Mould is not merely an aesthetic problem. It damages the structure of a building, can affect health, and regularly triggers a stressful dispute between tenant and landlord. How that dispute plays out depends heavily on how the tenant responds – and what they know about the law.
Mould as a Defect in the Tenancy
Mould impairs the habitability of a flat and therefore constitutes a defect within the meaning of § 536 BGB – regardless of who caused it. The question of causation determines who is responsible for remediation and whether the tenant is entitled to a rent reduction or damages.
Who Has to Prove What?
Under the so-called sphere-of-risk theory of the Federal Court of Justice (BGH), the burden of proof initially falls on the landlord. The landlord must demonstrate that it can be excluded from a technical standpoint that the mould originates in the building structure (BGH, WuM 2005, 5). Only if the landlord meets that burden must the tenant show that their heating and ventilation behaviour was contractually appropriate. For older buildings, the BGH has held that thermal bridges in exterior walls do not constitute a defect if the building was erected in accordance with the building regulations in force at the time of construction (BGH, VIII ZR 271/17 and VIII ZR 67/18, both 05.12.2018). No rigid ventilation rules apply; what is reasonable depends on the circumstances of the individual case.
Why Expert Evidence Is Almost Always Necessary
Courts routinely order an expert report in contested mould cases. Before bringing a claim, a tenant can also apply for independent evidence-preservation proceedings under §§ 485 ff. ZPO – useful when remediation must take place before any main proceedings conclude. A court-ready expert report currently costs approximately €2,500–3,000.
Notice of Defect under § 536c BGB
Tenants must notify the landlord of any defect as soon as it becomes apparent (§ 536c BGB). Failure to do so risks losing the right to a rent reduction for the period before notice was given – not retrospectively for the entire tenancy. Where the landlord claims damages for a missed notice, the landlord bears the burden of proving that the tenant failed to notify in time (BGH, VIII ZR 74/12, 05.12.2012). Written notice – recorded letter or email – is strongly advisable.
Tenant Rights
Rent reduction (§ 536 BGB): 5–15 % for a small localised area; 20–30 % for several rooms; 50 % or more for severe infestation. Courts decide case by case. Right to demand remediation (§ 535 BGB): The landlord must keep the flat in a condition suitable for contractual use, regardless of fault. Damages (§ 536a BGB): Where mould has damaged belongings or health, damages claims may arise if the landlord knew or should have known of the defect and failed to act.
The Cost Risk Without Legal Expenses Insurance
At a monthly rent of €900, a 15 % reduction amounts to €1,620 per year. A court-ready expert report costs €2,500–3,000; court and legal fees add further. Without legal expenses insurance, proceedings are difficult to justify economically even where the legal position favours the tenant. Insurance shifts the financial risk to the insurer and fundamentally changes the calculation.