Who lived in your flat in 1910? In Berlin you can usually find out, much of it free online. Here is the order I would search in, from the old address books to the building file.

With the seven state-owned housing companies, a flat swap keeps both net cold rents exactly as they are. With a private landlord it is a new lease, and the rent can jump. Here is how the swap portal, the 2025 downsizing offer and a private swap compare.