The California housing market has seen prices begin to rise and interest rates drop to unprecedented lows in 2012, but a low inventory of homes is constraining growth.

That fact had nearly 300 local real estate agents shaking their heads in frustration during an otherwise positive report on the state of the housing market presented by Leslie Appleton-Young, vice president and chief economist of the California Association of Realtors, to its members Tuesday at Embassy Suites in San Luis Obispo.

“We’ve had a really good 2012 ... the bottom is behind us,” Appleton-Young said, because home sales across California are up 6.5 percent year-over-year, and prices are up 15.5 percent in August year-over-year.

However, across California, there is only a 2.7-month supply of home inventory. In San Luis Obispo County, there is a 4.5-month inventory, according to the association. In comparison, there was a 10- to 15-month supply of homes in SLO County in the early ’90s, Appleton-Young told The Tribune.

Three factors are at play, she said: First, pent-up demand for homes is strong, as young professionals and families who were affected by the recession have put off the big purchase of their first new home.

“Affordability is at record highs, but a lot of people are unable to take advantage of it,” she said.

Second, investors are beating out first-time buyers to the inventory, and choosing to rent instead of re-selling the homes. In California, the demand for rental units is strong, so 83 percent of investors are buying to rent, Appleton-Young said.

Finally, nearly 30 percent of homeowners in California are underwater on their homes, meaning that they owe more on their home than it is worth. “People are stuck, not knowing what to do, and waiting for prices to improve,” she said.

And the number of lender-owned properties — for which demand is the highest — is dropping.

The supply problem led her to conservative predictions for continued growth in 2013: sales growth of 1.3 percent and a rise in median home prices of 5.7 percent, while interest rates should stay the same.