As per previous policy of development, the terraces and garages were not counted in F.S.I. The terraces are meant to be the common areas or non accessible areas and garages were meant only for parking.
However in old days developers used to sale those areas to the members. In case of garages the same used to get converted into shops and offices and used for commercial purpose.
Now as per new policy covered garages and terraces attached to the flats are counted in F.S.I. and hence if the members are to be given alternate terraces and garages the same are to be counted in F.S.I. which will be loss to the society. Against all odd once in possession of garages or terraces members are always reluctant to give away the benefit of the same.
There is no rule as such to compensate garages or terraces, however as a market practice to compensate terrace area, 1/3rdĀ of the terrace area is given against terrace and for garages, and alternate parking space in stilt area is given or monetary compensation is offered.

Posted in: Garage and Terrace Area