In one sentence
A rent increase is only valid if it stays within the rent cap, the lock-in period, and the comparable local rent — and you have until the end of the second calendar month after receipt to respond. Anyone who simply stays silent risks being sued for consent.
The three hard limits (Section 558 BGB)
Before you even think about formal defects, check the substantive limits. They tell you how much a rent increase may legally be:
- Rent cap: Within a three-year period, rent may rise by no more than 20 percent. In many tight housing markets, a state (Land) ordinance lowers this rent-increase cap to 15 percent – in larger cities in particular, that's the rule rather than the exception.
- Lock-in period: The new rent takes effect at the earliest 15 months after you moved in or after the last increase. A letter that arrives too early isn't automatically void, but it only takes effect later.
- Comparable local rent: This is the real ceiling. The landlord may raise the rent at most up to the amount that's customary for comparable flats in the area. If your rent is already there, there's no room to go higher — regardless of whether the rent cap would, on paper, still allow more.
Formal defects that invalidate the letter
The demand must be made in text form and must be substantiated. In essence, three routes are permitted as justification: the local rent index (Mietspiegel), naming three specific comparable flats, or an expert opinion. If the justification is missing entirely, doesn't hold together, or refers to a rent index that doesn't actually exist in that form, the demand is formally invalid. Courts have repeatedly ruled that a formally invalid increase demand doesn't trigger any obligation to consent. A formal defect is therefore often the fastest lever for rejecting a rent increase.
Your deadlines – and why staying silent is dangerous
A clear deadline applies to objecting to a rent increase: you have a consideration period running until the end of the second calendar month after you received the letter. Within that time you can consent, refuse, or only partially consent – for instance, to the portion that actually reflects the comparable local rent. Such partial consent is explicitly possible and is often a more realistic response than an outright no.
Important: if you refuse to consent or simply stay silent, the landlord can sue for consent once the deadline has passed. Your objection therefore doesn't stop the increase for good — it shifts the question to court. Using the deadline properly means, above all: respond with reasons, don't let it sit.
Special case: modernisation – and index-linked rent
Not every increase follows Section 558 BGB. For a rent increase following modernisation, the landlord may pass on 8 percent per year of the modernisation costs allocated to your flat — again with a statutory cap on the upside. Different requirements apply here (genuine modernisation, proper advance notice) than for a comparable-rent increase. A third track is index-linked rent: if the lease provides for a link to the consumer price index, the rent cap and the comparable-rent rule don't apply — the rent then follows the index instead. Which case applies determines which review is relevant in the first place.
Upload your rent-increase letter and have it checked
Whether it's 15 or 20 percent, whether the lock-in period was observed, whether the justification holds up — that's easier to establish from the actual letter than from general rules of thumb. Upload your rent-increase letter to SmartLegalPro: the document analysis checks the rent cap, the lock-in period, and the justification, and identifies which type of increase you're dealing with. You get an initial assessment in around 60 seconds, no account required; you then use the letter assistant to draft a factual response — consent, partial consent, or rejection. The glossary explains the terms behind all this, and you'll find the statutory text at Section 558 BGB. This doesn't replace legal advice in an individual case – but it tells you, before the deadline runs out, whether objecting is even worthwhile.
