CHICAGO—Even before Chicago's two new trophy office towers opened earlier this year, rental rates in the CBD's trophy spaces hit historical highs and had grown faster than in any other city in North America, according to CBRE Group, Inc.'s new Global Prime Office Rents survey. These top office properties, generally those constructed since 2000, contain about 14 million square feet, and saw rents rise 19.9% from 2015 to 2016, faster than every major market in the world except Belfast.

“When you dig into the data it's really a simple supply and demand issue,” Kyle Kamin, a Chicago-based executive vice president with CBRE, tells GlobeSt.com. “The overall vacancy rate for class A properties is about 10%, but within this subset it's about 4%.”

And much of the increase is driven by an even smaller subset of offices, he adds. “The vacancy rate for high-rise spaces within the prime office properties is less than 2%.” CBRE “typically defines high-rise spaces as those with great light, air and unencumbered views,” which for some buildings starts on the 30^th floor and for others on the 40^th. Therefore, this big increase is “really driven by a subset of a subset at the very top end of the market.”