Average asking office rents in Greater Boston plunged by more than 12 percent during the 12-month period ending June 30th, according to a recent report from the Boston office of real estate firm Lincoln Property Co.

Average asking office rents declined to $28.11 per square foot at the end of the second quarter, down from $32.02 per square foot at the end of the same quarter in 2008, according to the Second Quarter 2009 report from Lincoln Property Co.

On average, the report states that overall asking rents for office space fell by 12.2 percent during the same period.

The Lincoln Property report tracked office data in Boston proper, Cambridge and suburban markets, totaling 157 million square feet of office space. The report does not take into account asking rents for sublease space, the firm said.

"If asking rents for sublease space were included, rent decline would be much steeper," said Scott D. Faber, director of research at the Boston office of Lincoln Property Co. "Year-over-year net absorption for Greater Boston office space fell to negative 2.9 million square feet, as direct vacancy rose to 13 percent. The combination of these two trends, with an increase in sublease space, directly influences the decline in asking rates and therefore actual rental rates."

Suburban office submarkets were not immune to the declining rents, with a 12.5 percent decline in year-to-date asking rental rates. Route 128 West market saw asking rates drop by 13.5 percent in a year’s time due to the 5.6 percent of sublease space that populates that market as of the second quarter of this year.