5Where by reason of the structural alteration of any building relevant lands and heritages become or become part of different lands and heritages, the relevant lands and heritages shall be deemed for the purposes of this Schedule to have ceased to exist on the date (as determined in pursuance of the foregoing provisions of this Schedule) of the completion of the structural alteration and, in particular, to have been omitted on that date from any valuation roll in which they were then included ; but nothing in this paragraph shall be construed as affecting any liability for rates under section 24 of this Act in respect of the lands and heritages for any period before that date.