15Where, after the time at which the rateable value of a dwelling-house is material for the purposes of any provision of Part I of this Schedule, the valuation list is altered so as to vary the rateable value of the hereditament of which the dwelling-house consists (in whole or in part) or forms part and the alteration has effect from that time or from an earlier time, the rateable value of the dwelling-house at the material time shall be ascertained as if the value shown in the valuation list at the material time had been the value shown in the list as altered.