Index rents Sep 2026: Germany CPI 2.9% — §557b BGB rules and pitfalls

What changed on the data side

On 10 September 2026, the Federal Statistical Office (Destatis) confirmed Germany’s consumer price index (CPI) rose by 2.9% year‑on‑year in August 2026. The overall CPI index level stood at 125.8 on the 2020=100 base. These are the latest official figures available as of 20 September 2026. For landlords and tenants with index‑linked residential leases, these readings define the permissible adjustment corridor under §557b of the German Civil Code (BGB).

How §557b BGB converts CPI into a lawful rent change

The law allows parties to agree an index rent that tracks “the price index for the cost of living of all private households in Germany,” i.e., the national CPI published by Destatis. During an index‑rent term the base rent must remain unchanged for at least 12 months. Any adjustment must be demanded in text form and must state (a) the CPI change used and (b) either the new rent or the increase as a euro amount. The higher rent becomes payable from the beginning of the second calendar month after the notice is received.

Calculation principle. You do not apply the latest year‑on‑year inflation rate to the rent. You must compare the CPI index level at the time of the last effective index adjustment (or, if there has been no adjustment yet, at the start of the tenancy) with the CPI index level you rely on in your notice. The proportional change between those two index levels is the only lawful factor to apply to the net cold rent. Formula: new base rent = current base rent × (CPI_now ÷ CPI_base).

Example (illustrative). If the CPI at the last adjustment was 120.0 (2020=100) and the August 2026 CPI you cite is 125.8, the permissible uplift factor is 125.8/120.0 = 1.0483. A base rent of €1,000.00 may be increased to €1,048.33. If the notice is received on 25 September 2026, the new rent is due from 1 November 2026 (the beginning of the second month after receipt).

Which index counts — and which does not

Only the national CPI for Germany compiled by Destatis may be used. Regional indices, private indices, or the EU’s harmonised HICP are not permitted for residential index leases under §557b BGB. Destatis publishes the monthly CPI index levels in its GENESIS database (table code 61111‑0002) and in press releases. If Destatis rebases the CPI in future, use a consistent series (the ratio of two index levels remains valid regardless of base year) and cite the table and month used in the notice.

Clauses that fail compliance

- Automatic adjustment clauses. §557b(3) requires a text‑form demand; rent cannot change “automatically” without a landlord’s notice stating the CPI change and the new amount. - Upward‑only language. Clauses that describe increases but omit the tenant’s symmetrical right to a reduction when the CPI falls have been held invalid. Index rents must work in both directions. - Multiple adjustments inside 12 months. Any attempt to allow more than one change within a 12‑month period conflicts with §557b(2). - Mixing mechanisms. Under an index rent, increases to the local reference rent (§558 BGB) are excluded. Running a staff rent (Staffelmiete, §557a) concurrently with an index rent for the same period risks invalidity; if used at all it must be clearly sequential, not overlapping. - Wrong index reference. Tying the clause to the HICP, a state CPI, “wages”, or a non‑Destatis series violates §557b(1). - Vague or hidden clauses. Courts and commentators criticise index clauses that are buried under “miscellaneous provisions” or that fail to specify how the index maps into euros; such drafting invites AGB‑law scrutiny and invalidity.

Practical effect: if a clause fails, the landlord’s demand is ineffective and the tenant can continue paying the previous base rent (and reclaim overpayments).

Interaction with other rent rules

- One‑year freeze: The base rent must remain unchanged for at least one year. The 12‑month clock runs from the effective date of the last index change (or from lease start until the first change). - Notice timing: The higher rent is due from the start of the second month after receipt of a proper notice. Keep delivery evidence. - Modernisation: Increases under §§559/559e BGB are generally excluded during an index rent; narrow exceptions apply only where works were mandated or caused by circumstances beyond the landlord’s control (with specified carve‑outs). Seek legal advice before attempting any pass‑through. - Rent‑brake (Mietpreisbremse): §§556d–556g BGB apply only to the initial rent of an index lease; later CPI‑driven adjustments are not capped by those provisions. - Operating costs: Changes to service‑charge advances follow §560 BGB and are separate from indexation of the base rent.

Checklist for September 2026 notices

- Verify eligibility: at least 12 months since the last effective index change. - Cite the correct series and month: “CPI for Germany (2020=100), month and year.” Attach or link to Destatis figures. - Show the math: state both index levels, the ratio, and the resulting euro amount to two decimals. - Date control: a notice received in September 2026 makes the higher rent due from 1 November 2026. - Keep mechanisms separate: do not combine CPI indexation with a §558 increase or overlap with a staff rent period. - Risk note: non‑compliant notices and clauses are routinely struck down. Landlords should have counsel check wording; tenants should consult a Mieterverein or a Fachanwalt before withholding or reclaiming rent.