It is illegal to keep an apartment empty for more then three months. So if you are aware of any empty housing, feel free to denounce it (https://www.stadtentwicklung.berlin.de/wohnen/zweckentfremdu...). In 2018, 1,9% of the flats in Mitte were subject to an ongoing official procedure on those grounds. The owner can simply claim, they are reforming, but I presume there are limits to that too.
How many flats are really empty is unclear.
I've read that according to the Senat,
- there is an estimated 0.8%-2% of the flats unoccupied
- down from 3.5% in 2011
- ideally, it would be between 2-3%.
The Taz is not particularly strong on keeping opinion from facts separated, but that's where I got it from: (https://taz.de/Spekulativer-Leerstand-in-Berlin/!5749397/)
...The high housing prices.