On your policy documents, you will see the value referred to as ‘the buildings sum insured’ or the ‘buildings declared value”. If you under-insure the buildings, the insurers could reduce your claim in proportion to the under-insurance. For example, if you insure for 50% of the correct value, only 50% of your claim might be paid.
Most policies make provision for re-building cost increases over time, but if the original value is wrong this does not help much. We recommend you get the buildings valued by a surveyor with experience of insurance valuations. The valuation exercise should be repeated every three to five years, as although insurers may index-link the sum insured each year, this is based on national indices and the actual cost changes do vary on a regional basis. So over time, your rebuild value may vary from the buildings true valuation.