Individual floors will spread over more than an acre each, rental rates will set records and two lighted semi-artriums are likely to become Connecticut Avenue landmarks.

For years the best-kept secret in Washington and the subject of intense conjecture, plans for Washington Square, a $100-million office-retail complex, finally will be disclosed this week.

Construction work is well under way at the key intersection of Connecticut and L Street NW on what will be one of the District's biggest and most expensive private business centers and a focal point of architectural excitement by day or night.

The 64,000-square-foot site formerly housed such diverse businesses as Duke Zeibert's restaurant, Brooks Brothers and Scholl's cafeteria on several properties that were assembled doggedly by veteran Washington developers T.N. Lerner and Albert Abramson.

The 12-story, one-million-square-foot, marble-and-glass building will be distinguished by two hexagonal semi-atriums at either end of the 190-foot frontage on Connecticut Avenue. There will be another 380 feet of frontage on L Street, with retailers having street-level access.

The larger of the two glass-enclosed, 110-foot plaza shafts will dominate the Connecticut and L corner. A smaller semi-atrium will bound the building midway between L and K streets on Connecticut. Both will be illuminated at night and will be designed to attract passersby, according to veteran Washington architect Chloethiel Woodard Smith.

The building, with a facade of alternating, horizontal bands of pink Tennessee marble and glass, will be recessed from the walk at intervals to reduce the giant-box effect.

Lerner, who already has developed four major shopping malls in Virginia and Maryland, said that Abramson suggested nearly five years ago that they consider development of the corner property over a Metro station. The other three corners are sites of new buildings, two of which also were designed by Smith.

Initially, Abramson and Lerner made complex arrangements to acquire building rights on the site of the old, recently razed LaSalle Building on Connecticut Avenue. Later they acquired six other properties to amass the north half of the block between Connecticut and 18th Street.

By the time the total site acquisition was completed with the purchase of an Exxon Corp. gas station on 18th Street, the developers had an extraordinary site that may hve commanded a record land price here -- possibly $65 million at $1,000 a square foot.

Washington Square office space, which will total 600,000 square feet, is expected to be leased at rates of about $35 a square foot a year, a new top for downtown commercial leasing. With each of the upper floors of the building having more than 50,000 square feet of net space, full-floor tenants actually will be leasing space by the acre.

Lerner said that one tenant whom he did not wish to name had put in a reservation for a sizable chunk of space some months ago, regardless of price. He added that law firms and oil companies wanting prestige addresses are considering to be prospects for tenancy. Occupancy is scheduled in September 1982. Prime space will be on the 11th and 12th floors where one tenant would have exclusive use of a glass-walled hexagonal room -- actually the crown of the semi-atrium below.

Although basic retail space will total a comparatively modest 60,000 square feet on ground level, one principal "name" tenant is likely to be among the 25 or 30 stores. Talks are in progress with specialty retailers for a store of up to 30,000 square feet; Garfinckel's, Neiman-Marcus and Bonwit Teller are thought to be among the companies interested, while Saks Fifth Avenue (rumored earlier to be a candidate) apparently is not talking.

Bloomingdale's, the high-fashion Federated Department Stores chain with two D.C. area units, once was mentioned as a possible Connecticut and L anchor but also will not be in Washington Square. However, retail sources have said Bloomingdale's still may open a downtown store at another location.

Classy restaurants and cafes are expected to be tenants on the second level of the new center with outdoor balconies and restaurants seating overlooking Connecticut Avenue crowds and traffic.

Designed to attract noon-time shoppers who pile into the Connecticut Avenue corridor, the building's concourse level below grade will have convenience and service shops, some fast-food facilities, full-service restaurants and an international food marketplace.

The next level down will provide only windowless office space, such as that used for computer rooms. Parking for 500 cars will be provided on the third and fourth underground levels. Access to the parking garage, designed primarily for use by weekend shoppers and top executives in the building's offices, will be on 18th Street. Developers are assuming that most persons working in and near the building are likely to be subway riders.

Lerner specifically complimented officials of District government for cooperative aid. He said an assistant to Mayor Marion Barry Jr. persuaded Exxon, the nation's largest industrial enterprise, to sell its 18th and L corner as a benefit to the city and thus permit the development to achieve its near-epic proportion. Exxon subsequently agreed and turned down a higher offer for the property.

Voluntarily, the developers decided early to install a full sprinkler system throughout Washington Square and to provide a sophisticated security system that will protect tenants and provide pragmatic aids for fire-fighters. The building will have 10 office elevators and additional elevators and escalators to serve retail areas exclusively.

Trees will be planted in boxes to enhance the plaza areas of the building. Although it was unstated, both the developers and architect Smith appear determined to have their center be a contrast to the brick building designed by Skidmore, Owings and Merrill for the diagonal corner, also atop the adjacent Metro entrance and abutting the landmark Mayflower Hotel.

Architect Karl Kaufman worked with the Chloethiel Smith firm as a representative for the owners. Developer Lerner praised Smith for her creative design and patience in accepting new ideas and design requirements. For instance, the special British-made glass used in the exterior atriums will be engineered as connecting pieces with no horizontal seams to break the vertical openness.

Construction financing is being provided by American Security Bank on a five-year basis. No permanent mortgage financing has yet been arranged.

George Hyman Construction Co., a Clark Enterprises subsidiary that is the area's largest and one of the nation's fastest-growing general contractors, is building Washington Square.