Check a Mieterhöhung: Is the rent increase legal? (2026)

What does "checking a Mieterhöhung" mean?

Checking a Mieterhöhung (rent increase) means working out whether your landlord is allowed to raise the rent at all – and whether they have stayed within every limit. The most common form is the increase up to the ortsübliche Vergleichsmiete (local reference rent) under § 558 BGB.

The most important fact first: you do not have to consent immediately. You have until the end of the second calendar month after receiving the letter to check it and decide (Zustimmungsfrist, the consent deadline, § 558b BGB). If the letter arrives in March, the deadline runs until 31 May.

When is a Mieterhöhung actually allowed? (§ 558 BGB)

For an increase up to the ortsübliche Vergleichsmiete to be lawful, three conditions must be met:

1. The 15-month rule. At the point the increase is meant to take effect, the rent must have been unchanged for at least 15 months (§ 558 Abs. 1 BGB). The request itself may be made at the earliest one year after the last increase.
2. The Kappungsgrenze. The rent may rise by no more than 20 % over three years – only 15 % in areas with a tight housing market (Kappungsgrenze, § 558 Abs. 3 BGB).
3. A justification. The landlord must support the new rent – via the Mietspiegel (local rent index), three comparable flats, or an expert appraisal (§ 558a BGB).

Only when all three are in order is the increase formally valid. If even one is missing, you do not have to consent.

Important: Silence does not count as consent (§ 558b BGB). But that does not mean you should do nothing. If you do not respond at all, the landlord can sue for consent. Check the increase and reply within the deadline – whether you consent or object.

Step by step: how to check the increase

1. Note the date. When did the letter arrive? That sets your Zustimmungsfrist (end of the second month that follows, § 558b BGB).
2. Check the 15-month rule. Has the rent been unchanged for at least 15 months? If not, the increase is premature.
3. Do the Kappungsgrenze math. Compare the new rent with the rent three years ago. More than 20 % (or 15 % in a tight market) is not allowed.
4. Check the Vergleichsmiete. Is the rent demanded above the ortsübliche Vergleichsmiete in the Mietspiegel? Then it is excessive.
5. Check the justification. Is a valid justification attached (Mietspiegel, three comparable flats, or an appraisal, § 558a BGB)?
6. Decide and respond. Consent, object, or – if you want to move out – use the Sonderkündigungsrecht (§ 561 BGB).

Which increase follows which rule?

Not every Mieterhöhung runs through § 558 BGB. First work out which type it is:

Type of increase	Legal basis	Does the Kappungsgrenze apply?
Up to the ortsübliche Vergleichsmiete	§ 558 BGB	Yes (20 %, tight market 15 %)
Modernisierung (modernisation)	§ 559 BGB	No – its own ceilings
Staffelmiete (stepped rent, agreed in advance)	§ 557a BGB	No
Indexmiete (tied to consumer prices)	§ 557b BGB	No

Common mistakes tenants make

• Consenting too quickly, even though the deadline still runs for two months.
• Assuming silence is fine – the landlord can sue for consent.
• Overlooking the Kappungsgrenze, even when the Vergleichsmiete appears to be met.
• Confusing Modernisierung with § 558 – different rules, different limits.
• Missing the Sonderkündigungsrecht (§ 561 BGB) when you want to move out anyway.

With HausMaus you check this in minutes

HausMaus is free for tenants. The app checks a Mieterhöhung against the Kappungsgrenze and the ortsübliche Vergleichsmiete under § 558 BGB and shows you clearly whether the increase is lawful. It then helps you decide within the Zustimmungsfrist (§ 558b BGB) – consent or object – and prepare a fitting response letter.