Germany has fairly strong protection against rent increases for existing tenants: during a tenancy the rent can only increase up to the "ortsübliche Vergleichsmiete" (average rent of comparable properties in the same town), and by a maximum of 15%/20% (depending on location, 15% in Berlin) over three years.
In Berlin and many other cities there's also rent stabilization ("Mietpreisbremse") for new tenants: new leases on old properties may not exceed the maximum of 110% of the "ortsübliche Vergleichsmiete" and the previous tenant's rent. This falls away if the property is substantially renovated.
Renovations can stop both of these from working: in the former case by increasing the value of the apartment so that the set of "comparable properties" is much more expensive, possibly forcing the current tenant out so a new, wealthier tenant can be found to negotiate a new contract; in the second case, the control falls away completely if the value of renovations in the last five (?) years is at least a third of the total value.
The Mietpreisbremse is widely ignored: advertised rents exceed the limit for more often than not, and the landlords choose tenants they think are least likely to sue over it. The worst case for the landlord is that they have to pay back the difference from the point where the tenant complained; the best case is that the Federal Constitutional Court invalidates the law (which is fairly new and not yet judicially tested).
In Berlin and many other cities there's also rent stabilization ("Mietpreisbremse") for new tenants: new leases on old properties may not exceed the maximum of 110% of the "ortsübliche Vergleichsmiete" and the previous tenant's rent. This falls away if the property is substantially renovated.
Renovations can stop both of these from working: in the former case by increasing the value of the apartment so that the set of "comparable properties" is much more expensive, possibly forcing the current tenant out so a new, wealthier tenant can be found to negotiate a new contract; in the second case, the control falls away completely if the value of renovations in the last five (?) years is at least a third of the total value.
The Mietpreisbremse is widely ignored: advertised rents exceed the limit for more often than not, and the landlords choose tenants they think are least likely to sue over it. The worst case for the landlord is that they have to pay back the difference from the point where the tenant complained; the best case is that the Federal Constitutional Court invalidates the law (which is fairly new and not yet judicially tested).