fitting roof window in maisonette

longwalks1

I live in a maisonette, 2 floors and theres a flat beneath me. I have my own front door and a huge loft, that when I bought the property 4 years ago had been boarded out, insulated, plastered, lighting and heating added and carpeted. It wasn’t sold as an extra bedroom as there was no staircase to it (still had to use the loft ladder), and no fire escape or velux window. I’ve been using it as a loft and storage area and occasionally use my weights up there, if I wanted to fit a roof window would I likely be refused permission as its leasehold? Also, a little worried about asking about fitting one and then opening a can of worms regarding the ‘conversion’ (term used lightly) that has already been done

Any advice greatly recieved