The redevelopment of the 10-acre old Convention Center site into what is being described as the “heart and soul” of a vibrant new downtown D.C. will kick off in 2009, District leaders announced Monday.

Long-term lease agreements and zoning approvals for the $850 million project are finished, Mayor Adrian Fenty said, allowing the project to move into the design and permitting phase, and construction to be completed by 2012. The development is slated to include 670 apartments and condominiums — 20 percent “affordable” — 250,000 square feet of retail, 456,000 square feet of office space and 1,700 parking spaces.