Rent Reduction (Mietminderung) for Mould: How Much Money You Are Entitled To

Mould as a defect in the rented property

Mould in a rented flat is more than an aesthetic problem — it constitutes a significant defect and can endanger your health. Under § 536 BGB, tenants have the right to reduce the rent (Mietminderung) if the flat has a defect that limits its fitness for use. Mould infestation meets this requirement in the vast majority of cases.

How large can the rent reduction for mould be?

The size of the rent reduction depends on the extent and location of the mould infestation. Courts have recognised various reduction rates in the past:

• 5 to 10 percent: Minor mould in secondary rooms (cellar, storage room)
• 10 to 20 percent: Visible mould in the living room, bedroom, or kitchen
• 20 to 30 percent: Extensive mould infestation across several rooms
• Up to 50 percent: Health-endangering mould (e.g. black mould) in living and sleeping areas
• Up to 100 percent: In extreme cases, when the flat is uninhabitable

Important BGH rulings on mould

The Bundesgerichtshof (Federal Court of Justice, BGH) has strengthened tenants' rights in cases of mould in several landmark rulings:

• BGH, judgment of 11 July 2012 (VIII ZR 138/11): The landlord bears the burden of proof that the mould was caused by the tenant's improper ventilation. The tenant does not have to prove that they ventilated correctly.
• BGH, judgment of 5 December 2018 (VIII ZR 271/17): In older buildings that met the standards applicable at the time of construction, thermal bridges do not automatically constitute a defect. In these cases, the tenant can be expected to heat and ventilate appropriately; a reduction cannot then be justified by the building fabric alone.
• BGH, judgment of 18 April 2007 (VIII ZR 182/06): The rent reduction takes effect by operation of law — the tenant does not have to apply for it. What matters is that the defect is reported to the landlord.

How to proceed correctly

To avoid jeopardising your right to a rent reduction, you should proceed methodically:

• Document: Photograph the mould with a date. Measure the affected area and note down the room.
• Defect notice: Inform the landlord promptly and in writing (registered letter with acknowledgement of receipt). Describe the defect precisely and set a deadline for remedy (14 days is reasonable).
• Pay rent under reservation: Continue to pay the rent under reservation for now, or reduce the rent by the appropriate reduction amount. Caution: an excessive reduction can justify a Kündigung (notice of termination).
• Obtain an expert opinion: If there is a dispute about the cause, an independent expert opinion is advisable. The person who commissions it initially bears the cost; if the reduction is justified, the landlord can be required to reimburse it.

When is the landlord right? The question of ventilation habits

Landlords often argue that the mould was caused by the tenant's improper ventilation. It is true that mould can be encouraged by insufficient ventilation. However, the BGH has made clear: the landlord must prove that the mould is attributable solely to the tenant's behaviour. In the case of structural defects — such as missing thermal insulation or thermal bridges — the landlord is liable even if the tenant did not ventilate optimally.

Conclusion: Act quickly

Mould in your home is a serious problem that requires immediate action. Document the infestation, inform the landlord, and make use of your right to a reduction. KlarMiet can check your rental contract for potentially relevant clauses and give an initial assessment of the possible rent reduction. Do not hesitate — your health and your rights are at stake.