This might not what it appears to be. Household size is actually increasing so people actually moving in together. I believe what might be happening is that as people lose their homes/townhouse their moving into rental units or with other family members. However, that house that was foreclosed upon is temporarily not available for occupancy.
National vacancies fell from 7.8% to 7.2% in the third quarter of this year, one of the sharpest quarterly declines in vacancy on record. Occupied stock increased by over 84,000 units, the highest quarterly number for net units absorbed by the overall market on record. That in turn pushed asking and effective rents up 0.5 AND 0.6% respectively. With rents pushing even higher in larger cities, where home ownership has become more difficult thanks to the credit crunch.
And there is corresponding increasing in multifamily units
But now with increased demand and low inventory, builders who can get financing are responding.
Witness a 32 percent jump in multi-family housing starts in August, compared to just a 4 percent increase in single family