The market value of a property is based on the price 'which might reasonably be expected to fetch on a sale in the open market' and is based on the market value of the date of sale. For assets owned before April 1982, the date for the market value is the 31st March 1982. HMRC have the right to question your market value. Under such circumstances, you or they can complete a CG34 Post-Transaction Valuation Check. HMRC have specialist valuers to value land, and you’ll be able to discuss your valuations with the HMRC valuer.