Average asking rental rate in Bala Cynwyd/Narbeth is $30.28 per sf, among the highest in the greater Philadelphia area. It is exceeded only by the Main Line and Conshohocken submarkets, both of which have double-digit vacancy rates that rose during the first half of this year.

Overall vacancy rate in the Main Line submarket is 15.4%, according to the research. Conshohocken experienced its largest increase in vacancy over the past year. The rate now stands at 25.3%, including sublease space. "Historically, Conshohocken benefited from the lowest vacancy rates of all submarkets," Shelly says. "With additional buildings coming on line, there will continue to be downward pressure on rental rates."

Average asking rental rate in the Conshohocken submarket is $30.77 per sf, compared to an average of $32.50 per sf in the Main Line submarket. Though the Main Line average has stayed firm, Shelley predicts it will soften as Conshohocken competes for space.