The Chairperson, when sanctioning the erection of a building or execution of a work, shall specify a reasonable period after the commencement of the building or work within which the building or work is to be completed and if the building or work is not completed within the period so specified, it shall not be continued thereafter without fresh sanction obtained in the manner hereinbefore provided, unless the Chairperson on application made therefor has allowed an extension of that period.