The cost of the houses being built is affordable. It is the price of the land they are being built on that is so expensive - and as the demand for housing grows so will the price of that land so even though the cost of building the houses remains in line with costs, selling prices will inevitably rise. Public housing is as much a victim of land prices as commercial house builing.
I have no idea what the solution is. No one, not as an individual, a corporate body or a political party has yet put forward any satisfactory solution to this problem.
Meanwhile, if you want to build a small 4 bedroomed detached house on a very narrow plot cheek by jowl with the adjoining small dull terrace and facing the main road, come to our village, you can buy just such a plot. A snip at £200,000, plus the cost of building said house.