Construction is to begin next week on a $75 million office and retail complex embodying open space at its center on a site three blocks south of Dupont Circle in an area now experiencing a major office building boom.

According to the developers, a prime tenant in the three-building, three-stage project is expected to be part of the regional headquarters of the American Telephone & Telegraph Co.'s Long Lines Division.

One spokesman for the developers said a lease agreement for an eventual 165,000 square feet in the 1 million square foot project is expected with AT&T within a few days. A spokesman for Long Lines declined to confirm or deny this last night and would say only that the corporation currently is negotiating in Washington for space.

About 2,400 persons now are attached to the eastern area headquarters of Long Lines, the arm of AT&T that provides long distance telephone service. Most of the 2,400 are located in downtown Washington at 20th and L streets and at 2035 L St. NW.

Ground is to be broken Wednesday in Fairfax County for a new regional headquarters that will house about 1,500 of the employes, a Long Lines spokesman said last night.

Among features of the new project to be built on the block between 20th and 21st and L and M streets, is an east-west pedestrian passageway, an atrium and a galleria.

The first of the three buildings, to be known as Lafayette Centre, is to be started on the west side of 20th Street NW, on a parking lot that is the site of the former Pohanka automobile dealership. The dealership was moved nine years ago to Marlow Heights.

John J. (Jack) Pohanka, the auto dealer, is a principal developer, along with James E. Farr and Charles A. Jewett, partners in real estate businesses here.

Farr specializes in assembly of downtown property, Jewett, is a mortgage banker who arranged financing for the project with the American Security Bank and Equitable Life Assurance Society of the U.S.

While the complex, with its 10-story buildings, is to be built between existing structures on L and M streets, buildings on the 21st Street side of the site are to be razed for later stages of construction.

Jay Hellman, a limited partner in the project, said both the landscaped interior plaza areas and two additional stories in height were made possible through use of article 75 of the city's zining code.

The article, Hellman said, makes possible zoning variances on a large site in exchange for such amenities as public spaces and extra landscaping.

The first building, which is to include 10 shops and a restaurant, is expected to be ready for office tenants by the fall of 1979. Office leases in the complex are expected to average $13 a square foot.

The design of the buildings was described as a French-styled combination of traditional and contemporary motifs Facings are to be of red-brown brick, and entrances to the arcade areas are to be through three and four-story archways. Two levels of underground parking are planned.

Turner Construction Co. has been named as general contractor for the complex.

The general area of the new project now is checkerboarded with construction sites. Their board fences are a familiar part of the landscape to noontime strollers in what experts say has become one of Washington's most desirable office locations.

Among other projects now under way are two office buildings at the northeast and northwest corners of 18th and L Streets NW. Another building is being completed above the Metro subway entrance at the northeast corner of Connecticut Avenue and L Streets.

In addition, two office buildings are being erected just south of Dupont Circle. One's between Massachusetts and Connecticut avenues, and the other is nearby on New Hampshire Avenue.

Also under way in the area is the second phase of another large private downtown development --International Square, on the site between 18th and 19th and I and K streets.

Asked last night to account for the building boom in the area around Lafayette Center, Farr, one of the developers, cited "so many factors," including the start of subway service and the "general attitude that downtown Washington is becoming tremendously revitalized."

During the last year, new downtown office space has been in short supply while the market that services many lawyers, accountants and trade associations has been expanding.

With the construction boom now under way, however, some leasing specialists admit that by 1979 a surplus of space may again exist in a market in which supply and demand often are out of phase.

Completion of Lafayette Centre is scheduled for 1981. An athletic facility, including squash courts and a bar, is contemplated for one of the later phases.

A spokesman for the architects, Welton Becket Associates, said the complex will "blend totally with the fabric of the city, yet form a unified environment of comfortably warm, human proportions."