Berlin lacks housing units – given the high inflow of new residents in the German capital, around 10,000 new flats would have to be created to satisfy demand. In addition to sponsoring the construction of new housing, Berlin's Senate intends to raise the potential dormant in existing stock. After all, many apartments hypothetically available for residential use are not on the market because they are used for commercial purposes, e.g. as doctors' offices or as holiday homes. According to the Senate, around 15,000 residential units serve as holiday domiciles in Berlin.
Source : Michael Schick Immobilien