Fungible Floor Space Index (FSI) is the additional floor area developed by the builder over and above the FSI limit set by the government
The existing flat owners and in the other redevelopment like slum, cess buildings etc, the existing tenants will be given fungible FSI for construction free of FSI .
Under the new DCR , areas for balcony, flower beds, voids, terraces, niches would be counted in the FSI.
These were not considered in the earlier FSI calculations.
To compensate for this loss in FSII the government has allowed compensatory fungible FSI up to 35% for residential developments.
Thus it is a rule hence you may decide whether it is compulsory or not.