Three hard limits on every increase (§ 558 BGB)
An increase to the local reference rent (Vergleichsmiete) requires the rent to have been unchanged for 15 months by the time it takes effect, and it may be demanded at the earliest one year after the last increase. On top of that sits the Kappungsgrenze: at most 20% within three years — 15% in tight housing markets. The demand must also be justified, typically via the local Mietspiegel (§ 558a BGB).
Stepped or indexed rent blocks § 558 entirely
If your lease contains a Staffelmiete (stepped rent, § 557a BGB) or Indexmiete (indexed rent, § 557b BGB), reference-rent increases are excluded while it runs. A demand sent anyway is void — landlords overlook this surprisingly often. Mietsicht detects the clause in your lease and cross-checks it against the increase letter automatically.
Your deadlines after receiving the demand (§ 558b BGB)
You don't have to react immediately: your consideration period runs until the end of the second month after receipt. If you don't consent, the landlord must sue for consent within three further months — otherwise the demand lapses. Mietsicht computes both deadlines as concrete dates and can remind you by email.
Consent, partial consent, or objection?
An increase can be partially justified — up to the cap but not beyond, for example. Partial consent is possible. And every increase triggers a special termination right (§ 561 BGB). Mietsicht shows the computed limits, drafts a response if you want one, and points to a tenants' association or lawyer for contested cases.
How Mietsicht checks your increase letter
Upload the lease and the increase letter into one case. Mietsicht reads both documents and checks the waiting period, the cap, and the Staffel/Index exclusion against each other — every check shown with its arithmetic, statute, and confidence level, computed deterministically rather than estimated by an AI.