U.S. Home, a country's largest producer of homes in 1980, recently has reorganized its Washington-area operation, and its top executive here has resigned to start his own firm with a friend.

The Houston-based corporation, which has 10 projects in this area and plans to open several more this year, recently split its District of Columbia division into District and Maryland divisions. The new Maryland division is headed by Robert Sithens, formerly a vice president. The District division is headed by Michael Steten, who came here from the firm's Houston office.

Richard Sullivan, who was president of the former District division, has resigned to join builder-developer Herman Porten in a new firm, Porter-Sullivan. The new firm plans to begin building homes here next year on land which already has been acquired.

Porten, who sold Page Construction Co. to U.S. Home six years ago, has been building expensive contemporary town houses at Strathmore Lane and Rockville Pike. Sullivan said that he and Porten are planning developments that cater to the upper-income and lower-income ends of the market.

Concerning his resignation from U.S. Home, whose total residential production in this area increased from 150 units in 1977 to about 500 units last year, Sullivan said he wanted an opportunity to "go on my own" with Porten, a long-time friend and business associate. "I started with Herman 11 years ago as an accountant at Page and now am back with him as a partner." Sullivan succeeded Porten as head of U.S. Home's division office here in 1977.

Sullivan said that Charles Barbot and Ronald Garshag, who had headed land development and marketing for U.S. Home here, also have joined Porten-Sullivan.

Despite the strictures of what he called a "difficult mortgage market," Sithens predicted that U.S. Home will continue to expand its building in this area in 1981 with a variety of housing choices and financing plans.

In a recent report to stockholders, U.S. Home reported that new orders for the first three months of 1981 totaled 4,973, an increase of 25 percent over house orders taken in the first quarter of 1980.

Earlier, U.S. Home reported that Societe des Maisons Phenix, the largest manufacturer of single-family houses in France, had purchased 400,000 shares of common stock from the company at a price of $25.25, bringing the foreign firm's holdings in U.S. Home to 17 1/2 percent of outstanding shares.

The two firms have begun a joint venture to build single-family houses in Florida.

U.S. Home now is building residences in 271 subdivisions and communities in 121 cities in 18 states. It built 11,237 single-family houses in 1980.

Sithens said that U.S. Home will continue to have its office in Wheaton. As head of the new Maryland division, Sithens has jurisdiction over the Mazza development in Potomac of homes in the $360,000 range, the Timberlawn houses on Old Georgetown Road and the Woodlake town houses in the Germantown area. He said the division plans to open a single-family-home development in northeast Baltimore next month and a community of homes on Bonifant Road in Silver Spring next fall.

U.S. Home's District division has six residential projects in Maryland and Virginia -- none in the District. These include Heritage Harbor, a senior adult community near Annapolis, the Stonewood development of four-plex homes in the Laurel area and the Beacon Place town houses in Silver Spring.

Three other ongoing residential communities are in Burke, Va.: the Spring Oaks fourplexes, the Lakeside town houses and the Forest Landing single-family houses. This division also plans to open Glen Ora town houses next fall on Greenbelt Road in Maryland.

Aside from its area home-building operations, U.S. Home recently set up what it calls a Shenandoah Valley land development division in Falls Church, where Glenn Sudek is division president.