(Bloomberg News) Single-family home prices are increasing fast enough in the U.S. to justify taking on debt for purchases, according to Liz Ann Sonders, Charles Schwab Corp.'s chief investment strategist.

A "real" mortgage rate was calculated by subtracting the year-to-year percentage change in the median price of single-family homes, as compiled by the National Association of Realtors, from Freddie Mac's average rate on 30-year fixed mortgages. Since April, the real rate has been negative. The reading in June was minus 4.3 percent, the lowest since 2005, when the U.S. was in the middle of a housing boom. June's median price was $190,100, a gain of 8 percent from a year earlier.

"It makes sense again to borrow to buy a house," Sonders wrote in her commentary on Schwab's Web site. That wasn't the case three years ago, when the housing market's collapse sent the price-adjusted rate as high as 22 percent, the New York- based strategist added.

Freddie Mac's 30-year mortgage rate dropped last week to 3.49 percent, a record. That was 17 basis points lower than the average at the end of June, which was used to determine the real rate for the month. Each basis point is 0.01 percentage point.

Another plus for housing is that new households are starting faster than new homes are being finished, Sonders wrote. About 750,000 households are forming at an annual rate, according to a February estimate from Mark Zandi, chief economist at Moody's Analytics Inc. in West Chester, Pennsylvania. Builders completed 447,000 single-family houses last year, according to the Commerce Department.