[neokya]

I am quite not sure how it would be expensive for existing residents even after sale to so called foreign investor. As far as I understand, in Germany, rents are fixed for existing residents and landlord can not evict them (few exception exists, probably won't work for investors).

That 10% increase rule is only for new tenants.

[detaro]

Rent can be adjusted up to match prices in the area (with a limit of 15% increase per 3 years), and modernizations are a valid reason to increase prices above that. Which is the typical gentrification scenario Berlin is worried about: old buildings are bought, improved to a more luxurious standard and then rented at accordingly higher prices.