Section 180(2)(iv) in Kolkata Municipal Corporation Act, 1980
(iv)[when a new building is erected or an existing building is] [Substituted by section 11(2)(a) of the Calcutta Municipal Corporation (Second Amendment) Act, 1984 (West Bengal Act 13 of 1984), w.e.f. 15.5.1984, for the words "when it is" .] redeveloped or substantially altered or improved during the period the annual valuation remains in force; or