For Washington retailers, this will be remembered as the winter that Neiman-Marcus came to town and the Giant Department Stores left.

The opening of Neiman-Marcus two months ago was the latest - and by no means the last - immigration of a big-ticket business to the affluent Washington market.

The closing of Giant's discount stores a monthe from a will be the latest - but by no means the first - indication that competition for that affluence in increasing.

As the Metropolitan Washington Board of Trade's retailing specialist, Leonard Kolodny explains it: "The pie is being cut intomore pieces."

Merchants and developeers say there is growqing that retailing reality is coming to the hottest consumer market in the country, the $10 billion-a-year-plus buying spree that is metropolitan Washington.

Despite the chance to move in next door to Neiman-Marcus, despite the prediction that Friendship Heights eill become the best shopping area inside the Beltway, other stores have not yet filled uop the Mazza Gallerie.

Nearly a year after it opened, the White Flint mail in Rocksville still has storefronts covered with plywood. I, Magnin, California's answer to Saks Fifth Aveneue, is talking - so far only talking - about building at White Flint as the center's third achor store.

Some White Flint merchants grumble that what the center needs is not another rival to Lord and Taylor and Bloominmgfale's but a middle-range merchant that will built up the traffic.

"If it were my places. I'want J.C. Penney instead of I. Magnin," said one rival developer.

Even Ted Lerner, builder of Tysons Corner, the area's most successful shopping mail, is talking about putting up office building on his Tyscons 11 site instead of another mammoth shopping center.

Businessmen here to talking only about the boom now are worrying about how much sales will increase this year and who will be the next Neiman-Marcus or Bloomiongdale's on the block. Now, like narrow ties or short skirts, an old word is coming into fashion - overstored.

Few will argue that the Washington market is now overstored, not even in the brutually competitive discount square foot have come to be the lest a successful operation can get buy with and profits for of more than 2 per cent are more than most can hope for. Giant went back to the business it knows best not only because there are too many K marts and Mamcos and Zayres, but also because a four-store, $45 million-a-year operation is a mom and pop outfit in that league.

Income are growing faster than population, however. Sales Management magazine projects that the Washington metro area will grow in populaion by 48 per cent and in income by 145 per cent between 1970 and 1982.

In utter affluence, the area's $21,076-per-household buying power ranks second only to Nassau and Suffolk countries on New York's Long Island.

Those numbers will continue to draw merchants this year. Petrie Stores, a popular-prices apparel chain that is one of the fastest growing sepcially shop operations in the country, is looking for locations for its first stores in the Washington area.

Marshalls, a 37-stores discount apparel chain that opened in suburbun Virginia last year, says it wants more units here.

Conran's, a British home furnishings and housewares chain that landed in New York with much hoopla several months ago, will be a neighabor of Garfinckel's in the high-ticket George Park Project.