The building sum insured is not the value if you were to sell the property but should represent the re-build cost of the buildings which should take into account the full & current replacement cost of the buildings, inclusive of all professional fees.
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This should be reviewed annually to take into account inflation costs. It is also recommended a professional valuation for the rebuild cost is carried out at least every 5 years or possibly more frequent if a heritage listed building to ensure adequate insurance in in place and you are not underinsured.