Neiman-Marcus Co. yesterday announced that it would expand its operations in the Washington area by opening a store in the Tysons II shopping center now under construction.

The $20 million store, which will have 120,000 square feet of space on three floors, is scheduled to open in the fall of 1989, joining R.H. Macy & Co. and Saks Fifth Avenue, the two other anchors in the new mall.

"It's a market we have long wanted to enter, ever since we opened the first store in Washington, D.C., 10 years ago," Richard Marcus, chairman of the posh Dallas-based retailer, said in an interview yesterday. "We have always thought of this city as a two-store strategy. The whole metropolitan area is one of the most exciting, fastest-growing and affluent markets in the country. We're not doing ourselves justice by serving it only with one successful store."

In addition to opening a new store here, Marcus said Neiman-Marcus also will launch an extensive renovation of its existing store at Wisconsin and Western avenues NW in the District. The multimillion-dollar renovation will begin early next year and take 18 to 24 months to complete.

Once the new store is finished and the renovation is completed, "I think you'll find one and one will add up to more than two," Marcus said. "We will probably share a few customers between the two stores, but also reach many more customers than we are now serving."

The addition of another Neiman-Marcus store will come at a critical time for Washington retailers, who already are bracing for the entry of Macy's and Nordstrom Inc. next year. Nordstrom's, a West Coast merchant well-known for its customer service and deep inventories, is scheduled to open the first of several stores here in Tysons Corner -- across Rte. 123 from Tysons II -- next March.

Meanwhile, two other major retailers -- New York-based Henri Bendel and Paris' Au Printemps -- are reported to be looking for sites in the area.

Marcus said he is aware of the increased competition. But he said, "It's going to be healthy. That's the stuff good retailing is made of. It's more fun to be in a competitive environment than one that is not."

Ever since it opened its first store here 10 years ago, Neiman-Marcus has said it was interested in opening another. But disappointing sales for the first few years at the Wisconsin Avenue store kept those plans at bay. "We really wanted to get our tap root down in this market" before expanding, Marcus said.

That took longer than expected. "When we came into this market, we had a location that was terrific, but a center {Mazza Gallerie} that was badly leased, and a street that suddenly went into the war zone with the subway construction," Marcus recalled. "It took us a while to get through all that. Within the last six years, however, we have been very pleased with the growth of the District store, which has often posted the highest gains, in percentage terms, in growth. It has grown a lot faster in the last several years than we expected."

The Tysons store is part of the company's overall plan to open one or two stores a year, beginning in 1989. Neiman-Marcus also plans to renovate many of its 22 existing stores.

The expansion -- the first since 1985 -- reflects the new ownership of Neiman-Marcus, which -- with Bergdorf Goodman and Contempo Casual -- was spun off by its parent Carter Hawley Hale Stores earlier this year as part of a corporate restructuring.

Now part of a smaller company that bears the name of the premier store -- Neiman-Marcus Group Inc. -- the chain "is looking at expansion and renovation in a more aggressive way," Marcus said.