Construction has been completed on the third and final building of the Fifty/Sixty-Six Office Plaza, the first office park to be developed in the Fair Oaks section of Fairfax County.

The $2.1 million, 30,000-square-foot building is located at 11130 Main St., near the intersection of Rte. 50 and Interstate 66. It is the final phase of the three-tower office plaza.

"We were one of the first to recognize the growth potential of the Fair Oaks area and pioneered its development as early as 1973. As developers of the first office park in that region, we are excited by the continued expansion," said R. Lide Glenn, president of Weissberg Corp.

Weissberg Corp. began development of the complex in 1979, before I-66 opened the area for major expansion and turned it into the hub of Fairfax's business community.

Tenants leasing space in the complex include Consumer Housing Libraries, Fireman's Funds/American Express, the State of Virginia, Time Flex Inc. and American Medical Labs Inc.

Coldwell Banker is the leasing agent, while Sheridan Behn Eustice is the architectural firm and E. H. Glover is the general contractor.

Other recent developments in commercial realty in the Washington metropolitan area:

*The George Hyman Construction Co. completed construction of One Westin Center, a 400,000-square-foot office building at 2445 M St. NW. The building is adjacent to the recently built Westin Hotel at the corner of 24th and M streets.

*Doak and Shrum, a consulting firm for political campaigns, leased 4,000 square feet from M and Potomac Street Associates at 1200 Eton Ct. NW. The aggregate value is in excess of $150,000.

*A lease was signed between 2112 F St. Associates and Dr. Maureen Y. Chua for 2,600 square at 2112 F St. NW. The lease value is in excess of $680,000. -- M.B. Regan