Your rent increase may not exceed 5%. If the increase still exceeds this percentage, you may complain to your landlord, which can be done by e-mail. They will then be obliged to adjust the increase to a maximum of 5%.
A draft mail you can send to your landlord:
Dear [x],
I hereby object to the rent increase I have received from [x] with effect from 1 July 2025. The proposed increase amounts to [x]% and thus exceeds the legal maximum rent increase of 5% for rooms from 1 July 2025, as set by the central government.
I request you to substantiate this increase and would like to hear on what legal basis it is based. Until then, I will assume that the increase is unjustified and ask you to adjust it to the applicable regulations.
I would like to receive a written response to my objection within four weeks.
Yours sincerely,
[First and last name][Address]
[Postcode and city]
[Telephone number]