Is basement a common area? Can we claim mesne profit, if other people of building use it commerciall

We have 6 flats in a builder floor along with basement. People bought these flats when the building was constructed in 2000s. Since then some people have sold their flats to others. Over time a single person has bought both units of ground floor and has claimed the basement area as well for his personal use. Stating that just like roof rights in any building, he has acquire basement rights from the previous owners. He has now converted the basement into an office and given it on rent. I want to know, if the basement could be sold like this to one person? Or do ground floor owners have exclusive right on basement because of their ownership of ground floor? Can it be put on rent for commercial purpose if the property is on notified commercial street? Are these things legal? These offices are encroaching on all the common areas. We need some recourse against them. Please help.