@fristi @gettie @mametsuko tying rent to a value of a house is a nice recipe for overinflated house values as well....
Germany generally wants the okay of the tenant other than one loophole.
We have like indexes where you can check out how much a flat/house costs in relation to the mean of the city and for stuff that is comparable to yours. (comparable rent)
The landlord could come to you with that every 15 months and no more than 20% in 3 years based on said index. And It can never be more than the rent index. So mostly you can get more rent by having new tenants.
Obvs. the index also generally has upward trajectory but not as hard as basing on value of a home alone.
If all conditions are met the landlord demand your okay until a certain deadline. If you not okay it they could sue you to pay them the index rent (not some fantasy number)