The Mason Hirst Cos. have announced that they have signed an agreement to sell the Federal Home Loan Mortgage Corp. (Freddie Mac) two buildings totaling more than 100,000 square feet in the Lake Fairfax Business Center at Reston.

Freddie Mac will house its computer operations and loan accounting departments at the Center, which will contain 1.2 million square feet of space in a dozen buildings when completed.

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

*Eden Hannon and Co., a local financial services and investment banking firm, has acquired the recently completed contemporary-style office building at 112 South Alfred St. in Old Town. Eden Hannon plans to occupy the third and fourth floors of the 23,700-square-foot building by early next year.

*Carr Properties has acquired and is currently renovating Magnolia Commons, 197-unit rental community located less than 2.5 miles from the Pentagon on Columbia Pike in Arlington.

Both the interior and exterior will be renovated, at a cost of approximately $3 million. Completion is scheduled for this Janurary.

*Jackson-Cross has announced that Sperry Corp., Global Research International and Defense Systems Inc. have each leased 22,000 square feet at 8500 Leesburg Pike, Vienna, for a total lease value of nearly $9 million.

*Blake Construction Co. Inc. has announced the following leasing transactions at 1150 Connecticut Ave. NW.: The Proprietary Association has leased 12,023 square feet; The Voluntary Hospitals of America have leased 6,809 square feet; Reed, Smith, Shaw, and McClay have leased 4,909 square feet, and Hershey Foods has leased 825 square feet at 1730 Rhode Island Ave. NW. The four leases have an aggregate lease value in excess of $5.5 million.

*The DANAC Real Estate Investment Corp. has announced that the Earth Observation Satellite Co. has signed a lease for approximately 48,000 square feet of office space at 4300 Forbes Blvd., Lanham.

The EOSAT will be occupying the entire first building in the DANAC Corporate Center complex, which is located near the intersection of Rte. 50 and George Palmer Highway.