You must always insure your home cover is based upon the rebuild value of the property and not the purchase value.
The purchase value of your property isn't necessarily the same as the cost of rebuilding your property. If you had a survey or valuation completed at the time you bought the property, the valuer should have calculated the rebuilding cost.
If the rebuild cost exceeds the sum insured, the buildings policy would need to be cancelled.
If you're unsure of rebuilding costs, you can get some information from the ABI website http://abi.bcis.co.uk/ which may be of help, particularly with regard to measuring your home. Alternatively, you should seek professional advice.