Terrace Room Construction

I own a Terrace Flat at the 5th Floor of a Building which is a Housing Co-operative Society from the Year 1983. I live in this Flat for the last 40 Years.... My Flat is registered in the Co-operative Society in my name plus I am a Share Holder of the society too.... I have my Flat's Registration Deed registered from the High Court of my City and the same is fully registered in the Co-operative Housing Society....I have a Terrace measuring 260 Sq.Ft. on the South Side of my Flat which is being given to me while I purchased the Flat from the Builder and the same is being given through an ALLOTMENT LETTER both by the Builder and the Chairman of my Housing Society drafted by signature and seal.... Now my question is that I want to construct a room in that terrace by fabricated structure like Steel sheeting and sliding windows....Can I make the room in the terrace by erecting the temporary structure??.... The common terrace is on the other side of the building and there is no common way or any other way to enter my separate terrace of my flat being allocated to me....