An area considered by some to be the last prime development site on the Potomac along I-66 in Rosslyn is being developed by Park Tower Realty Corp., a New York company.

Park Tower Realty has begun building the $80 million Potomac Tower at 1001 19th St. in Rosslyn. It is the firm's first project in the Washington area.

Potomac Tower was designed by I.M. Pei & Partners, the well-known architectural firm. When completed, it will have 257,000 square feet of space on 19 floors.

"There will be tenants who will want this building and this quality," said Thomas J. Regan Jr., managing director of Park Tower's Developments Ltd. new office in the Willard Office Building. Regan was previously executive director of the Pennsylvania Avenue Development Corp., where, among other things, he helped oversee the preservation of the Willard Inter-Continental Hotel.

Potomac Tower will have a stone and concrete exterior and a glass curtain wall facing the monuments of Washington. Two glass towers will top the roof.

The building will have concierge service, open-air terraces, marble-finished elevators and a two-story lobby with natural granite and marble. There will be parking for 500 cars.

Annual lease rates will range from $34 to $40 per square foot, higher than Rosslyn's current market rate of the low $30s.

Regan is aware that suburban vacancy rates in the area, unlike those downtown, have risen recently.

"There are very weak segments of the D.C. market," Regan said. "We're concerned, but that's just prudent."

Omni Construction is general contractor for the project. Completion is scheduled for January 1989.

Park Tower is considering other sites in D.C., Baltimore and Northern Virginia, Regan said.

In other developments in commercial realty:

Hazel, Thomas, Fiske, Beckhorn & Hanes, P.C., Northern Virginia's largest full-service law firm, leased 53,000 square feet at 3110 Fairview Park Dr. from Cadillac Fairview Urban Development Inc. Cadillac Fairview Urban Development Inc. was the leasing agent.

The Montgomery County Board of Realtors, an association of real estate agents, leased 28,700 square feet at 1355 Piccard Dr., Rockville, from Foulger-Pratt. Carey Winston was the leasing agent.

Reese Communications Cos., a marketing company, leased 21,250 square feet at One Colonial Place, 1903 N. Key Blvd., Arlington from Colonial Village Center Inc. and Galbreath-Ruffin Corp. Coldwell Banker was the leasing agent.