Formally, you submit a building application which the neighbor complains about. I am not sure at the moment whether there is an objection procedure, so initially a conversation/written communication between the neighbor and the building authority for clarification/resolution of the matter. If this exists, that is the first step and only if no agreement can be reached there, it proceeds to a lawsuit.
However, the building authorities are not stupid either; they do not approve applications that are obviously unlawful, especially since the neighbor is already known in this case, meaning the lawsuit is already on the table. If a caseworker in this situation tells you that the building application is okay, that already carries weight – unless the caseworker has lost all common sense.
Nevertheless, the matter is then processed in the above-mentioned procedure, which can simply take time. Ultimately, a judge has to make a decision at some point if it cannot be resolved amicably beforehand.
Best regards
Dirk Grafe