At the moment the company is looking for a potential site for construction of the facility.
According to available information, currently, the company has chosen the region of North Rhine-Westphalia and the German state of lower Saxony. By the way, Elon Musk has announced that Germany will be a priority in Europe for the construction of industrial area in July.
Note that North Rhine-Westphalia is the most densely populated part of Germany which borders the Netherlands and Belgium. In turn, lower Saxony shares borders with the Netherlands.
According to the forecasts of Western experts, Europe needs to become the second largest global market for electric cars in the next decade. Recall that Tesla finishes building a factory in China, where they will collect Model 3 and Model Y with the end of 2019.