The financial advantage Southern California home buyers hold over renters has rapidly shrunk in recent months as home prices and mortgage rates have risen, according to a new report.

It is still cheaper to buy a home rather than rent a similar property in Southern California, though, according to a Trulia analysis released Thursday. If a buyer purchased a house in Los Angeles County this summer, lived there for seven years then sold it, he or she would have saved 21% over renting a similar property, the real estate website said.

That's down from a 35% savings in the winter. In upscale Orange County, a buyer would save 20% if he or she purchased this summer; in San Diego County 21%; in Ventura County 22%.

This year's rapidly rising prices - and more recently an increase in mortgage rates - have frustrated many would-be home buyers, causing some to bow out of the marketplace and remain renters.

Although the Southland's affordability has faded fast, it's nothing compared with the San Francisco Bay Area, where heavy demand from wealthy technology workers has added even more fuel to the red-hot housing recovery.

It's only 4% cheaper to buy than rent in San Jose and 9% less expensive in San Francisco, nearing a tipping point when potential home buyers may be prudent to call it quits.

Trulia assumes several things in its calculation: the buyer receives a 30-year mortgage at a fixed rate of 4.8%, puts 20% down, is in the 25% tax bracket and itemizes his or her federal tax deductions.

In some areas of Los Angeles County, a potential buyer would save less than the region as a whole, according to the Trulia data.

Buyers in Pasadena, the San Gabriel Valley and the Westside would have saved only 11% over renting a similar property.

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