I used to think that all work done on a flat in England required the approval of the local authority Building Control team. I know now this is not quite true.
As I said in the News section, I am allowed to insulate my ceiling without approval because there is another flat above me, and it counts as an internal structure. However, I would need approval to insulate my floor which lies over the outdoor car-parking area because this is a “thermal structure”.
I am going to research this in more detail in the hope of finding a general guidance note.