You don't necessarily need price controls or rent controls. You could put a special tax on second homes which would make renting and investing economically unfeasible. You could even make it so that speculation was enabled by allowing the sale of derivative housing futures (a young person would buy a house at a reduced price, and then have to pay the investor every time the house's price increased over the next 10 years). That would allow both investors to speculate and young people to buy.
