An annual allowance under section 266 of the Act of 1952 in respect of capital expenditure incurred after the 5th November 1962 on the construction of a building or structure, or under section 17 of the Finance Act 1956 in respect of capital expenditure incurred after that date on dredging, shall be equal to one twenty-fifth of the expenditure instead of (as heretofore) one-fiftieth thereof, and accordingly, as respects any such allowance, for the word " one-fiftieth " where it occurs in sections 266(1) and 268(5) of the Act of 1952 and section 17(1) of the Finance Act 1956, there shall be substituted the words " one twenty-fifth ", and for the word " fiftieth " where it occurs in sections 266(2) and 267(1) of the Act of 1952 there shall be substituted the word " twenty-fifth ".