Short answer? Divine approval. Longer answer? It's a fickle magic that has its roots in the ancient Celtic goddess Nemetona. And I do mean ancient. The kind of ancient in which the Celts were still living around Germany. How such places are made depend how far the religious rabbit-hole you're willing to go, but it's not something you can really ask for. You're more likely to get smited with a tail or something than get a sacred space for magic-working.