It's in § 25 LuftVG (due to the lack of English version here is the German quote)
(1) Luftfahrzeuge dürfen außerhalb der für sie genehmigten Flugplätze nur starten und landen, wenn der Grundstückseigentümer oder sonst Berechtigte zugestimmt und die Luftfahrtbehörde eine Erlaubnis erteilt hat.
Freely translated: Aircrafts may start and land outside of authorized airports only when the land owner or otherwise authorized person agrees and the aeronautical authority has issued a permit.
Same article, Section 2 (2), deals about precautionary or emergency landings.
I had to go down in a field in Germany last December, because of a door popping open. After I was back on base, I checked regulations to see what they say about this unscheduled precautionary landing. Art. 25 S 2 (2) LuftVG is your friend, says you can land and subsequently takeoff for safety reasons (note, you cannot take off again if it was an emergency landing). However, what you have to do is to leave your contact and insurance details with the landowner, so that he can claim damages in case there are any. He then cannot prevent you from taking off again. Seems this paragraph also deals with hot air balloons ("when the landing site cannot be predetermined because of the type of aircraft").
Being in Germany, I was waiting for the police to call in and ask about the incident. Thankfully didn't happen.
This no-offsite landing rule in Germany (Austria as well, by the way) sucks, but on the other hand there are plenty of small (and cheap) airfields in Germany and you are welcome almost anywhere with a helicopter. Even PPR ones are usually happy for anybody to come in and may even assist you with transportation etc...