If you rent the property out then you are liable to pay the stamp duty charge that an investor buying the same property at the same time in 2003 would have paid. You need to find out what the investor rates of stamp duty were for the applicable period and then calculate what's due based on the original price of the property. As far as I know the stamp duty rates for investors have not changed since 2003 (you should verify this though!) so you would be liable for the relevant rate specified in the Residential Property - Rates of Stamp Duty table, Full Rate column in [broken link removed].