standard bank

I only have an R500 000..00 bond . The municipality value is R850000.00. An agent from SEEFF Property's valued our house for R1.5milj. Standard bank insists to ensure it for R***.00. We tried everything - write numerous letters with no luck. The answer is always that they decide with their assessors what the value is and we have to pay the premie. Some of the building is not even on the plans and they told me that they do not cover that, but it is still part of the insured amount!. I feel that this is not fair