The legal position is that all open spaces in a building, including the terrace is a common easement for the benefit of all the members who are entitled to enjoy the same. The terrace along with the parking space cannot be used by anyone solely as it is termed as “Common Area / Space”. All the members of the society have an equal right & share on the terrace and on the parking and one single member cannot enjoy that right exclusively. No one can deprive the other member of the open space rights.
When the Municipal Corporation sanctions a building plan the terrace is not taken into consideration for the purpose of calculation of Floor Space Index as it is for the use and benefit of all the members (flat owners) of that society.
Therefore, anyone who indulges in such an unfair and illegal practice of excluding others from exercising their rights does so at his own peril. He/she has got no legal sanction to indulge in such an act.
You can file a writ petition before the concerned High Court from claiming your legal rights. Prior to that, you can give a legal letter to the building society stating your rights and claims.
Hope this helps you.