When can the tenant demand a rent reduction?
The tenant has the right to have the rent reasonably reducted, if the rental apartment is in worse shape than what has been agreed upon. There's no such right, if the worsening of the conditions is caused by the tenant.
In most cases the right to a reduction appears during a renovation. Renovations often cause disturbances to living, which gives the tenant the right to an according reducton of the rent.
The right to reduction starts from the timepoint when the tenant has informed their landlord of the disturbance.
The tenant cannot begin paying the reduced rent without agreeing upon it with their landlord first. It would be good to have the agreement in written form. If the tenant has already paid the full rent for the time for which they later agree on a reduction, the landlord should return the sum equivalent to the reduction.