Far better to charge CGT on the gain when someone sells with the difference in buy price and sell price minus mortgage payments and the costs of house improvements deducted from the 'profit' figure.
So if someone buys a property for £250k, does nothing to it, and property prices double (say over 20 years) and they decide to move somewhere else, to a similar property - they sell for £500k, they buy for £500k, they pay costs of moving, legal fees, estate agent fees, they also pay £12.5k stamp duty. So it's cost them a fair bit to move to an equivalent house.
But you now want them to pay some Capital Gains Tax too, when they haven't actually gained anything?