Problem with brick-built shelf

Good evening everyone.

I want to replace the old shower cabin and shower tray because the previous tenants really messed them up.

The problem is that there is a continuous built-in shelf, and from 1.30m there is a 16 cm wide "gap" if I want to install a standard 80 x 80 shower cabin.

Does anyone have an idea how to solve this problem without an expensive custom-made solution?

Do you have a picture? I can't quite imagine what you mean.

You could probably install the normal shower cabin and place a "piece of plastic" at the wall recess. Connect the cabin and the "piece of plastic" with a strip and silicone to make it waterproof and done.

I would first inform the landlord, because for existing installations, I as a tenant am initially not responsible or they do not belong to me. Therefore, I would at least want to obtain approval. If the cabin even has defects (that go beyond appearance), I would also try to have the cabin renewed by the landlord.