FSI stands for Floor Space Index and is calculated as the ratio of the total built up area to the total area of the plot. It is calculated by local municipal corporations to provide ideal living conditions for residents of an area while keeping in mind the area’s population density and availability of open spaces.  Since a terrace is not a built up area, it is not included in FSI if it is less than 1.5m wide. It is considered an open area. Terraces bigger than 1.5m in width however, are included in FSI. Any staircases on the terrace are also included in FSI since they are built up.