Jodi Raffa has been searching for a roommate for over a year. Her husband passed away five years ago, and compounding her loss was a 75 percent reduction in her household income.

The 76-year-old lives in a sunny three-bedroom, two-bathroom home overlooking a lake in a 55-and-over community in Groveland, Fla. "I live on a very strict budget and am not able to indulge in any extras at all,” said Raffa, who worked in administrative jobs before she and her late husband retired in 2010. Raffa now views that move as a “hasty decision” in light of her financial circumstances.

The growing interest in home sharing, especially for those boomers who are house-rich and cash-poor in expensive housing markets, is being cultivated by nonprofit and commercial programs as well as municipalities. Since 2015, New York, Seattle, Denver, Tucson, Northern California and the metro Wa...