Less than 18 months after moving into a 1,300-square-foot office building in the Westpark office complex in McLean, the BDM Corp. announced this week plans to double its floor space by expanding into a nearly identical building next January.

Earle Williams, president of the Washington-based professional services firm, cited BDM's continuing growth, which has exceeded 25 percent annually throughout this decade, in explaining the firm's need for additional space.

"Our reputation is helping us get new clients, and the marketplace is growing." Williams said "We are probably growing faster than the marketplace."

BDM provides professional services in such fields as advanced technology, applied sciences,system analysis and policy research to the government and private businesses on a contract basis. Its primary focus is in five areas - energy. environment. communications. transporation and defense.

About 85 percent of BDM's business is with the Defense Department. Williams said most of the company's new business is with private companies, but it is also seeking government contracts.

Most of BDM's 650 Washington employes are not housed in the 130,000 square-foot BDM building in the Westpark office complex, where the company moved in early 1977 to consolidated its operations. But by the end of 1977, the company was already over-flowing from that building into nearby office buildings.

The new BDM building, being constructed next to the first building, will house employes who now work in three separate office buildings in Westpark. The new building will also provide space for 28 new employes, as the company expect to have 1,000 local employes by 1979.

The new office building will be nearly identical to the first one, with seven floors arranged in three wings in a "Y" shape.

The fast-growing company's sales are now running at an annual level of about $45 million, compared with just under $33 million in 1977 and $24 million in 1976. The company's earningshave been increasing similarly. Net income exceeded $1 million in 1977, up from $670,000 in 1976.

In addition to its Washington operations, BDM has a large technology applications center in Albuquerque, N.M., as well as offices in Alabama, Arizona, California, Kansas, Kentucky, Nevada, New Jersey, Ohio, Virginia. West Germany and the Netherlands.

BDM was founded in 1959 as Braddock. Dunn and McDonald Inc. by Drs. Joseph Braddock. Bernard Dunn and Daniel McDonald. Bratdock and McDonald now serve as vicepresidents for the company and Dunn serve as corporate chief scientist.