House purchase, illegal attic expansion? Dismantling?

The difference is the permanent stay. I do not stay permanently in the storage room.

I hope the ceiling can hold up properly, a bookshelf is not exactly light.

Yes, exactly: the usage is the only way to turn the legal storage space into a presumably illegal living space. But who is supposed to prove/document the usage and how? Therefore, the initially proposed dismantling is not necessary.

A colleague bought a condominium with an attic room. However, this was not approved and cannot be authorized and is no longer accessible. There was a cease and desist order regarding this. There was no second escape route.

I would also suspect that the expansion is not approved and cannot be authorized. Simple reason: the windows look too small to serve as a second escape route.

That is not correct, I am building a single-family house and it is a requirement!

This applies to living rooms. If they are official, the floor needs a second escape route.
When renting, such an apartment is offered without specifying the room within the stated square meters but with an additional room, converted attic space, or basement room.

The difference is permanent occupancy. I do not stay permanently in the storage room.

Exactly.

And of course, a TV room is a living room, but since it is not rented out, it does not matter in what condition this room is, nor who uses it and how.
For household insurance, I would include it, otherwise educate the children not to play with fire up there, etc. One should also not do so oneself... yes, of course that goes without saying, but there are enough situations where the worst case happens.

No, you do not have to carry out a dismantling.

This applies to common rooms. If they are official, the floor needs a second escape route.

No, from the beginning the attic was designated as "storage space" and nevertheless our building authority gave the requirement for the escape window, i.e. that I have to install a window with the dimensions 0.9m x 1.20m and at least 1.20m above the finished floor level! Or is it then not an escape window?