What is it and when to do it?
The Kündigung is the formal notice to your landlord to end your tenancy. Under §573c BGB the minimum notice period for tenants is 3 months, running from the first day of the month after the letter is received. Missing the deadline means paying three extra months of rent.
Where and how much does it cost?
Write a simple letter in German: your address, the landlord’s address, the phrase “hiermit kündige ich den Mietvertrag”, your desired move-out date, and your signature. Send it as an Einschreiben mit Rückschein (about €4–5 at Deutsche Post) — far cheaper than an unplanned extra month of rent.
Common mistakes
Sent the notice by ordinary post — the landlord claimed nothing arrived. Had to pay three more months. Always use Einschreiben mit Rückschein: a €4 receipt can save you €3,000.
Wrote the notice in a language the landlord understood — but the tenancy agreement was in German, so the letter had no legal force. Always write your Kündigung in the language of the contract.