The number of apartments being renovated and sold as condominiums is escalating around the nation at a time when single-family homes are beyond the economic grasp of many Americans.

Although no exact figures are available on how many renters are affected, the number of apartments sold to condominium redevelopers nationwide rose almost tenfold from 7,800 in 2002 to 70,800 in 2004, according to Real Capital Analytics, a Manhattan-based research consulting firm. As of June 1, 43,900 units have been sold to redevelopers this year, says Dan Fasulo, director of market analysis for the firm. There are an estimated 19 million apartments in the U.S.

The conversions are occurring most rapidly in Southern California, Northern Virginia and the Miami and Las Vegas areas. The converted condominiums, often much cheaper than houses in the same market, are the only way for many to acquire their first home.

In San Diego County, for example, the median price for a converted condo in May was $250,000 less than that of a single-family home, says DataQuick Information Systems, a company in La Jolla, Calif., that tracks property information.

"We take people normally collecting rental receipts in shoeboxes and make them homeowners," says Mark Lewis, mayor of the San Diego suburb of El Cajon, where 1,000 rental units are currently being converted.

Local officials and housing advocates are grappling with how to help tenants pushed out by the conversions. Many people are unable to buy and need rental housing in markets that have few vacancies.

"Condominium conversions provide entry-level ownership opportunities," says Elizabeth Morris, CEO of the San Diego Housing Commission. "But there are concerns about what it does to the available supply of rental housing."

While a record 69 percent of households owned homes last year, lower-cost rentals are dwindling, according to a study released last month by Harvard University's Joint Center for Housing Studies.

About 43 percent of rental units built in the past five years are renting for more than $800 a month, compared with 25 percent built before then.

Some real estate analysts say conversions are not significantly depleting the supply of apartments.