Checking a rent increase: permitted amount and deadlines in minutes

A letter from the landlord, a new figure, an uneasy feeling: is that even allowed? The good news is that a rent increase under the German comparative-rent procedure follows clear rules – and that you can roughly assess it yourself in a few minutes. The trick is to separate two entirely different questions: is the demanded amount permitted? And is the letter itself formally valid? Both hinge on the same decision – consent or not – but follow very different rules.

If you're in a hurry, enter your figures straight into the checker. It calculates the cap limit exactly, compares it with your estimated comparative rent and derives every deadline from the delivery date. How the review behind it works – and where its limits lie – is in the rest of the article.

Two reviews, one decision#

Most guides and almost all online calculators focus on a single question: by what percentage may the rent rise? That matters – but it's only half the review. An increase demand can be attacked in two fundamentally different ways, and the legal consequences are opposites.

A formal defect concerns the letter itself: if a valid justification is missing, the text form is not observed, or the blocking period was not respected, then the entire demand is invalid. You need not react at all; the landlord must draft a new, error-free letter.

A material defect, by contrast, concerns only the level: if the landlord demands more than the cap limit or the comparative rent allow, the demand is invalid only in the excess portion. Up to the permitted level it remains in force. This distinction is the thread running through this article – and the point at which most quick answers from the web become imprecise.

The amount: two ceilings, the lower one wins#

On the amount many make the same error: they treat the cap limit as the limit. In fact § 558 BGB imposes two ceilings simultaneously and independently Quelle. Only what stays within both is permitted – so the lower of the two values always wins.

The cap limit: 20 or 15 percent over three years#

The cap limit is a relative ceiling: within three years the rent may rise by at most 20% (§ 558(3) BGB) Quelle. In areas with a tight housing market the state government may lower this to 15% by ordinance. Whether your municipality is covered depends on the specific ordinance and is time-limited – the lists change constantly and are municipality-specific. That is why the checker above defaults to 20% and lets you switch to 15% if you know your city is affected. A fixed list of cities would be outdated within months – better to check the value yourself with the tenants' association or your city.

This limit is purely arithmetic: 800 € starting rent becomes at most 960 € at 20%. That is exactly the part a tool can state precisely – unlike the second ceiling.

The local comparative rent: the absolute cap#

The local comparative rent is an absolute ceiling: it is never exceeded, no matter how low the starting rent was. It is the average of the rents agreed or changed over the past six years for comparable housing – comparable by type, size, fittings, condition, location and energy characteristics (§ 558(2) BGB). The reference window was extended from four to six years in 2020; older guides therefore sometimes still say "four years".

The catch: the comparative rent is a question of fact, not a value you can calculate yourself. It follows from the rent index, an expert report or comparable flats. That is why the checker honestly treats it as an estimate you enter – not as a calculated result. Once you enter it, it shows you whether, in your specific case, the cap or the comparative rent caps the increase.

How do you get a reliable estimate? The first port of call is your city's rent index – many municipalities publish it online or as a brochure, often with a simple form that turns year of construction, size, location and fittings into a range per square metre. Multiplied by your floor area, you get a rough comparative rent for the checker. Watch two things: use the current edition, and classify your flat honestly – anyone who talks themselves into the most expensive column checks themselves into the wrong answer. If unsure, a short consultation with the local tenants' association helps; they know the rent index and the typical points of dispute.

Two further details are set by law: third-party funds such as public subsidies must be deducted from the calculation (§ 558(5) BGB), and agreements to the tenant's disadvantage are invalid – the rules are therefore semi-mandatory (§ 558(6) BGB). Anything in the tenancy agreement that deviates from § 558 to your detriment simply does not apply.

A worked example: sometimes one binds, sometimes the other#

Take a starting rent of 800 € and a municipality without a 15% ordinance. The cap limit allows up to 960 €. If the local comparative rent is 1,000 €, the lower value wins – 960 € is permitted, not 1,000 €. If the comparative rent is 900 €, however, it caps the increase even though the cap still had room up to 960 €: only 900 € is then permitted. This shows how sometimes one, sometimes the other limit binds – and why you need both.

The form: here a single error voids the whole demand#

Once the amount is clear, look at the form of the letter – that is where the sharpest lever lies. With a formal defect it is not just part but the entire demand that is invalid.

Text form and justification#

The increase demand must be "declared and justified in text form to the tenant" (§ 558a(1) BGB) Quelle. Text form means: a letter or an email suffices, a handwritten signature is not required. What is decisive is the justification – without one, or with an unsuitable one, the demand is formally invalid.

The means of justification: the four common ones#

What the landlord may use to justify is listed in § 558a(2) BGB. The catalogue is introduced by the word "in particular" – so it is not strictly exhaustive, but in practice these are the four standard means:

1. 1

   Rent index

   A simple or qualified rent index of the municipality. The qualified one carries special weight (see below).

2. 2

   Information from a rent database

   A recognised database that records comparable rents.

3. 3

   Expert report

   A report by a publicly appointed and sworn expert.

4. 4

   Three comparable flats

   Naming at least three comparable flats with their rent.

The qualified rent index is worth a second look: it must be drawn up according to recognised scientific principles and be recognised, must be adjusted every two years and redrawn at the latest every four years (§ 558d BGB) Quelle. It carries a presumption: the rents it lists are presumed to reflect the local comparative rent. But if the landlord relies on an outdated index that was not adjusted on time, that presumption can be challenged.

The double blocking period: why both 12 and 15 months are correct#

Two numbers circulate around the blocking period – 12 and 15 months – and both are correct, because they measure different points in time (§ 558(1) BGB). Separating them cleanly is where many accounts become imprecise:

Combined, that means: the landlord may send the letter roughly one year after the last increase. But because the new rent only takes effect at the start of the third month after delivery, the old rent has then in fact been unchanged for about 15 months. The checker above takes this confusion off your hands: enter the date of the last increase and it reports whether both periods are met. If one is breached, that is a formal error – and the whole demand falls.

The deadlines: how much time you have#

Once the form holds, it's about your own clock. From the delivery of the letter, three dates follow deterministically (§ 558b BGB) Quelle – exactly the ones the checker calculates:

An example: if the letter arrives on 10 March, your consent deadline runs until 31 May; if you consent, you owe the higher rent from 1 June. If you do not consent, the landlord has until the end of August to sue for consent – if he doesn't, the increase demand lapses and he must start over. Important: the exact delivery can be disputed (drop-in versus actual notice). The deadline countdown is therefore an orientation, not a legally binding end date.

One point is often overlooked: consent need not be expressly declared. Anyone who pays the new, higher rent without reservation from the effective date may thereby consent by conduct – impliedly. So while you are still checking, or if you consider the amount too high, do not simply pay the increased sum. When in doubt, keep paying the old rent and respond in writing – or, if you pay at all, pay expressly "under reservation". Otherwise you create facts you actually still wanted to clarify. Conversely: mere silence is not consent – but it lets the deadline lapse and hands the landlord the next move.

Formal or material? That decides your reaction#

Now everything comes together. Whether a defect is formal or material determines what you must do:

The practical consequence of the material case is partial consent: if the increase is justified in principle but too high in amount, you can consent up to the amount you consider permitted and reject the rest. The increase then takes effect at that level. The landlord must litigate over the rest if need be. The checker above shows the excess amount directly – that is the sum at stake in a partial consent.

Your four possible reactions#

Colloquially it's "consent or object". In fact you have four paths until the deadline – and partial consent is missing from most accounts:

1. 1

   Consent

   Expressly or by paying the new rent without reservation. Sensible when amount and form are clean.

2. 2

   Consent in part

   Up to the amount you consider permitted; reject the excess specifically. The right path for a pure amount error.

3. 3

   Reject

   For a formal error or an unjustified increase. Then the move is with the landlord – suit within three months or lapse.

4. 4

   Do nothing

   Counts as non-consent, but is risky: no documented review. A clear written reaction is almost always better.

How to react correctly#

Once you've decided, it's about clean execution. Four things matter:

• In writing and provable. Formulate your answer – whether partial consent or rejection – in text form and send it provably. A verbal reaction is worthless in a dispute.
• Name it specifically. Say which amount you consent to or which point you contest. A blanket "I object" line doesn't help; give the reason (missing justification, blocking period, excessive amount).
• Meet the deadline. Your answer must reach the landlord by the end of the second calendar month after delivery. Don't wait until the last day.
• Don't confuse it with modernisation. A rent increase after modernisation (§ 559 BGB) follows entirely different rules than the increase up to the comparative rent described here. If your letter mentions a "modernisation surcharge", that's a separate topic.

And a note on tone: a rent increase is not an affront but a regulated procedure. Those who check calmly and respond calmly come out better than those who reject everything on a hunch – or sign everything.

The most common pitfalls#

1. 1

   Treating the cap limit as the only limit

   The most common shortcut of all. Even if the 20% still has room, the local comparative rent is the end – and vice versa. Always compare both values and take the lower one.

2. 2

   Confusing 12 and 15 months

   Both numbers are correct, they just measure different points in time. Anyone who lumps them together mistakes a correct deadline for an error – or overlooks a real one.

3. 3

   Rejecting everything over an excessive amount

   A pure amount error voids the increase only in part. Anyone who rejects across the board instead of consenting in part needlessly risks a dispute over the part that was justified anyway.

4. 4

   Missing a formal error and paying anyway

   If the justification is missing or the blocking period was not observed, the whole demand is invalid. Anyone who consents by reflex pays more than they must.

5. 5

   Reacting only verbally

   Whether consent, partial consent or rejection – without provable delivery in text form it's your word against theirs in a dispute.

What the checker can – and cannot – do#

So you read the result correctly: the checker above calculates two things exactly – the cap limit from your starting rent and the deadlines from the delivery date. Both are pure arithmetic and therefore reliable. The local comparative rent, by contrast, is only ever as good as your estimate; the checker cannot know it. And whether your municipality falls under the 15% reduction depends on an ordinance with a cut-off date that changes – which is why you must choose the cap rate yourself.

What the checker deliberately does not assess is the content of the justification: whether a specific rent index is suitable or an expert report convincing is a legal judgment in each case. The checker tells you whether amount and deadlines are within limits and which formal points you need to tap – the final assessment of a contested case belongs in the hands of the tenants' association or a specialist lawyer.

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