Mould.
Record it, report it, set a deadline.

The cause decides who is responsible. Structural reasons are the landlord's matter; if you ventilate or heat too little, you share responsibility.

• Structural reasons: thermal bridges, poor insulation, leaking windows, roof or pipes
• Water damage from pipes or neighbouring flats
• How you live: too little airing, furniture close to cold exterior walls

The more precise, the easier it is to show later since when and how badly.

• Dated photos, close-up and as an overview
• Room, spot and approximate area
• Since when you have noticed it
• How you air and heat the flat
• Every report and every reply from the management

Do not claim a cause you cannot prove, and do not simply withhold the rent. If you want to withhold rent, you must deposit it with the office designated by the canton after giving written notice – there are strict rules; get advice first.

Can I pay less rent because of mould?

From the report on, you can ask for a reduction as long as the defect exists. How much is appropriate depends on the extent and the rooms affected. Ask for it in writing; do not simply pay less.

How long does the management have?

The law sets no fixed number. Set a reasonable deadline; a shorter one if health is at risk. If that does not help, you can turn to the conciliation authority – it is free of charge.

May I remove the mould myself?

You can clean small surface stains, but the cause remains the landlord's matter. Report it anyway – otherwise you will lack proof later.

Art. 257g CO (duty to report), Art. 259a and 259b CO (remedy, deadline), Art. 259d CO (reduction from knowledge of the defect), Art. 259e CO (damages), Art. 259g to 259i CO (depositing rent).