A new report from a leading commercial real estate brokerage firm shows that Detroit's central business district buildings are just 7.5 percent vacant.

The analysis, the annual Skyline report, from the Royal Oak office of Chicago-based JLL (formerly Jones Lang LaSalle) covers 20 office buildings (see box) in the downtown core totaling more than 10 million square feet. The average asking rent for the properties is $22.55 per square foot, the analysis finds.

The report shows an improving Class A office market; last year's Skyline report found the vacancy rate was 9.4 percent, although rents were slightly higher at $23.23 per square foot.

Some of the improvement in occupancy can be attributed to the lease-up of the 1900 Saint Antoine building, owned by Dan Gilbert, who put Rock Connections in the 99,000-square-foot property that had been largely vacant for years.

The decline in average asking rent is largely attributable to much of the remaining vacancy being in the Renaissance Center and 211 West Fort Street, where asking rents are slightly less than some of the other prominent buildings in the Skyline report, said Harrison West, research analyst for JLL.

"It's just down a tick," he said.

Most of the buildings are full or nearly full, although there are large blocks of space available in the RenCen towers plus the 211 West Fort Street building.

Gilbert and other Detroit developers have been calling for the construction of new office space to meet what they say is growing demand and increasing employment bases in the downtown core.

Gilbert, the founder and chairman of Detroit-based Quicken Loans Inc. and Rock Ventures LLC, plans to include considerable office components in the developments he has proposed just east of the One Campus Martius building that he jointly owns with Meridian Health, as well as the former site of the J.L. Hudson's department store on Woodward Avenue.

The Ilitch family also plans office space as part of its District Detroit project, anchored by Little Caesars Arena and also to include retail, residential, hotel and other entertainment space.

A recent report by CBRE Inc. said there is 2.1 million square feet of office space in development in the greater downtown area, which is 7.2 square miles.