What a Nebenkostenabrechnung is (and why you got a back-payment)
In Germany your rent has two parts: the Kaltmiete (base rent) plus Nebenkosten (operating costs) — together the Warmmiete. You pay an estimated monthly advance (Vorauszahlung) for the operating costs, and once a year the landlord sends a Nebenkostenabrechnung that reconciles those advances against the actual costs (§556 BGB). If you underpaid, you owe a Nachzahlung (back-payment); if you overpaid, you get a Guthaben (refund). A surprise back-payment usually just means the advances were set too low — but it's always worth checking the statement is correct.
The 12-month deadline that can wipe out a back-payment
This is the single most useful thing to know. The landlord must deliver the statement within 12 months of the end of the billing period (§556(3) BGB). After that deadline they can no longer demand a back-payment — unless they genuinely weren't responsible for the delay. A late statement does not cancel a refund owed to you; the bar only blocks the landlord's claim. So if a Nachzahlung arrives more than a year late, check the dates before you pay. The law is on gesetze-im-internet.de.
Which costs a landlord can (and can't) charge you
Only the operating costs listed in the Betriebskostenverordnung (§2 BetrKV) can be passed on — there are 17 categories, including property tax (Grundsteuer), water and sewage, heating and hot water, refuse and street cleaning, building cleaning, garden, lift, communal lighting, building insurance, and the caretaker. "Other costs" are only allowed if named in your contract. What a landlord cannot charge you: administration costs (Verwaltungskosten) and repairs or maintenance (§1(2) BetrKV) — those stay with the landlord.
How costs are split: per m², per person, and the heating rule
Each cost is divided among tenants by a distribution key (Umlageschlüssel) — commonly by living area (m²), by number of occupants, or by metered consumption. If nothing else is agreed, the default is by living area (§556a BGB). Heating and hot water are special: by law (§7 Heizkostenverordnung), at least 50% and at most 70% must be billed by actual recorded consumption, with the rest by area — which is why your flat has heat meters.
Is your bill normal? The "second rent" benchmark
The Deutscher Mieterbund (the national tenants' association) publishes a yearly Betriebskostenspiegel. For the 2024 billing year it put average operating costs at about €2.67 per m² per month, rising to as much as €3.68/m² if every possible cost type applies — which is why Nebenkosten are nicknamed the "second rent." For an 80 m² flat that's roughly €3,500 a year on average. If your statement is far above this, it's worth a closer look.
If it's wrong: object, inspect the receipts, get help
You have 12 months from receiving the statement to raise objections (§556(3) BGB), and the right to inspect the underlying receipts (Belegeinsicht) to check the figures. If something's off — a non-allowable cost, the wrong distribution key, a late statement — raise it in writing; the Verbraucherzentrale and your local Mieterverein (tenants' association) offer template letters and individual help. Start from your lease, which sets what you agreed to pay — see the rental contract guide — and build your housing plan to keep the deadlines in view.