Mold in Rental Apartments: Liability under Art. 256/257g CO, Notice Obligations & Rent Reductions

1. The Law: Art. 256 CO vs. Art. 257g CO (The Fatal Trap of Delayed Notification)

Under Art. 256 para. 1 of the Swiss Code of Obligations (CO), landlords are obligated to maintain the leased property in a condition fit for designated use. Mold growth diminishes living quality and poses respiratory hazards according to the Federal Office of Public Health (FOPH) — legally classifying unquestionably as a defect.

The most perilous trap for tenants lies in Art. 257g CO (duty of care and reporting): if a tenant discovers mold spots (for example behind a bedroom wardrobe or along a window reveal) and conceals them for months or merely wipes them down superficially, they commit a material breach of contract.

• Liability for Consequential Damage: If a tenant fails to notify promptly, they become liable for all consequential remediation expenses arising from deep masonry saturation or cross-room contamination (Art. 257g para. 2 CO).
• Documentation via Registered Mail: Always notify defects in writing via registered letter with dated high-resolution photographs. Verbal phone calls are virtually impossible to substantiate before conciliation authorities.

2. Who Pays? Structural Defects vs. Tenant Ventilation Fault

In practice, property managers frequently attempt to blame tenant ventilation first. Before Swiss conciliation authorities and courts, however, the statutory burden of proof under Art. 8 CC applies:

• Landlord Must Rule Out Building Physics Defects: Before asserting tenant fault, landlords must prove that the building envelope is free of structural thermal bridges or masonry water intrusion. In older buildings with uninsulated exterior walls or following airtight window retrofits without ventilation provisions, building physics are almost always a contributing cause.
• Expert Assessment Costs: When landlords commission an independent expert, the landlord bears initial costs. Only if gross tenant negligence (e.g. drying laundry continuously in an unheated room with closed windows) is proven can costs be shifted.
• Cost Sharing in Mixed Causation: Parties frequently agree on conciliation settlements sharing costs (e.g. shared liability where thermal bridge and suboptimal ventilation coexisted).

3. Rent Reduction & Depositing Rent in Escrow (Art. 259a ff. CO)

As long as mold remains unaddressed, the property's utility is diminished. Tenants possess a statutory right to rent reduction commencing from the date the landlord received notice:

• Minor Infestation (e.g. small spots in window silicone): approx. 5% to 10% rent reduction.
• Moderate Infestation (e.g. corner of bedroom or living room affected, noticeable musty odor): approx. 10% to 20% rent reduction.
• Severe Infestation (hazardous spore concentration, room uninhabitable): 25% to 35% reduction for the affected room or total dwelling.
• Rent Escrow under Art. 259g CO: If landlords refuse remediation despite written deadlines, tenants may not unilaterally deduct rent from transfers. They must formally warn of rent escrow in writing and deposit monthly rent into the official escrow account of the cantonal conciliation authority. An action must be filed within 30 days.

4. FOPH Guidelines: When Mold Becomes Hazardous & Proper Remediation

The Federal Office of Public Health (FOPH) explicitly cautions: mold spores can trigger allergies, asthma, chronic bronchitis, and mucous membrane irritation. Infants, toddlers, and asthmatics are particularly vulnerable.

Infestations exceeding 0.5 m² qualify under FOPH guidelines as professional remediation projects for certified contractors. When cleaning small localized areas, use 70% to 80% ethyl alcohol or isopropanol. The FOPH advises against vinegar essence, as vinegar neutralizes alkaline lime plaster and leaves organic residues that feed future mold.