21. Where any person who carries on an undertaking for the construction and sale of houses, being an undertaking approved by the Commissioner for National Housing having regard to the housing policy of the Government, sells any house or flat, the construction of which was commenced by such person on or after January 1, 1977. such sale being the first sale of that house or flat-
(a) the entirety of the profits and, income arising from such sale, if the floor area of such house or flat does not exceed five hundred square feet; |
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(b) seventy-five per centum of the profits and income arising from such sale, if the floor area of such house or flat exceeds five hundred square feet but does not exceed one thousand two hundred and fifty square feet; and |
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(c) fifty per centum of the profits and income arising from such sale, if the floor area of such house or fiat exceeds one thousand two hundred and fifty square feet but does not exceed two thousand square feet, |
| shall be exempt from income tax. |