What can I do if I only discover mold in the basement three months after moving into my new home?

The 2026 mold check

In the event of mold, immediate notification of defects to the landlord is the most important step. In Switzerland, this defect must be reported in writing (preferably by registered letter) in order to maintain the right to remedy and a potential rent reduction. Anyone who only discovers the mold after three months is not automatically at a legal disadvantage as long as they can prove that the damage has only now become visible or is due to structural defects.

The immediate reporting obligation and the notification of defects

The first and legally decisive step after the discovery of mold is to immediately inform the administration or owner. In Swiss tenancy law, tenant are required to immediately report defects for which they themselves are not responsible. This so-called complaint serves to give the landlord the opportunity to make improvements and to limit the damage. Anyone who discovers the mold and remains silent for weeks risks being held liable for the subsequent damage, as the spores can spread unhindered during this time.

The written report should describe the situation precisely and set a reasonable period of time for inspection and repair. In 2026, it is also absolutely advisable to immediately document the damage photographically and to draw up a list of affected objects. Since the burden of proof when investigating the causes is often complex, this initial documentation forms the basis for all further legal steps. After receiving the complaint, the landlord is obliged to have the cause checked by a specialist instead of simply superficial painting over the defect.

Investigating the causes: Construction faults vs. ventilation behavior

One of the biggest points of contention is the question of why the mold developed. landlord often argue with incorrect ventilation behavior in the basement area, especially in summer when warm air condenses on the cold cellar walls. In 2026, however, many cellars were also thermally insulated by subsequent insulation of the façade, which changed the natural air circulation. If the mold comes deep out of the masonry or is due to a defective seal, there is a clear construction defect for which the tenant cannot be held liable.

If the landlord claims that the tenant is to blame, it is recommended to set up a hygrometer and record the values over a period of time. If it appears that humidity remains consistently high despite correct ventilation, this speaks for structural problems such as rising wetness or thermal bridges. In such cases, it is often necessary to call in an independent expert. Many tenant associations offer support in order to provide scientific proof that the cellar compartment is simply not suitable for use in accordance with the contract.

The right to rent reduction and compensation

As long as the cellar cannot be used or can only be used to a limited extent due to the mold, the tenant in Switzerland is entitled to an appropriate rent reduction. This reduction applies from the time the landlord is informed of the defect until it has been completely remedied. The amount of the reduction depends on the severity of the impairment; in the case of a completely unusable cellar in which no furniture or clothing can be stored, reductions in the single-digit percentage range of the gross rent are common.

In addition to the rent reduction, there may also be a claim for compensation if private property such as suitcases, furniture or sports equipment has been destroyed by the mold. However, care should be taken here: The tenant must be able to prove that the landlord culpably caused the defect or did not react in good time despite reporting. In addition, in 2026, household insurance companies check very carefully whether the items were stored in the cellar with adequate protection (e.g. on pallets instead of directly on the floor) in order to rule out complicity on the part of the tenant.

Going to the conciliation authority in case of disagreement

If the landlord denies responsibility or refuses to restructure, the next step is to go to the competent conciliation authority. In Switzerland, this procedure for rental disputes in the first instance is free of charge and designed to reach an amicable settlement between the parties. The authority can instruct the landlord to carry out the necessary repairs and decide on the legality of a rent reduction.

Another important strategic trick in 2026 is the payment of rent. If the landlord does not act despite the set deadline, the tenant can, after prior threat, deposit the rent with a cantonal authority instead of paying it to the landlord. This increases the pressure massively, as the rent is only released once the shortcoming has been resolved or an agreement has been reached. However, this procedure is subject to strict formal conditions and should never be initiated without legal advice.

Conclusion: Speed and collection of evidence secure your rights

What can you do if you discover mold in the cellar? In short: Complain immediately, secure evidence and insist on root cause research. A mould infection three months after moving in is not a reason for resignation, but a case for professional repair of the defect. In a modern real estate market, a dry cellar is a guaranteed property that you can count on as a tenant.

In summary, the combination of written documentation and the use of your tenancy instruments protects you from financial losses. Anyone seeking dialogue with the landlord, but at the same time clearly formulating their claims for rent reduction, will usually find a solution more quickly. In 2026, the energy quality of a property also includes a healthy indoor climate — and the owner bears primary responsibility for this.

glossary

• Complaints: The tenant's formal notification to the landlord of damage to the rental property.
• rent reduction: The right to a reduction in rent in the event of limited usability of the apartment.
• Arbitration Board: A body with equal representation that mediates in the event of conflicts between tenant and landlord.
• Burden of proof: The obligation of a party to prove the accuracy of an alleged fact (e.g. the cause of the mold).
• Tenant due diligence: The careful review of one's own obligations and rights as well as the building fabric to maintain the value of personal quality of life in the long term.