Published by legal principle
If a rented apartment (or house) is sold, then the old landlord leaves the lease and the new owner enters, Civil Code § 566, The change occurs the moment the buyer enters the land register as the new owner.
This rule protects the tenant. If the lease did not pass, the new owner could ask the tenant to let him have the apartment. Because he would not have a lease with him. So the tenant would have no right to the new owner opposite the apartment.
The tenant could then claim damages from the old owner at most, because this can no longer make available to him the rented apartment.
Exception for interior sales
With BGH landed now the question, whether that applies also with a Innenverkauf. The rented apartment belonged to a couple. The man later transferred his share of ownership to the woman, who thereby became the sole owner. The woman terminated the lease and the court argued that the termination was effective or should have been signed by the man. The legal question was whether the man was still landlord despite transferring his former ownership of his wife.
The BGH affirmed that (Az. VIII ZB 26/17). The legal requirement speaks of a sale to a "third party". The woman is not "third party" because she was previously (with her own co-ownership share) already landlord. The tenant needs no protection here, because between him and the acquirer of the share already a lease exists.
Consequences in practice
This decision has serious and currently unmanageable consequences for the practice.
In the future, the starting point is the following basic rule: if a rental property is owned by more than one person and transfers one share to one of the others, it still remains landlord.
Legally relevant declarations in the tenancy agreement must always be submitted by all contractual partners to all other contractual partners. If the tenant wants to cancel, he must do this to all landlords, otherwise the notice is ineffective. This also applies to rental price complaints, notification of defects, objections to utility bills and so on. Conversely, all landlords must declare when they charge operating costs, demand rent increase, modernize or file for payment. In the meantime you have to remember that the retiree will continue to be the landlord.
It gets really interesting at later resales. The landlord changes in sales only if he is identical with the owner. This is no longer the case after an internal sale, so that in other, now external, sales of the lease probably does not pass to the new owner. If you saw it differently, the resale would be a so-called contract at the expense of a third party, which is inadmissible. Thus property and lease fall apart in this constellation after all.
Examples in practice
With co-ownership of spouses, a share is often transferred in a divorce or separation to the other, to compensate for gain or to separate economically from each other. Here the retired ex-spouse remains with the landlord until the tenancy ends.
When heirs are involved, the shares of all others (usually in exchange for compensation) are transferred to one of the heirs. Here all heirs remain landlords even after the transfer of their shares.
In the so-called Münchener Modell, a GbR buys a house in order to allocate the apartments to individual partners if there is a prohibition on partitioning, for example in protected areas. Here, all GbR shareholders remain landlords in all apartments and the other shareholders.
I suspect that we will now have a mess in many objects. The BGH decision applies not only to the future, but also to all transfers that have already taken place. For any legal explanation, one must first recalculate the land register back to the time when the lease began, to see if there were interim breaks that are still going strong today.
Whether that was so wanted by the BGH, I dare to doubt.