If you have recently purchased the property the Home Buyers Report should include a surveyor's recommendation on the sum required for insurance purposes. This can appear under the heading of insurance, rebuild or reinstatement values.
If you have owned the property for a number of years, general guidance on the rebuild cost can be found at http://abi.bcis.co.uk/. This guidance has been prepared by the Royal Institution of Chartered Surveyors on behalf of the Association of British Insurers (the ABI). By entering basic details about your property using the Rebuilding Cost Calculater you will be given an estimate of the current rebuild cost for your home. Please be aware that this is designed to give you a good indication of the rebuilding cost of your property. If you are in any doubt then you should approach a surveyor to provide you with a rebuild valuation specific to your property.
If the building is a listed property, or of unusual or non standard design or construction we would recommend that you always instruct a surveyor to undertake a survey and provide you with the current rebuild figure.