For the purposes of this Chapter, "annual rateable value" means--
         (a) in the case of hotels, colleges, schools, hospitals, factories and any other buildings which the Chief Executive Officer decides to assess under this clause, one-twentieth of the sum obtained by adding the estimated present cost of erecting the building to the estimated value of the land appertaining thereto; and
         (b) in the case of building or land not assessed under clause (a), the gross annual rent for which such building exclusive of furniture or machinery therein or such land is actually let or, where the building or land is not let or in the opinion of the Chief Executive Officer is let for a sum less than its fair letting value, might reasonably be expected to let from year to year:
     Provided that, where the annual rateable value of any building is, by reason of exceptional circumstances, in the opinion of the President Cantonment Board, excessive if calculated in the aforesaid manner, the President Cantonment Board may fix the annual rateable value at any less amount which appears to him to be just.