Mould is the classic German housing dispute – and it almost always turns on one question: construction defect or ventilation habits?
Who must prove what
The Federal Court has ordered the burden clearly (VIII ZR 271/84, settled case law): first the landlord must prove the building fabric is sound – no thermal bridges, intact sealing, working heating. Only then must the tenant prove proper heating and airing.
"The tenant must notify the landlord without delay of a defect that appears during the tenancy."
— § 536c (1) BGB (translated)
What courts consider reasonable ventilation
Established case law expects cross-ventilation two to three times a day and heating to 18–20 °C. Not reasonable: airing four or five times daily, keeping furniture centimetres off exterior walls, or leaving rooms permanently unheated – demands like that point to a building defect.
The sensible sequence for both sides
- Report immediately and document (dated photos, affected area in cm²).
- Clarify causation: hygrometer log, expert if needed; remove small patches (< 0.5 m²) professionally.
- Rent reduction by room and extent: typically 10–20% for one affected living room.
- Document the remediation – in the ticket system with photos and the contractor's report, so recurrence is provable.