Have you received your operating costs statement (Betriebskostenabrechnung, also called Nebenkostenabrechnung, the annual utility and service charges statement) and are unsure whether everything is correct? This free check walks you through the most common formal errors and landlord tricks in just a few minutes. Simply answer the following questions. For this you will need your current statement and, if available, last year's. At the end you will receive a traffic-light assessment (green / amber / red) with explanations for each point and suitable next steps. It is best to also have your tenancy agreement (Mietvertrag) to hand, as it states which types of cost may be passed on to you at all. One note in advance: if you have already received court mail regarding this claim, the first question is especially important, as short deadlines apply there.
Look out for a yellow envelope and for the words 'Mahnbescheid' or 'Vollstreckungsbescheid' from the Amtsgericht (local court) (not to be confused with an ordinary reminder or a debt collection letter from the landlord). With genuine court mail, very short deadlines run: you must respond within 2 weeks of service, otherwise the claim becomes enforceable, even if it is unjustified on the merits. In that case a letter to the landlord is not sufficient.
For detailed background, see our guides Checking your service-charge statement and Tenant rip-off tactics.