It is necessary to build more housing stock, but if you simply do that alone while there is still significant incentive to buy investment properties then the developers will obviously sell to those that pay and it's typically those with means that will pay, which tends to be people who can afford multiple properties more than those who are struggling to afford one place to live in. Obviously if you're a developer looking for a return on your investment you'll price according to what those people will pay so that housing stock is quickly swallowed up mostly by landlords who will want to recoup their investment by charging higher rents and so on.