Have you noticed all the construction cranes in Downtown Phoenix? The Phoenix Business Journal explains part of it by reporting that office space in downtown Phoenix will grow by 42 percent through 2011. An Integra Realty Resources study states that current inventory sits at 4.94 million square feet, with another 2.05 million feet expected to be built between 2008 and 2011. That number puts Phoenix 11th among U.S. cities for downtown construction, which is expected to total 88.5 million square feet nationwide over the four-year period.

New York tops the list with an expected 20.8 million square feet of new construction, followed by Washington at 9.69 million. Rounding out the top 10 are Boston; Charlotte, N.C.; Chicago; Houston; Miami; Nashville, Tenn.; Salt Lake City; and Seattle.

The report also shows downtown Phoenix vacancies at 8.5 percent, compared with a national average of 11.1 percent. Detroit has the highest vacancy rate at 24 percent, while Charlotte has the lowest at 3.5 percent.

On Monday, the Federal Aviation Administration ruled that the three-square block Downtown Phoenix project, CityScape, poses no risk to aircraft that fly in and out of nearby Phoenix Sky Harbor International Airport. Monday’s FAA ruling helps clear the way for one building, with hotel rooms and condos, to rise as high as 510 feet.

“We initially issued a Notice of Presumed Hazard because the structure heights exceeded several FAA obstruction standards,” said FAA spokesman Ian Gregor. After further review, the agency concluded that the projects “punches up” into an airspace buffer, but doesn’t have a negative effect on flights.