River City Co. on Wednesday launched Chattanooga's Urban Design Challenge, kicking off the yearlong contest to develop sites that have fallen behind during the city's downtown revitalization.

Teams in the challenge, which includes a full redesign of six major downtown sites, include architects, developers and builders to create multi-use environments that connect the city and provide "context" for surrounding areas, said Ann Coulter, a principal at Kennedy, Coulter, Rushing & Watson.

"We'll see a lot of attention paid to a mix of commerce, and what enlivens a site during a 24-hour cycle," she said. "We're going to look at what the city looks like in the 21st century."

Team Dynamic Density, led by architect David Barlew, will kick off the challenge Sept. 1 with a new design for Market Street's 700 block, which is vacant and decrepit.

Every two months others will follow with full designs for the Patten Towers area, Patten Parkway, the Fourth Street corridor near U.S. 27, the Vine Street corridor between UTC and downtown, and a section of Main Street between Broad and Chestnut streets.

At a kickoff gathering Wednesday night, urbanist and planner Alex Krieger charged team members to invest in culture, stay green, create affordable living and "fill in the gaps" left by past urban designers.

"What we've been doing as a culture is abandoning the places we've built" in order to pursue "life by the acre," said Krieger, who has worked to revitalize downtown Boston and teaches graduate design courses at Harvard University.

But suburban living is no longer the singular ideal, thanks to high transportation costs and the high cost of providing government services. The nuclear family, for which suburban living was created, is now only 18 percent of all households, he said, leaving 82 percent to pursue housing elsewhere.

Suburbanites all over the country have found their way back to former mills and warehouses, re-created as convenient, affordable living.

"If you've grown up in the suburbs, you know that those aren't particularly sexy," Krieger said. "Cities are sexy."