When you rent a furnished apartment in Berlin, the rental agreement has a move-out date. It is not possible to give notice before that date.
The Crocodilian rental agreement is available in German and English. It can be concluded online, which is fast and very easy.
It already has a chapter dealing with the possible option to extend the lease. You only have to tell the owner if you need an option to extend, and if so, for how much longer you would possibly like to extend the lease. You should address this subject as early as possible.
If you would like to continue renting the furnished apartment, you get in touch with the onwer and with Crocodilian one month before your rental agreement terminates. Or earlier. The sooner, the better.
If you do not get back to the owner and Crocodilan in time, the flat may already be rented out to somebody else. So please note that it is you who has to make sure that you actually get the option to extend if you need it.
Is it possible to extend the Rental Agreement more than once?
Yes, it is. Owers know that tenants need to be flexible and cannot commit to a long rental period.
But hardly any owner will extend the lease on a monthly basis. It should always be two months, at least.