Labour keep insisting a capital gains tax will lead to more affordable housing. In fact I suspect it will lead to an increase in house prices as home owners will not want to sell houses they have, so they do not get clobbered with CGT. If you reduce the supply of houses on the market, prices will go up. There may be some less demand also, but as we see in Australia any hope that a Capital Gains Tax is a solution to housing affordability is a myth.
The major cost is not the house, but the land. We need more land available for urban development. Simple.
Data from this site.