Shares of Irvine-based Standard Pacific Corp. slumped on Tuesday, the same day that the homebuilder announced a proposed $150 million convertible notes offering and concurrent common stock offering of 12.5 million shares.

Orange County’s largest publicly-traded homebuilder said the proceeds from the proposed offerings would be used for land acquisition and development, home construction, and other related general corporate purposes.

Standard Pacific’s stock, which has nearly doubled in price in 2012, was down nearly 9% in mid-day trading on Tuesday.

The company’s nearly 200 million shares of outstanding common stock are currently valued at about $1.14 billion; factoring in preferred stock that can be converted in common stock and the company has a valuation close to $2 billion.

The new fund-raising plans for Standard Pacific came as the closely-watched S&P/Case-Shiller index on Tuesday reported that its composite 20-city home price index, a key gauge of U.S. home prices, was up 2.2% in May from the previous month and fell just 0.7% from a year earlier.

The new report suggests that home prices may be close to posting year-over-year gains.