Mieterhöhung Step by Step: Writing a Legally Valid Rent Increase Letter (2026)

Raising the rent on an existing tenant is one of the most procedurally fragile things a German landlord can do. The substantive rules are not hard, but a single formal mistake and the entire Mieterhöhungsverlangen is void, the rent stays where it was, and you cannot try again for at least a year. This guide walks through the rent increase process under § 558 BGB, the timing constraints, the four valid justifications, and the mistakes that most often defeat a rent increase in court.

This article covers the standard rent increase to ortsübliche Vergleichsmiete (local comparative rent). Index rents (Indexmiete), step rents (Staffelmiete), and modernisation surcharges (§ 559 / § 559e BGB) follow different rules and are out of scope here.

When you can ask for an increase

Two timing rules govern when an increase is even possible.

Jahressperrfrist (12-month lockout). You can only send a Mieterhöhungsverlangen if the rent has been unchanged for at least 12 months at the date the request is sent. This counts from the last rent increase, or from the start of the tenancy if it has never been raised. Modernisation surcharges and operating cost adjustments do not reset this clock.

15-month rule on effect. The new rent itself can only take effect at the earliest 15 months after the last increase. In practice, this means: send the request after 12 months, give the tenant the 2-month Überlegungsfrist plus the additional month before the new rent applies, and you land at 15 months total.

Practical sequence:

Last increase or tenancy start
│
├── +12 months ──► Earliest you may send the Mieterhöhungsverlangen
│
├── Tenant has Überlegungsfrist (end of 2nd month after receipt)
│
└── +15 months ──► Earliest the new rent can apply

If your last increase was 1 January 2025, the earliest you can send a new request is 1 January 2026, and the earliest the new rent can take effect is 1 April 2026.

The Kappungsgrenze: how much you can raise

§ 558 Abs. 3 BGB caps the increase at:

20% over any 3-year period in standard markets.
15% over any 3-year period in tight markets (angespannte Wohnungsmärkte) designated by Land ordinance.

The 3-year window is rolling and looks backwards from the date the new rent takes effect. Every increase counted, whether based on Mietspiegel, Vergleichswohnungen, or expert valuation. Modernisation surcharges under § 559 / § 559e BGB are not counted, they have their own caps.

Example. Berlin (15% market). Current cold rent is €1,000. The 3-year ceiling is €1,150. If you raised by €80 two years ago, the next raise within the 3-year window cannot exceed €70 (15% × €1,000 = €150 total, of which €80 is used).

The Kappungsgrenze is a hard ceiling, but it is also a floor for your strategy: you do not have to use the full cap. The increase still has to stay at or below the local ortsübliche Vergleichsmiete (see next section). The lower of the two limits applies.

The ortsübliche Vergleichsmiete (local comparative rent)

§ 558 lets you raise rent only up to the local comparative rent. This is the empirical median of rents being paid for similar apartments in the same area. You cannot exceed it, even if the Kappungsgrenze would technically allow more.

To prove what the comparative rent is, your Mieterhöhungsverlangen must reference one of four documented sources:

1. Qualifizierter Mietspiegel (qualified rent index)

By far the most common and lowest-friction option in cities that have one (Berlin, Munich, Hamburg, Frankfurt, Cologne, etc.). The qualified Mietspiegel is statistically grounded and has presumption of correctness in court.

You must:

Identify the correct Mietspiegel category for the apartment (year built, size, location, fittings).
State the resulting rent range, e.g., “€8.50 to €11.20 per m²”.
Place the apartment within the range based on Wohnwertmerkmale (positive features push toward the upper bound, negatives toward the lower).
State the resulting target rent and the math.

If the city has only an einfacher Mietspiegel (non-qualified), you can still reference it but the evidentiary weight is weaker.

2. Mietdatenbank (rent database)

A formal certified database maintained by some municipalities or industry bodies. Rare in practice. Treat it like a Mietspiegel reference if available.

3. Sachverständigengutachten (expert valuation)

A written, reasoned report from a publicly appointed and sworn surveyor. Costs €500 to €1,500 depending on apartment complexity. Used when no Mietspiegel exists or the Mietspiegel does not cover the specific property type.

4. Three Vergleichswohnungen (comparable apartments)

Three concretely identified other apartments at higher rents than yours, comparable in location, size, type, and condition. The most legally fragile option because tenants commonly challenge whether the comparables are truly comparable. Use only when no Mietspiegel exists.

What the request must contain (form requirements)

The Mieterhöhungsverlangen must:

Be in Textform. Written letter, email, or fax. WhatsApp messages are technically Textform but practically risky for evidence purposes. Use a signed letter sent by Einschreiben Einwurf.
Be sent by the landlord (Vermieter) or by a properly authorised agent (with original power of attorney enclosed if not the property manager listed in the lease).
Be addressed to every tenant on the lease. A request sent only to one of two co-tenants is void in its entirety.
State the concrete new total rent, not just the percentage or the increase amount. Example: “I request your consent to a new monthly cold rent of €1,150.”
Include the justification (Mietspiegel reference with category and range, the comparable apartments, or the surveyor’s report).
Show the math if you are using a Mietspiegel: which category, where in the range, why the apartment falls there.

Importantly, the request does not need to argue the Jahressperrfrist or the Kappungsgrenze. Those are objective facts the court can check independently.

The tenant’s response window

After the tenant receives the Mieterhöhungsverlangen, they have until the end of the second month following receipt to consent (Überlegungsfrist).

Example. Tenant receives the request on 18 March 2026. They have until 31 May 2026 to consent. If they consent within this window, the new rent takes effect on 1 June 2026 (the third month following receipt).

Three possible tenant responses:

Full consent. New rent applies as stated.
Partial consent (e.g., agreeing to a smaller increase). Treat as deemed rejection of the original. You can either accept the partial offer or proceed to court for the difference.
Silence or refusal. The Mieterhöhung does not take effect automatically. You must sue for consent (Zustimmungsklage).

If the tenant refuses: the Zustimmungsklage

You have 3 months after the end of the Überlegungsfrist to file a Zustimmungsklage (consent lawsuit) at the local Amtsgericht. Miss this deadline and the entire Mieterhöhungsverlangen becomes void, and you must restart the process from scratch.

The Zustimmungsklage is a civil suit asking the court to substitute its judgment for the tenant’s missing consent. If you win, the new rent applies retroactively to the original effective date.

In practice:

Most rejected increases are settled before judgment if the legal basis is solid.
Litigation costs are typically €500 to €1,500 in a typical residential case, recovered from the tenant if you prevail.
Tenants commonly challenge: the Mietspiegel categorisation, the Wohnwertmerkmale ranking, the comparability of the Vergleichswohnungen, and the formal validity of the request itself.

Common mistakes that defeat rent increases

In court, these mistakes account for most defeated rent increases:

Sending to only one of multiple tenants. Husband-and-wife leases, flat shares, anything where more than one signature is on the original Mietvertrag. The request must be addressed to all of them. Defective address renders the entire request void.
Insufficient Mietspiegel reasoning. Stating only the rent range without categorising the specific apartment within it. The tenant cannot evaluate the request and the court will agree.
Below the Jahressperrfrist. Sending the request 11 months and 2 weeks after the last increase. The clock starts on the date the previous increase took effect, not on the date it was agreed.
Mathematical errors. Computing 15% of the wrong base (it is the cold rent, not the warm rent). Reaching above the Mietspiegel range. Adding modernisation amounts that should be tracked separately.
Conditioning the request. “I will agree to fix the leak if you accept the new rent.” Conditional rent increases are entirely void under settled BGH case law.
Including a deadline shorter than the Überlegungsfrist. “Please confirm by 30 April.” This does not shorten the statutory window, but if it is taken as the actual demand, the court may treat the request as procedurally defective.
Forgetting the Mietpreisbremse interaction. In a tight market with a Mietpreisbremse, the new rent must still respect the 10% above ortsübliche Vergleichsmiete cap if the lease is a re-let scenario. For continuing leases, the Kappungsgrenze and the ortsübliche Vergleichsmiete are the binding constraints.
Missing the Klagefrist after refusal. This is fatal. Diary it the day you send the request.

Worked example: Munich, 3-year-old lease, qualified Mietspiegel

A landlord rents a 75 m² 3-room apartment in Munich-Schwabing. Current cold rent: €1,275 (€17.00 per m²). Last increase: 1 February 2024 by 8%. Munich is a tight market (15% Kappungsgrenze).

Step 1: Check timing.

Last increase: 1 Feb 2024.
Earliest send date: 1 Feb 2026 (12 months passed since 1 Feb 2025, the date the prior increase fully took effect, plus the 12-month freeze).
Earliest effect date: 1 May 2026.

Step 2: Compute Kappungsgrenze.

Cold rent at start of 3-year window (1 May 2023): assume €1,180.
Maximum increase: 15% × €1,180 = €177.
Already used: €1,275 minus €1,180 = €95.
Remaining headroom: €177 minus €95 = €82.
Maximum new rent under Kappungsgrenze: €1,275 plus €82 = €1,357 (€18.09 per m²).

Step 3: Check Mietspiegel.

Qualified Munich Mietspiegel range for this apartment type and location: €18.50 to €23.40 per m².
Apartment has good Wohnwertmerkmale (renovated bathroom, balcony, fitted kitchen, EPC class C).
Reasonable mid-range placement: €19.50 per m². Total target: €1,462.

Step 4: Apply the lower of the two.

Kappungsgrenze allows: €1,357.
Mietspiegel target: €1,462.
Use €1,357 (the lower limit). Increase amount: €82.

Step 5: Draft the letter.

Address to both tenants on the lease.
State new total cold rent of €1,357.
Reference the Munich Mietspiegel category.
State the resulting range, list the Wohnwertmerkmale, place the apartment at €19.50 per m².
Note that the actual increase is constrained by the Kappungsgrenze (no obligation to argue it, but useful for transparency).
Send by Einschreiben Einwurf.

Step 6: Wait for the Überlegungsfrist. If the tenant receives the letter on 5 February 2026, they have until 30 April 2026 to consent. New rent applies 1 May 2026.

Step 7: If refused, file Zustimmungsklage by 31 July 2026.

Mietpreisbremse: when it applies

The Mietpreisbremse (§ 556d BGB) caps the rent at 10% above the ortsübliche Vergleichsmiete on new lettings (Wiedervermietung), not on continuing leases. For an existing tenant, the binding constraints are § 558 (Kappungsgrenze) and the ortsübliche Vergleichsmiete itself, the Mietpreisbremse does not directly limit you further.

There is one indirect link: if the previous tenant was already paying above the Mietpreisbremse-permitted level due to a grandfather clause, you may be able to claim that grandfathered level. This is highly fact-specific and worth a separate consultation if relevant.

What about the Mietrecht II reform?

The 2026 Mietrecht II draft (still in legislative process as of April 2026) primarily changes index rents (Indexmiete) by capping them at 3.5% per year in tight markets, and tightens furnished surcharges. It does not directly change § 558 BGB. The Kappungsgrenze, the Mietspiegel reference, and the procedural rules covered here remain unchanged.

If you have an Indexmietvertrag instead of a regular lease, the post-Mietrecht II rules (3.5% cap, tighter justification) will apply once the law passes. The mechanism is different from § 558.

A clean Mieterhöhungsverlangen checklist

Before you send the letter, verify all of these:

How this affects your investment math

Rent increases are not just a tenant relationship issue, they directly compound your long-term yield. A €82 monthly increase like the Munich example above adds €984 per year to gross rental income, which over a typical 15-year holding period is €14,760 of additional revenue, before any subsequent increases. The same property’s rental yield calculation shifts meaningfully when the realised rent tracks the local market rather than lagging it.

For tax purposes, the increased rent flows through to higher Anlage V income and a higher AfA-offset position, with the relative cost depending on your marginal rate. Use the AfA calculator to model how depreciation absorbs part of the increased rental income at your tax bracket.

The pattern most landlords miss is consistency. A landlord who sends a § 558 letter every 3 years like clockwork, even at modest amounts, dramatically outperforms one who waits until tenant turnover, because the Kappungsgrenze keeps resetting and the rent never falls more than 15 to 20% behind the market. The procedure feels heavy the first time. After the second cycle, it becomes routine.