People are leaving the dense NE cities and going to the less dense south and west ones.
To pick the largest city in the US, the population density of Manhattan went from 21 in 1910 to 12 today. And Lower East Side went from 70 to 16.[1]
Also look at construction statistics, new home construction outpaces population growth, people are moving into less dense areas and need new housing.
They build a bit less than 1 million homes every year, and most of those are single family homes.[2] Population growth is about 2.8 million each year, but that's people not families.
[1] http://www.nytimes.com/interactive/2012/03/01/realestate/man...
[2]http://www.economicpopulist.org/content/new-residential-cons...