As a percentage what are talking about? There are homes and apartments aplenty, the biggest issue is the economics of it. What would cause rents to go down? An oversupply? What would cause an oversupply? Barring some regional crash it would have to be overbuilding. People aren't dumb and they won't overbuild. They will build as demand increases but they certainly won't lower prices. If anything they will raise them. It's obvious.