The opposite has occurred, David Jarjisian, senior vice president of the local office of CB Richard Ellis, tells GlobeSt.com. "We're approaching the lowest vacancy rate since 2000." He predicts, "the vacancy rate will drop another 2% to below 10% this year and average rent rates will rise about $2 per sf, more in some cases."

Jarjisian attributes much of the progress to a thriving University City, where the office vacancy is 1% and new buildings have a rental rate of $32 per sf. "University City is the new frontier," he says, noting, "600,000 sf of Center City office space was absorbed by companies moving in from University City." That is one, but not the only contributor to the CBD market's success, say others.

"The Philadelphia CBD submarket, defying the expectations of some, has handled the additional two million sf of new office space introduced to the market in the past year," says Tim Pulte, executive managing director of GVA Smith Mack, referring to Cira Centre and Comcast Center. "Additionally, CBD has seen a relative surge in investment activity as nine Center City office buildings have changed hands during the year, including one, 1760 Market St., which traded twice in the span of two months."