The Evans Co., a Northern Virginia-based commercial real estate development firm, has announced plans to build a new office building, One Fair Oaks, on a site adjacent to Fair Oaks Mall at Legato Road, Route 50 and Interstate 66.

Construction is slated to begin in September.

The 12-story building will have 200,000 square feet of office space, a mirrored glass exterior and a three-story lobby and atrium, the developer said. Surface and covered parking space for 675 cars also is planned.

Davis & Carter P.C. are the architects for the project, and Omni Construction Inc. is the general contractor.

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

* Winchester Homes Inc. has formed a new commercial division to operate in the Baltimore and Washington areas.

Winchester Commercial, as the new division will be called, plans a full range of commercial and industrial projects and was formed to take advantage of "the prime market" in the Baltimore and Washington areas, the firm said. Initially, the firm will concentrate on the areas north of Washington.

The division will be managed by Peter G. Byrnes, who has 11 years experience with Winchester Homes and its parent company.

Winchester Homes is part of the Weyerhaeuser Real Estate Co., a wholly-owned subsidiary of the Weyerhaeuser Co., which is based in Tacoma, Wash.

* Perpetual American Bank will open its 57th branch office in Germantown's North Lake Commerce Park I. The new branch, which will offer a complete line of the bank's services, will be located at 12850 Middlebrook Road. The lease for 2,500 square feet has an aggregate value of $500,000.

* Telefocus Inc., a telecommunications firm, has moved its offices to a new nine-story office building at 8280 Greensboro Drive in Tysons Corner, according to Rouse & Associates, developers of the building. Telefocus had been headquartered at 8260 Greensboro Drive.

* Work has begun on a new three-building complex for Ted Britt Ford Sales Inc. in Fairfax. The new structure, which will include a showroom, a service building and a used car building for a total of 47,400 square feet, is slated for completion in early 1986. Jack Bays Inc. of McLean has won the construction contract for the complex.

Other firms involved in the project include Strang and Samaha, an Annandale architectural firm; Gannett, Fleming Inc., a Fairfax engineering company, and Butler Manufacturing Co.