2 flats purchased on ground floor along with external space, society claiming the place now

My father had taken two adjoining flats on ground floor of a society back in 1998. The builder then gave us a offer that if we take two adjoining flats together we would get a space outside (of around 125 sq ft) right outside our flats. He even opened a door from that place. He marked the space in the our agreement plan with different color but didn't clearly mention the same in agreement. Now after so many years the society is threatning us to break the compound wall that had built then and that it's illegal since it is not mentioned in our agreement. This had happened once 10 years back too. Then too society specifically the chairman and secretary of society then had threatened us. We had sent legal notice to them but didn't receive it n it came back to us. Now again the society comittee members are planning to take action and we even presented them the letter given to us by builder on his letter head along with his stamp and sign mentioning that the place belongs to us. I intend to finalize this matter. Can anyone advice me what can be done? Anything that legally solve the problem once and for all without intervention of society, since society won't do anything for us as they want to turn it into parking place?