Thomas Properties Group Inc. said it has signed an agreement with affiliates of Blackstone Real Estate Advisors to acquire 3.5 million square feet of office properties in Austin for $1.15 billion.

The Los Angeles-based real estate company said about 75 percent to 80 percent of the purchase price will financed through property-level mortgage debt, with the remaining funds coming from equity investments by Thomas Properties as well as some of its investment partners, the company said. The deal constitutes around half of the Class A space in the city’s central business district.

“We believe this is a truly extraordinary opportunity for our company and our investment partners,” said Chief Executive James Thomas in a statement. “Austin is expected to exhibit the strongest job growth in the nation over the next five years and this portfolio is well positioned to benefit from that growth.

Approximately 82 percent of the space is currently leased, the company said, noting that Class A rents in that market increased 18 percent between 2005 and 2006. Austin’s vacancy rate is currently at 12.7 percent.