The office vacancy rate in Detroit's most prestigious addresses has notched slightly upward as increasing rental rates are driving some tenants out of the downtown core, according to a new report.

It is now 8.8 percent across 21 buildings, according to the Royal Oak office of JLL's annual Skyline report, which tracks leasing and rent trends in downtown's best-known and priciest office properties. It was 7.5 percent a year ago, still a vast improvement over the 20 percent and higher vacancies from 2008-2011.

Rental rates in those buildings have jumped nearly $1 per square foot, from $22.55 per square foot to $23.47 per square foot, a 4-percent bump, the report says.

Those are likely to continue to climb as billionaire mortgage mogul and real estate developer Dan Gilbert has a pair of projects that will alter Detroit's skyline in the coming years: the 800-foot residential/hotel tower under construction on the site of the former J.L. Hudson's department store on Woodward Avenue, and the Monroe Blocks project that is expected to break ground this year and add a 35-story, 518-foot office tower to downtown. The rents at those office properties are expected to be at least in the high $30s per square foot.

The Hudson's project is slated to cost $909 million, while the Monroe Blocks project east of Gilbert's One Campus Martius building is expected to cost $830 million.

In addition, new office development in the Ilitch family's The District Detroit project is expected to have a role in driving up office rental rates in the future.

Nonprofit tenants like United Way of Southeastern Michigan and the Local Initiatives Support Corp. and others have flocked north to areas like New Center where rents are more affordable.

"Other companies (SEMCOG) have publicly stated their uncertainty as lease expiration approaches," Harrison West, senior research analyst for JLL in Royal Oak, said in an email. "As rents continue to climb in the coming years, we expect more of this activity."

Here are the buildings included in the report:

• 211 West Fort: 450,000 square feet, 79.4 percent leased

• 150 West Jefferson: 500,000 square feet, 94.1 percent leased

• One Woodward: 360,000 square feet, 91.4 percent leased

• Ally Detroit Center: 980,000 square feet, 96.3 percent leased

• Guardian Building: 643,000 square feet, 95.9 percent leased

• Chase Tower: 505,000 square feet, 100 percent leased

• First National Building: 850,000 square feet, 92.8 percent leased

• Chrysler House: 416,000 square feet, 97.4 percent leased

• One Kennedy Square: 247,000 square feet, 100 percent leased

• 1001 Woodward: 305,000 square feet, 97.9 percent leased

• One Campus Martius: 1.08 million square feet, 100 percent leased

• Renaissance Center, Tower 100: 588,000 square feet, 84.7 percent leased

• Renaissance Center, Tower 200: 593,000 square feet, 53.8 percent leased

• Renaissance Center, Tower 300: 585,000 square feet, 100 percent leased

• Renaissance Center, Tower 400: 576,000 square feet, 94.2 percent leased

• Renaissance Center, Tower 500: 307,000 square feet, 100 percent leased

• Renaissance Center, Tower 600: 307,000 square feet, 63.3 percent leased

• Ford Field (office space): 331,000 square feet, 100 percent leased

• 1900 Saint Antoine: 99,000 square feet, 100 percent leased

• Brewery Park: 371,000 square feet, 82.1 percent leased

• Fisher Building: 635,000 square feet, 87 percent leased