A Board, when sanctioning the erection or re-erection of a building as hereinbefore provided, shall specify a reasonable period after the work has commenced within which the erection or re-erection is to be completed, and, if the erection or re-erection is not completed within the period so fixed, it shall not be continued thereafter without fresh sanction obtained in the manner hereinbefore provided, unless the Board on application made therefor has allowed an extension of that period:
     Provided that not more than two such extensions, each for not more than one year, shall be allowed in any case