The office building in the award-winning Flour Mill complex on the Georgetown waterfront has been sold by Weissberg Development Corp. to a European investment group known as Cowen Mountain Inc. for just under $20 million.

Cowen Mountain is the same group that is building the $50 million Trans Potomac Plaza project in Alexandria and plans further investments in local real estate, said developer Marvin Weissberg. "We'll be doing more ventures with these people," he added.

Weissberg Development is continuing to manage both the office and condominium portions of the multi-million-dollar Flour Mill. The developer said his firm decided to sell the office building, but not to dispose of the adjacent residential building.

The developer disputed suggestions that he sold off the offices because of slow sales in the condominium project.

"We'll keep the condos" Weissberg said. "We think we're the only game in town in the luxury condominium business." he added, saying present plans now are to retain eight of the 55 units in the costly condominium, rent them out and sell them later, hopefully at higher prices. Prices there now range from $200,000 to $545,000.

Condominium sales have been stagnant for months, mostly because of high interest rates, Weissberg said only three or four apartments remain for sale in the Flour Mill.

Weissberg's Flour Mill project won one of two first place awards in the 1980 preservation program of the Washington Metropolitan Chapter of the American Institute of Architects.

Until 1977 the home of the Wilkins-Rogers Milling Co. (makers of Washington flour and mixes), the complex was renovated and expanded into a contemporary mixed-use project designed by ICON, the International Consortium of Architects.

Rather than attempt to restore the structure to its original 1847 appearance, the architecturs stripped the building to its concrete core, then sheathed the condo building in a contemporary brick skin with angular balconies that have become the symbol of the complex.

The buyers of the Flour Mill offices--who paid cash for it, Weissberg said--are a low profile organization that manages pension funds for unspecified European firms.

Working through Savage-Fogarty Inc., a Dutch-owned Alexandria realty operation, European pension funds have invested in several local projects, including the huge Trans Potomac Plaza in the northern part of Old Town Alexandria.