NEW YORK CITY-Helping to get the third quarter off to a good start for Manhattan office leasing, digital marketing agency LBi US is nearly doubling its headquarters space with a move to 11 W. 19th St. While the 52,000-square-foot deal goes somewhat against the grain for office relocations in that the tenant is increasing its footprint as it consolidates from three other sites, it illustrates a pair of trends.

One is the popularity of Midtown South, which Cushman & Wakefield says has more than doubled its year-to-date leasing volume over 2009. The other is the “densification” of office space, as firms seek not only to fit as many employees as possible into a given amount of square footage but also to increase efficiency and interactivity.

At its midyear press briefing Wednesday, C&W, whose Jamie Katcher and Frank Coco coincidentally figured in the LBi deal, released figures showing that YTD leasing in Midtown South is 236% over last year at 2.3 million square feet. That compares to a 102% increase in Midtown and a 14.3% gain in Lower Manhattan.

More than that, though, the submarket’s strength thus far in 2010 speaks to its growing cache. “With neighborhoods such as the Flatiron District, Midtown South has become a destination,” Joseph Harbert, COO of the New York region at C&W, commented at Wednesday’s briefing. Two of the largest deals in the second quarter—the 280,762-square-foot relocation for Local 32BJ at 620 Ave. of the Americas and Tiffany’s 260,994-square-foot deal at 200 Fifth Ave., the former International Toy Center—occurred within the Chelsea-Flatiron District, as did the LBi deal.

“This is a testament to the continued demand for quality, well located Flatiron District properties, particularly among growing firms in creative industries,” says Bob Savitt of Savitt Partners in a release. Savitt represented the building’s ownership, 11 West 19th Associates, along with colleagues Michael Dubin and Carol Sacks.

The lease brings the 280,000-square-foot property’s office component to 100% occupancy. It does not, however, represent the largest lease at 11 W. 19th, a Savitt spokesman tells GlobeSt.com. That distinction belongs to Nautica, the spokesman says. Just before the July Fourth holiday weekend, the New York Post reported that the property had signed on a retail tenant, Italian restaurant Zio, for 5,000 square feet.

For LBi, 11 W. 19th “stood out for its location, quality ownership, proximity to transportation and its larger floor plates, which offer efficiency while enhancing the collaborative environment for employees,” says Coco, who represented the tech firm along with Katcher, in a release. That trend toward open plans and interactivity is extending even to such traditionally office-heavy tenant sectors as law firms, C&W experts said at Wednesday’s briefing.

In the case of LBi, the move to 11 W. 19th will bring it to two consecutive floors at the 11-story, 108-year-old office property. Currently, it’s situated in 27,000 square feet at 295 Lafayette St., and has a total of 11,000 additional square feet at 235 Park Ave. South and 45 Main St. in Brooklyn.

The move will give the agency room for future growth, Katcher says in a release. It merged with Bigmouthmedia, the largest search engine marketing specialist in Europe, this past February.