Since the housing crisis, most new construction has been luxury in nature, giving developers the best return on their investment. With a demographic surge of young millennial professionals, the largest demand for luxury apartments in Cook County has come from millennials with a household income of $74,000 to $123,000, according to the Institute for Housing Studies at DePaul University. From 2007 to 2014, the number of 25- to 34-year-old renters surged 40 percent, the institute found.