1) builder could not have sold adjacent area to ground floor flat owners as it forms part of common area for benefit of all members
2) in addition you dont have any documentary evidence of sale of additional area to you
3) all payments were made in cash
4) safety wall is illegal
5) no OC would be issued if any illegal construction has taken place
6) if any unauthorised construction has been done by building residents you can also complain to muncipal corporation against them