The insurance hereby extends to cover alterations, additions and improvements (but not appreciation in value) in excess of the sums insured for buildings and machinery specified in the policy for an amount not exceeding 10 % of the sums insured thereby or Rp………………………………. Whichever is the less it being understood that the insured undertakes to advise the insurer each quarter of such alterations, additions and improvement and to pay the appropriate additional premium thereon.