Su Development has broken ground on a 21-story apartment tower in downtown Bellevue, the city's first new high-rise since the real-estate market collapsed.

The building, at Northeast Second Street and 106th Avenue Northeast, is the first phase of a project Bellevue-based Su calls Soma Towers.

While rising rents, declining vacancies and a rising tide of young adults have prompted developers to start building thousands of apartments — including several high-rises — in and around downtown Seattle, downtown Bellevue has been relatively quiet.

No new apartments are scheduled to be completed there this year, according to research firm Dupre + Scott Apartment Advisors, and just 75 units are in the pipeline for 2013.

Downtown Bellevue's last high-rise was 24-story Avalon Towers Bellevue, also an apartment project, said Liz Stead, the city's urban design planning manager. It broke ground in 2008 and was completed in 2010.

"We're all very excited to have it going up," Stead said of Soma Towers.

The 21-story tower will have 142 apartments, about 10,000 square feet for shops and restaurants, a 3,400-square-foot exhibition hall, 5,300 square feet of office space and 198 underground parking stalls, according to city records.

An 18-story apartment building also is planned at Soma Towers. That second phase would have 124 more apartments, another 15,000 square feet of retail and restaurant space, and underground parking for 197 vehicles.

Su officials did not return calls. But Dupre + Scott says the first tower is scheduled for completion in spring 2014, the second tower two years later.

The vacancy rate in Bellevue west of Interstate 405 has fallen from 6.78 percent in fall 2009 to 3.25 percent this spring, according to the research firm, and the average monthly rent has jumped nearly $200, to $1,596.

The 443 luxury apartments at The Bravern, built as condos but converted upon completion in 2010 to rentals after the market turned, now are more than 90 percent leased, Dan Ivanoff, of developer Schintzer West, said recently.

The employment picture in Bellevue also appears to be improving, which could lure more residents downtown. Paul Sweeney, a principal with commercial brokerage Broderick Group, predicted at an industry gathering earlier this month that downtown Bellevue's office vacancy rate would drop from 12.9 to 8.6 percent over the next year.

Companies are moving to downtown Bellevue from other Eastside locations because they like its amenities, particularly its transit options, he said.

All this sounds good to the Bellevue Downtown Association. "We're clearly seeing signs of life in the development community," said Patrick Bannon, its vice president.

Soma Towers' groundbreaking is another affirmation of downtown Bellevue's livability, he said.

Su Development has been a major developer in downtown Bellevue. Its previous apartment projects include 989 Elements and Elements Too, high-rise projects just west of I-405, and Soma, an 8-story complex on Main Street that opened last year.

Eric Pryne: 206-464-2231 or epryne@seattletimes.com