When developer James W. Rouse revealed plans in 1963 to build a "new city" in Howard County, he predicted that it would have more than 100,000 residents by 1980.

Nearly five decades later, Rouse's planned community — named Columbia in 1964 — still hasn't officially crossed that numerical threshold, federal and local planners say. But it appears poised to do so within the next few years.

According to both the U. S. Census Bureau and Howard County's Department of Planning and Zoning, Columbia had between 99,000 and 100,000 residents as of December 2011, and that number has essentially held steady for the past several years because of a lack of housing construction.

An 817-unit development planned for Columbia's town center is expected finally to bring Columbia's population above 100,000, possibly within 18 months to two years.

"That will do it," said Jeff Bronow, chief of the research division of the county's Department of Planning and Zoning. "That will push it over the 100,000 mark."

What's important is not only that Columbia is finally reaching the mark but that the next wave of development will bring more residents after the population had been relatively constant for the past decade, said Jessie Newburn, director of communications and community engagement for the Columbia Association.

"Reaching 100,000 is a pivot point," she said. "It's not just the number. It's the phenomenon of having more residents. It's literally the closing of one phase of development and the launching of a new chapter."

Representatives for the Howard Hughes Corp., the Dallas-based company that now serves as the master developer of Columbia's town center, are scheduled to appear before Howard County's planning board April 12 as part of an effort to gain approval to build up to 817 housing units and retail space on land next to The Mall in Columbia.

The meeting comes two years after the Howard County Council adopted a redevelopment plan that calls for construction of up to 5,500 additional residences in Columbia's town center. The Howard Hughes team, which also includes Kettler and Orchard Development Corp, is the first group to seek permission to build housing in Columbia's town center since the redevelopment plan won approval.

If each new dwelling in Howard Hughes' as-yet-unnamed project contained 1.8 residents, developers and planners say, that would add more than 1,400 residents. That figure, on top of the 99,000 people who live in Columbia today, would bring the population above 100,000.

"We are very excited to begin the new development of downtown Columbia," said Howard Hughes' senior vice president, John E. DeWolf. "This is the initial step of a comprehensive plan to revitalize the town center, and one that has been highly anticipated by the community."

Bronow said Columbia's population can be difficult to determine because its geography does not neatly follow U. S. Census Bureau geography, largely because it has numerous "outparcels" that were never acquired by the Rouse Co. Some of those outparcels nevertheless have residents who live within the general boundaries of Columbia, and both the Census Bureau and the county include residents of the outparcels in their figures.

The county does not include all of the residents of one of Columbia's 10 villages, Dorsey's Search, in its population figures for the Columbia regional planning district, Bronow said. Instead, Howard County splits Dorsey's Search between its Columbia regional planning district and its Ellicott City regional planning district, he said.

Planners say the last time Columbia saw a substantial increase in population was when its newest and final village, River Hill, was completed about 10 years ago.

If 5,500 residences are built in the redeveloped town center over the next several decades, and each one houses an average of 1.8 people, Columbia would gain about 9,900 residents, planners say. That would put Columbia's population close to 110,000.

Columbia already accounts for more than one-third of all the residents in Howard County, which had a population of 292,997 as of December 2011, according to county figures. The county's second-largest population center is the Ellicott City regional planning district, with about 63,500 residents as of December 2011, according to county figures.