The 60-acre Tackett's Mill retail, office and residential complex at the entrance of the Lake Ridge planned community in Prince William County has completed several preliminary stages of its decade-long construction plan. A seven-story apartment building for the elderly and the handicapped and a children's day-care center will be ready for occupancy in July and dredging for two lakes at the center of the mixed-use complex has begun. Ground will be broken in July on the reconstruction of an 18th-century grist mill to be incorporated in the overall design.

Upon scheduled completion in the late 1980s, the $50 million complex will include 155,000 square feet of retail and office space and 100 to 200 one- and two-bedroom rental apartments.

Hackett's Mill's developer, the Ridge Development Corp., a subsidiary of Weaver Brothers Inc., plans to reconstruct the working mill on the north shore of the lower lake as the focal point of the complex.

The two lakes will feature a fountain, a bandstand and a New England-style covered footbridge.

The lakes, which will cover four acres, will be connected by a waterfall.

Residential apartments will be along the south shores of both lakes and the apartment building for the elderly is on the upper lake. The colonial village-style retail plaza with specialty shops and two 10-story office buildings will be on the lower lake.

The 100-unit Fellowship Square apartment building was designed exclusively for the handicapped and for adults over 62 years of age. The $300,000 Children's World day-care center can accomodate 150 children in six classrooms and a playground.

The Lake Ridge community, with a current population of 20,000, and capacity for 34,000, opened in 1969 with single-family homes, townhouses and garden condominiums in the $40,000-to-$200,000 price range. The 65,000-square-foot Lake Ridge Shopping Center will be refurbished, expanded and incorporated into the Tackett's Mill area. Two phases of an executive office park containing 52,000 square feet of space also have been completed.