Basically: If it is registered in the land registry but none of the parties involved were involved in the original construction, the likelihood that a new neighbor could successfully sue for demolition is almost zero. Of course, it would be interesting whether the garage was approved by the builder or not, or whether it complies with current regulations. Here in Lower Saxony, boundary construction is permitted as long as the building height does not exceed 3m and, I believe, it is not longer than 9m (I would have to check that number). That of course does not mean that such a structure is not subject to approval or at least notification requirements. So as long as you do not want to convert or replace the garage, I wouldn’t worry about it. Especially since the new buyer can see the current state and should/could raise concerns before purchasing. But if it is drawn in the cadastral plan, why would a potential buyer think to investigate whether the structure was properly applied for? From my point of view, that makes absolutely no sense and then the thing would have to be really ugly for me as a neighbor to concern myself with it. PS: You cannot just “do whatever you want” even in agreement with all neighbors. That certainly makes many things much easier, but for most construction projects at least a notification requirement applies and the city/municipality can very well object or impose conditions.