International Business Machines Corp. will begin construction this month on a 340,000-square-foot office building in Prince William County, bringing the office space occupied by IBM in the county to a total of 1.6 million square feet.

The new facility will employ about 1,000 people in the company's federal systems division. The cost of construction is estimated at about $20 million to $25 million, according to the county's economic development office.

The new IBM building is a part of the 7,000-acre Prince William Industrial Complex, which also contains a new engineering facility for Atlantic Research Corp.'s propulsion division.

Recent developments in commercial real estate in the Washington area:

* Satellite Business Systems has leased 20,000 square feet of office space in the Fairfax Executive Park to house the business operations department for its public long distance telephone service. The 170,000-square-foot park, located at the intersection of Route 50 and Interstate 66 in Fairfax, is being developed by Rouse & Associates.

* The law firm of Schwalb, Donnenfeld, Bray & Silbert has sublet from Syscon Corp. about 11,000 square feet of space at 2828 Pennsylvania Ave. NW. The five-year lease has a total aggregate value of $1.3 million. The firm's offices were previously at 1333 New Hampshire Ave. NW.

* The Kaempfer Co., an investment building firm, has leased 2,500 square feet of space at 2828 Pennsylvania Ave. NW. The value of the lease is $150,000. At the same address, the marine engineering firm of Ship Analytics Inc. has leased for a three-year period over 1,000 square feet of space. The total aggregate lease value is $55,000.

* The General Services Administration has sublet 17,000 square feet of space in Crystal City from USAir. It was erroneously reported in this column last week that GSA was the lessor.

* Johnson's Garden Center has signed a lease to open a new location near Rockville Pike and Randolph Road in Montgomery County.

* Magnavox Government and Industrial Electronics Co. has purchased a 76-acre tract of land on Route 7 in the Ashburn Junction area of Loudoun County. The property was formerly owned by county resident Marcus J. Bles. Magnavox plans to develop the land within the next five years into an engineering and software design center.

* Construction began recently on a $14.6 million, 296-bed nursing home being developed in Northeast Washington by the Grant Park Nursing Home Limited Partnership. The facility received a $13.1 million mortgage insured by the Department of Housing and Urban Development. It will contain a medical and dental clinic, pharmacy, gift shop and pathology, physical therapy and x-ray labs.