Some info below from
Civilian drones and the legal issues surrounding their use -
(This is 1.5 years old and I suggest to confirm the rules haven't changed)
My comments:
Frankfurt has a LOT of air traffic, so know where the airports are and fly accordingly.
While it would be cool to film a cathedral, castle or other structure, keep in mind these areas attract a lot of people. I have found that some places that are crowded during the day can be virtually abandoned at sunrise. But I'd still check with the authorities prior to flying.
Read up on the copyright private property and surveillance protections in that link.
Your not in the US and its easy to transfer the mindset of the protections we have from unjust prosecutions to our time in other countries where the laws maybe vastly different.
I consider my time traveling in other countries or cities a valuable commodity. As such, if I plan on flying there, I research the regulations well in advance.
From that link:
Permit to fly
The reverse conclusion of this definition is that civilian drones which are used for the purposes of sport or recreation are subject to less stringent rules under the German Aviation Act.
Where drones are used for purely private purposes, an obligation to apply for a permit to fly them applies if the drone weighs more than 5 kilogrammes. Otherwise there is no obligation to apply for a permit.
Furthermore, pilots of private civilian drones less than 5 kilogrammes are subject to no specific aviation legal requirements. For example, pilots are not required to prove knowledge of aviation rules. There are also no age restrictions, meaning that any person, including children and teenagers, can fly such unmanned aircraft.
Restrictions
Some restrictions do apply to flying civilian drones for private use: a minimum distance of 1.5 km has to be kept from airports; special no-fly zones apply in the government district in Berlin; and some federal states also prohibit flights over military bases and nuclear power plants. Flights over industrial complexes, accident scenes and public gatherings require official permission from the relevant authorities (§ 6(4) no. 1 LuftVO).
The drone has to stay within the sight of the person flying it, i.e. in an open area and at a maximum distance of 200 – 300 metres away. Some federal states also restrict the height of such flights to between 30 and 100 metres.
Risk to aviation safety
These restrictions the free use of airspace by civilian drone pilots shows that the legislature has sought to guarantee air traffic safety. Where unmanned aerial vehicles have the potential to endanger aviation safety, the restriction on the use of unmanned aircraft fulfils the purpose of aviation law.
Unmanned aerial vehicles pose an increased risk in comparison to other air traffic. An unmanned flying system consists of the drone itself, the ground station and the communication technology. There are therefore three possible sources of error. The legislation reflects that the safe integration of civilian drone traffic into the available airspace is fraught with uncertainty and still needs to be mastered.