Their premise is that the free market drove up the price of houses. Deregulation that is to say end of social housing programs led to this.

Yes and no - lack of supply led to this. It doesn't really matter where the supply comes from. More social housing would certainly be good though.

Is that not neo-liberal? High construction costs come mainly from high cost of labor. The market driving up costs isn’t free market?

Do... do you think that business owners like having higher labour costs? I don't think I've ever seen someone on the left complain about higher worker wages.

What protectionist policies are there?

Try building a residential property and see how many hoops you have to jump through.

When have property values been under threat of dropping.

If supply were to increase, prices will drop. This is good for everyone except those who think their house is a retirement fund.

They’ve been ever increasing due to market demand.

You forgot half of the "supply and demand" equation.

You say “lack of good investment”. Others say investors determined housing was a prime investment.

Exactly... Whereas housing shouldn't be an investment. If its value outpaces inflation, it becomes less and less affordable to new buyers.