What is it and when to do it?
The Mietkaution is a security deposit that protects the landlord against damage or unpaid rent. Its maximum amount is set by law (§551 BGB): no more than three net cold rents. It is paid before or on move-in and can be split into three instalments.
Where and how much does it cost?
Pay by bank transfer to the landlord’s account, or use a specialist product (Mietkautionskonto or Mietkautionsbürgschaft — a surety bond instead of cash). A surety costs around 3–5% of the deposit per year but avoids tying up your savings.
Common mistakes
Paid the deposit in cash without a receipt — at move-out the landlord claimed to have received nothing. Court proceedings, lost time and stress. Always transfer by bank: your bank statement is your proof.
The landlord hadn't returned the deposit after 8 months, citing 'ongoing checks'. It turned out the money was not in a separate account — which is illegal. Sent a formal demand letter with a deadline, and the money arrived two weeks later.