The owners of the Landmark Shopping Center in Alexandria will spend $84 million to enclose the 20-year-old complex and add an underground level to triple the number of stores there.

The shopping center is one of the few in the Washington area not already covered.

Developer Dusco Inc. will be in charge of the project at the center, which is owned by International Income Property Inc., Sears, Roebuck & Co., The Hecht Co. and Woodward & Lothrop.

There are now 43 stores at Landmark Center, situated at Duke Street and Shirley Highway. When proposed renovations are completed, there will be more than 150, developers said.

"We will turn Landmark into a new center. It will have no resemblance to the existing center in the way it now looks," said James P. Egnew, senior vice president of Dusco, which is the development and property management arm of International Income Property.

Project attorney Henry Thomas said the necessary permit applications will be filed with Alexandria officials in late September.

The site already is zoned to accommodate the proposed building changes, but Dusco will ask Alexandria's Board of Zoning Appeals for a variance in setback requirements along Shirley Highway to allow for parking garages.

The site plan will have to be approved by the Alexandria Planning Commission, Thomas said.

The new space will be added below ground. The three major anchor department stores -- Sears, Hecht's and Woodward & Lothrop -- all have basement levels with retail spaces. The renovation plan calls for excavating the remainder of the shopping center down to 17 feet below ground level to create another level of shopping facilities. The lower level will be open to the present level of shopping, which will be topped by a food service picnic-style garden area, creating a three-level mall, developers said.

Speculation about what changes might come to Landmark has intensified since IIP purchased the site in 1984. Sears, Hecht's and Woodward & Lothrop own the buildings and the land they occupy.

Egnew said Dusco and IIP specialize in "finding shopping centers that are in terrific locations that need to be expanded and upgraded."

The renovations will be done in two phases so that existing tenants can move into new facilities while their old buildings are demolished and rebuilt after excavation work is done, developers said.

Developers predicted the expansion will produce an additional $110 million in annual revenue and said retail sales taxes will increase by $4 million annually. The renovation is being designed by the architectural firm of Cope, Linder Associates.

The Hecht Co. already has completed major renovations of its Landmark building, and Woodward & Lothrop and Sears are considering renovations, developers said. Developers estimated that the expansion will create approximately 900 permanent jobs.

Traffic improvements are planned as part of the project, including work on the existing flyover bridge that now provides access into the shopping area. Structured parking is planned, and access roads will be improved to "provide greater flexibility and increased accessibility for those who use the shopping center," developers said. If Alexandria officials approve the necessary variance and site plan, Dusco plans to start construction on the initial phase of the project in January.