'Carpet Area' refers to the total usable area within the four walls of dwelling unit. In other words, it is the area on which a carpet can be laid if required by the owners. The stamp duty is payable based on the carpet area.

'Plinth Area' refers to the entire carpet area along with the thickness of the external walls of the apartment. It obviously includes the thickness of the internal walls and the columns, if any, lying within the four walls of an apartment.

'Built-up Area' refers to the sum of actual area available for living with wall thickness and demarked common area.

'Super Built-up Area' refers to the plinth area of an apartment as added by the balconies and other common areas like corridors, staircase, lift room, motor room, security room, meeting hall, gymnasium and area reserved for indoor games, if any. 
The super built-up area includes the carpet and built-up area of the apartment and its proportionate share in the common spaces of the building, like lobby, staircase and elevator. In some cases, the terrace is also included in this. The total area of all these is divided by the number of apartments in proportion to the size to calculate the Super Built-up Area.

This break up is extremely essential as builders can place anywhere from 60 per cent to 80 per cent of the super built area as carpet area. That means, if the sale price is quoted on 2,000 sq ft, the carpet area could be anywhere from just 1,200 sq ft to 1,600 sq ft. If this break-up is not mentioned in the agreement, demand that the builder mentions it in the sale deed.

Bureau of Indian Standards (BIS) defines various important terms like plinth area, carpet area, rentable area, balcony, stair cover (mumty), loft and porch. It stipulates that the areas of basement, floor without cladding, floors including top floor which may be partly covered, mezzanine floor, and garage shall be measured separately.

Apart from plinth and carpet areas, the Standard also gives the method of measurement of rentable areas for residential and non-residential buildings by adding certain areas to the carpet area. It mentions the areas to be included and excluded while working out the rentable areas. Architects, engineers, contractors, builders, developers, promoters and all others concerned should adopt/implement this Indian Standard for uniform implementation of the standard method of measurement of areas of buildings, especially in the interest of common consumer.
What is sold and how? 
Generally, the area that is sold to a prospective purchaser of a residential flat in a society is the super built-up area. The prospective purchaser not only purchases the plinth area along with the balconies but also purchases a proportionate share in the common areas as mentioned above. While selling a residential apartment, the super built-up area is sold along with the proportionate undivided share, right, title and interest in the land on which the residential apartment building has been constructed.

The undivided share of land pertaining to a residential apartment is generally arrived at by the formula i.e., the super built-up area of the apartment as divided by the total super built-up area in the building, which in turn is multiplied, by the total land area. For e.g., if the land area is say 4,000 sq. ft, the total super built-up area is 8,000 sq. ft and the super built-up area of the apartment concerned is 2, 000 sq. ft, the undivided share of land pertaining to the apartment will be 2000 divided by 8000 and multiplied by 4000 which is equal to 1000 sq. ft.

Precautions while buying

While purchasing a house/flat, keep an eye out for its carpet area. These days, the prices of most newly constructed houses are worked out on the basis of the super built-up area. Even though you may pay for a flat on the basis of the super built-up area, it is the carpet area that you actually use, so ask your builder that how much carpet area you are getting.