The redevelopment of one of the most valuable urban parcels in the country, D.C.’s Old Convention Center site, has slowed as the designs for a half dozen buildings to be clustered on the site idle in the District’s development pipeline.

City leaders still anticipate that the site, currently a giant parking lot in the East End of downtown, will become a vibrant mixed-use neighborhood featuring residential, office and retail space. But Mayor Adrian Fenty’s hope of breaking ground within four months of his inauguration, a goal found in his 100-day plan, was utterly unrealistic, said officials familiar with the undertaking.