A feller opposite me had a mate of his round and he checked out his gutter's with one, the good thing was they needed cleaning and he asked me.
Boys & their toys spring to mind. Bless him
Can you just imagine turning up at a house to check the gutters with a drone. The look on their face would be classic...WTF 
Whilst it seems a good idea, here is a summary in a short video from the UKCAA website on the rules for flying drones, probably most worthy of note - drones not to be flown within in 50 meters of buildings or people, cameras can break privacy laws and commercial use requires permission.
https://www.youtube.com/watch?v=A6uU1LTdI8MNow I know many people regard the law and the real world as two separate things, but is it worth the risk ?
And back to the original post, considering the above , how can a commercial use such as window cleaning drones be allowed to operate or is there a license ?
edit- just realised they are German, laws maybe different there, but for the UK I don't see it happening any time soon.
Any body else see this on the tv just before xmas ?
https://www.youtube.com/watch?v=LXjvIZTp-mI