To speed recovery of the housing market, Fannie Mae in March will begin purchasing and guaranteeing mortgages for borrowers carrying loans on as many as nine other properties, up from the current limit of three.

However, the number of months of reserve payments that must be held by investors will rise to six in June from two currently.

"One of the things that leads the economy out of a housing crisis is when prices get cheap enough that investors start moving in and buying things," says Joe Garrett of the Berkeley, Calif.-based consulting firm Garret, Watts & Co.