1) if you want to carry on additional construction on terrace you would need consent of other owners . please note that terrace forms part of common area and every flat owner has undivided interest in it irrespective of agreement to contrary
2)Delhi Apartment Ownership Act, 1986 introduces
into the title/rights to immovable property, the concept of common areasdefined in Section 2(j) thereof inter alia as roofs, yards, gardens, parking spaces and by Section 4 thereof confers on every apartment owner anundivided interest in the said common areas; Section 24 makes the
provisions of the said Act applicable to all apartments, notwithstanding anything contained in any other law or in any contract.