National Real Estate Development is doubling down on housing at its East Market project in Center City, with plans for a second residential tower at the complex, which aims to turn a quiet-after-dark area into a lively community of businesses, shops and homes.

The Washington, D.C.-based developer said in documents posted Wednesday to the website of the city's Civic Design Review board that the new 21-story tower would add 240 residential units to the 322 rental apartments already under construction at the East Market development site.

The newly added tower would be built atop a two-story retail podium now rising at 1199 Ludlow St., between Market and Ludlow Streets on 12th Street. The Design Review Board, which makes non-binding suggestions on Philadelphia's biggest development projects, is scheduled to consider National Real Estate's proposal for the additional units at a meeting Sept. 6.

The company did not immediately respond to a message seeking more details about the proposal. National Real Estate managing director Daniel Killinger had previously said that some sort of additional development was always contemplated atop the 1199 Ludlow St. retail podium, and that more residential units were shaping up as the best choice for the market.

"I think the highest and best use for that location is some sort of mixed-use residential over retail, which is what they're doing," said Andrew Townsend, vice president for investments at commercial real estate brokerage Marcus & Millichap in Philadelphia.

An office tower at the site would have been more difficult because such projects generally require an anchor tenant to be in place before construction can begin, Townsend said.

"You can build apartments and rent them up in six months or a year," he said. "If you build an office building that's 20 stories, that's not the easiest thing."

The newly planned residential units will add to a wave of housing in Center City's Market East district — bounded by Seventh and 13th Streets, Filbert and  Chestnut — that will more than double the number of homes in the area.

Other projects there include the 322-unit apartment tower under construction  at 1213 Walnut St. by Houston developer Hines and the locally based Goldenberg Group, as well as Brickstone Cos.'s 1112-1128 Chestnut St. development, which includes 112 apartments over Center City's first Target store and other retail.