More good news on the housing market. This morning, the S&P/Case-Shiller Home Price index posted its biggest annual increase since 2006—just before the housing market crash.

Home prices in all 20 metro areas included in the index rose for the second month running. Phoenix led, with a 23% annual increase followed by San Francisco (18.9%), Las Vegas (17.6%) and Atlanta (16.5%).

Still, Robert Shiller, co-creator of the index, is cautious. “There’s a lot of excitement in the housing market now but it might be just short term,” he tells The Daily Ticker.

Related: The Recovery in Housing is Behind Us: David Rosenberg

Shiller says the housing market is operating in an “abnormal economy” where the Federal Reserve is buying $40 billion worth of mortgage securities and $45 billion worth of Treasury notes each month. This has driven mortgage rates to record lows.

According to Freddie Mac’s weekly survey out last Thursday, the average rate for a 30-year fixed rate mortgage is a record low 3;4%; for a 15-year fixed rate mortgage it’s a record low 2.61%.

The Fed will eventually stop buying these securities, says Shiller, and mortgage rates will rise.

Related: Here Comes a Millennial Housing Boom: PulteGroup

Shiller, also an economics professor at Yale University, says the biggest home price increases now are seen in multifamily rather than single family homes which reflects a shift from home ownership to renting. The buyers are investors who rent their properties.

“Most of the increase in households in this country has been met by an increase in renting,” says Shiller. “My own survey data with Chip Case confirms that people feel more positive about renting.” He suggests that those investing in real estate buy homes that are most suitable to convert to rentals.

Another shift that Shiller observes in the housing market is the growing popularity of urban areas and suburbs near those areas.

Shiller says people don’t want to commute long distances because of relatively high gasoline prices and in the current “ideas economy,” many want to live in close proximity to others.

There are also demographic shifts: aging baby boomers who no longer enjoy working on the upkeep of their homes and the increase in single-person households.

When asked where this all leaves the housing market 10 years from now, Shiller says home prices will be “about where they are now” after adjusting for inflation.

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