According to Kerala Building Rules, the Area of Residences shall be calculated as follows (excerpts from KMBR 1999) -
'Carpet Area' means the usable floor area excluding the area of staircases, lift wells, escalators, ducts, toilets, air condition plant room and electrical room.
[Note:- For calculating carpet area, twenty percent of the floor area shall be deducted from the total floor area in each floor;]
'Plinth' means the portion of a structure between the surface of the surrounding ground and surface of the floor, first above the ground;
'Plinth Area' means area of the building at the plinth level, does not include the area of open porch [(not enclosed by wall)] uncovered staircase and the like;
'Floor Area' means the built up area of a building at any floor level;
More links on Floor Area-
ASKIN- Architect Shahdad Karim INdia