Humidity in basement apartment

You can create as many expert reports as you want, but it only becomes legally binding when it is ordered by the court.

I have experienced the ventilation issue myself

Lived as a single person for 5 years in a 2-room apartment without any problems, then my wife moved in with me.
Double the humidity from washing and drying clothes inside the apartment, more cooking, etc. no longer ventilating regularly because I often worked nights, so hours without ventilation and 1 year later mold.

The same today: anyone who builds a KfW house without controlled residential ventilation is to blame themselves.

I would have done it now regardless of the currently not 100% understandable fault, so that I pay for the renovation, the tenant pays the rent and for his furniture.

We already told him weeks ago, when it started to mold, that he should move the couch away from the wall. What he did there is already grossly negligent.

that is 500 Euros warm rent per month.

Lived as a single person for 5 years in a 2-room apartment with no problems, then my wife moved in with me. Double the humidity from washing and drying clothes in the apartment, more cooking, etc. No longer ventilating regularly because I often worked at night, so no ventilation for hours, and one year later mold.

Just to be fair to you: How am I supposed to imagine that?

Drying clothes and cooking in the apartment without moisture exhaust? (How can clothes even dry like that?)

There are activities in the apartment where, if no other option is available, the window must be open. Cooking without exhaust and drying clothes are such cases. And yes: the window in the laundry room (bathroom, for example) should be left open continuously during drying.

I don’t want to deny that otherwise you can get mold in the place. But you have to be doing something wrong—sorry to say so.

I lived for years with two people in a relatively small apartment, also doing laundry there, etc. No ventilation during the day due to both working. Exception: when clothes were on the drying rack, the bathroom window was tilted open—at every time of year. Never got mold. And the roller shutter boxes were cool spots inside with condensation risk. Still nothing happened.

That you couldn’t exhaust moisture from laundry and/or cooking just because you’re not there during the day is not logical.

Insufficient moisture removal! Incorrect ventilation, e.g., tilt open windows, etc. Our bathroom, for example, had no window. And only windows on one side. We then bought a dryer, stopped ventilating with tilted windows, and kept the heating slightly on even when it was warm in the apartment.

And that is exactly what the tenant practiced here, it's mentioned somewhere earlier, and then furniture against the external facade.

Always blaming building defects is easy.

The fun costs €7,800. For an apartment that brings in 450 euros in rent.

Crazy.

It's still harmless. As a WEG, we spent €12,000 on roof windows. That's just how it is.