Lewis & Thos. Saltz, the 41-year-old Washington retail firm that has cornered a significant share of the local men's wear market by selling proper clothes to proper men, is about to launch an areawide expansion under new ownership.

Stanley H. Rosensweig, new board chairman of the company, revealed in an interview yesterday that Saltz will open its first new branch since 1970 next summer at White Flint Mall, on Rockville Pike.

Within the next two years, the company plans to build two new stores at regional malls in Northern Virginia, Saltz's first ventures in that state.

Moreover, Rosensweig and company president Henry W. Steinbach said, Saltz will add women's departments at all new units (a branch in Chevy Chase is the only current seller of women's clothing) and will broaden its lines of men's clothing for young professionals.

Steinbach described Saltz as a store that seeks to maintain its image as "progressive-conservative" and neither high-fashion-oriented nor trendy.

Since Sept. 1, the Washington-based firm has been owned by a local group that includes Rosensweig, Sonny Abramson and shopping center developer Ted Lerner.

They purchased the Saltz firm - with two downtown stores, the Chevy Chase branch, and annual sales of $5 to $10 million - for about $3 million in cash from the family of Richard Sleen, which had taken over the local firm from the brothers, Saltz in 1955. Steen has remained with the firm as chairman of the executive committee.

Rosensweig said the new owners are prepared to invest cash in opening new suburban branches, at the clip of more than $1.25 million per store.

The Saltz expansion comes at a time when the company will face initial retail competition here for its premier line of Oxford suits. Neiman Marcus of Dallas, which plans to open a D.C. store near the Saltz branch in Chevy Chase in November, also features the Oxford line.

To direct the expansion, management of the company has been reorganized with 34-year Saltz veteran Steinbach remaining as president and becoming chief operating officer.

Howard J. Johnson, chief financial officer, is vice president and controller; Richard C. Beehtel was promoted to vice president and manager of men's clothing, and Georgia Young was elected a vice president and merchandise manager for ladies apparel.

No other changes are expected in operations of Saltz, officers and yesterday.

The new store at White Flint, a mall developed by Lerner's company, will be located in 13,500 square feet on the first floor near Lord & Taylor. Like other Saltz stores, it will feature an English "pub" atmosphere of wood exteriors and fixtures.

An existing branch on lower Connecticut Avenue NW, will be renovated, the officers said. In the year ended Aug. 31, the company experienced its greatest sales in history with sharp gains at both downtown units - Connecticut Avenue and 1409 G St. NW - both of which will be maintained, they added.

Rosensweig, a long-time Washington businessman, real estate investor and former president of the Gem stores, said he telephoned Steen earlier this summer and expressed an interest in buying the firm. The Steen family has retained ownership of Bullock & Jones, another quality retailer in San Francisco.

The company was started by two youngsters, Lewis and Thomas Saltz, who came to Washington in the Depression years from Crisfield, Md., where their father operated a general store.