Been doing a few off-airport landings lately in northern Germany in a Danish R44, here is what to do ..
You have to obtain a written permit from the German authorities. The German CAA, called LBA, is not the one to contact. They deal with like major aviation stuff for the entire country, not so much local stuff in each state within Germany ..
You need to contact the local state. I don't know all of their address'es, but if you are gonna land off-airport in Schleswig-Holstein, your contact is
www.lbv-sh.de. Rüdiger Hildebrandt in this state would be your chap to help you out ..
You then pay a fee for this document they make and send you. It will state what aircraft and what pilot is allowed to land at what location at what specific time/date or time/date period. Its Germany,
"ordnung um sein" 
..
NB: Do notice they might not fancy, meaning allow, a lowtimer PPL to land off-airport, especially in certain places ..
And of course you also need the permission from the landowner
(public or private) to land on location ..
If you are coming directly from another country, you also need to file a flightplan to land such a place ..
- madman