Raleigh Stores Corp., metropolitan Washington's largest independent retail apparel firm, is planning a major expansion - including an additional story atop its lower Connecticut Avenue NW store.

The two top floors will be devoted exclusively to menswear, when the expansion is completed. Raleigh's also is planning to build an outside, glass-enclosed elevator for access to men's selling area.

Inside the current four-level Connecticut Avenue DeSales Street store, which has more than 35,000 square feet, women's clothing will be expanded to cover three floors and the company plans to move its corporate headquarters from the building this weekend, to permit additional selling.

Local business sources confirmed yesterday that Raleigh will move its corporate offices to the Longfellow Building, north on Connecticut Avenue at M Street. Raleigh president Sidney Lansburgh could not be reached for comment on his firm's plans.

Raleigh is the dominant menswear retailer in metropolitan Washington, with 10 stores and approximately one-third of the men's clothing market, according to retail sources. The company's revenues in 1976 were estimated at between $35 million and $40 million, up from $20 million in 1971.

Although men's clothing has accounted for the major business volume at Raleigh's in the past - up to 70 per cent of annual sales - the company is moving to expand its share of the women's market as well, partially in reponse to growing fashion interest attributed to the arrival of New York's Bloomingdale's at Tysons Conner last fall.

Raleigh's sells medium and higher priced men's and women's clothing and is not a direct competitor for Bloomingdale's high-fashion customers. But the Connecticut Avenue store also is located in the heart of a booming commercial office building center, close by a subway stop, and its women's business has grown because of purchases by the nearby office workers.

In a few months, Raleigh's will open its 11th store in the new White Flint Mall on Rockville Pike, joining a shopping center that includes Bloomingdale's second branch here and Lord & Taylor's third area store.

According to industry sources, Raleigh's also plans renovations at other stores around the Beltway and plans to maintain its business in the city's older downtown business district - either at its current F Street location or at a new site near 12th and G Streets NW.

A closely-held private firm, Raleigh's expects to finance its own expansion, trade sources said. Lansburgh, two brothers and a sister purchased the Washington retail firm in 1952 from the estate of Clarence W. Grosner, who began business with a haberdashery in the old Raleigh Hotel on Pennsylvania Avenue in 1911.