Adding a capital gains tax to some houses will not make prices affordable unless it comes with a long warning period before implementation, such as announcing it in 2017 to take effect say the end of 2018.  Just springing it  without warning, effective immediately, on potential sellers will cause them to withdraw from offering a property for sale unless they have no choice, much like people hang onto stocks to avoid the tax hit when they sell (if they have a paper profit up to that point).