A friend of mine has an undeclared sublet on a one bedroom flat that is big enough to accommodate a small extra bedroom. She is thinking of installing two tall Ikea bookcases across one end of a large room with two entrances. Then she will fill the gaps (between bookcases and wall, ceiling, and each other) with insulation and plastic trim. The bookcases will have one or two screws into the wall for stability but otherwise will not be affixed to the walls, ceiling, or floor. The room will not have a window.
I assume this not exactly permitted. But I also assume that flat subdivision is relatively common here.
Any thoughts or advice would be most welcome.