New construction in my flat in my absense

I moved to US in 2014 and since then my flat is locked with all my belonging in packed condition. Flat is in 4th floor and the society is still under construction. The building my flat is didn’t had approval for 5th floor however builder already constructed it earlier so completion formality were not done. To get the approval he had put additional staircase and elevator. The area where he planned the starcase and elevator was passing through lobby of my flat including all 5 flats in that building. He earlier approached me about the proposal in 2011, I didn’t agree at that time as he didn’t provide me any better offer. There was another flat owner who he was talking to and rest of 3 flats he owned. My family visited the flat yesterday and found that he already installed elevator passing through the lobby and moved the entrance door of my flat after the lobby. He had society approval but not mine citing I was unreachable. Please guide me on next course of action. I am in US and was planning to sell it.