Berlin from Sunday began restricting private property rentals through Airbnb and similar online platforms, threatening hefty fines in a controversial move meant to keep housing affordable for locals.The German capital fears that the growing trend of people letting out apartments to tourists through sites such as Airbnb, Wimdu and 9Flats is cutting into a limited property supply and driving up rents. From May 1, the city-state will enact a new law known by the German mouthful of "Zweckentfremdungsverbot," or prohibition of improper use.Rents in Berlin shot up 56 percent between 2009 and 2014, although at around 10 euros per square meter this year, they are relatively low compared to other major European cities.