...The high housing prices.
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/)
What's wrong with that? Enjoy the little green space you have left while it lasts. Have a BBQ with friends over there, maybe? The gentrifiers/developers won't be so long to remove every last bit of green and freedom in your neighborhood, as they did everywhere else.