect and document the facts as carefully as you can, also with photos if possible.