A letter with a new rent: check first, then pay
With an index rent, the rent increase arrives by letter. Unlike an increase to the local comparative rent, you do not have to consent: if the declaration is correct, you owe the new rent from the stated date. That makes it all the more important to check the letter carefully. The following six points cover the most common mistakes. The index rent calculator compares the amount and date for you in its "Check landlord's letter" mode.
1. Is there a valid index clause at all?
An index rent must be agreed in writing and refer to the consumer price index for Germany calculated by the Federal Statistical Office (§ 557b(1) BGB). Look at your lease. If it names another index or the wording is unclear, have the clause checked before you pay.
2. Is the text form observed?
The declaration must be made in text form (§ 557b(3) BGB). Under § 126b BGB this means a legible declaration on a durable medium that shows who is making it. A letter or email is enough; no signature is needed. A verbal announcement or a quick phone call, however, is not sufficient.
3. Are the old and new index values stated?
The letter must state the change in the price index. Usually it gives the old and new index values with their months, often with the percentage change as well. Check:
- Is the old value taken from the month of the last adjustment or the start of the lease, or from the month your lease prescribes?
- Is the new value a monthly value that has already been published?
- Do both values come from the same series, currently base 2020 = 100?
The calculator lets you pick the official monthly values, so you can see immediately whether the figures in the letter match the published values.
4. Is the amount correct?
The new rent equals current rent × new index ÷ old index. The letter must state the new rent or the increase in euros. Do the maths: if your rent is €800 and the index rises from 122.3 (August 2025) to 125.8 (August 2026), the result is €822.89. If the landlord demands €830, that is €7.11 per month and €85.32 per year too much.
Small differences of a few cents often arise because a rounded percentage was used. Strictly speaking that is also too much, but it rarely justifies a dispute. Larger differences deserve a closer look: common causes are a wrong starting month, mixed base years, or a point difference treated as a percentage.
5. Is the date right?
The new rent is only payable from the start of the month after next following receipt of the declaration. In addition, the rent must have remained unchanged for at least one year since the last change (§ 557b(2) BGB). Example: the letter reaches you on 24 September and the last increase has applied since October of the previous year. The earliest possible date is 1 November. If the landlord names 1 October, you do not have to pay the higher rent for October.
So note down the date of receipt. For a letter, this is usually the day it is in your letterbox at a time when it would normally be emptied.
6. Is something else being increased as well?
During an index rent, an increase to the local comparative rent is excluded (§ 557b(2) BGB). If a letter combines the index adjustment with a reference to the local rent index (Mietspiegel), something is wrong. Modernisation surcharges are only possible to a limited extent, and service charges are handled separately.
How to respond if something is wrong
- Reply in writing and state specifically what you object to, for example the wrong starting value or the date being too early.
- Pay the undisputed part. If only the amount is too high, pay the correctly calculated rent from the correct date.
- Keep the letter, the lease and your calculation.
- If in doubt, tenants' associations or tenancy lawyers can help. This article is not legal advice.
More background on the formula, deadlines and the cap is in our main guide Index rent in Germany. If the higher rent strains your budget, take a look at the housing benefit calculator.