Anderson & Cramer Inc., a heating, plumbing and air-conditioning mechanical contractor, has opened its new corporate headquarters and assembly plant in Reston's 1,000-acre Business Center.

The company's brick facility is located on a 4.5-acre site at 11109 Sunset Hills Rd. The structure contains 40,000 square feet of office and warehouse space, of which 20,000 square feet is slated for future expansion but is presently available for leasing.

The relocation from the company's former 10,000-square-foot facility in Chantilly, Va., was necessitated by an increase in the company's contracting business, said Anderson & Cramer President John Loveless.

The two-level building was constructed by Anderson & Cramer.

The Fairfax County Economic Development Authority granted the company a $2.8 million industrial revenue bond, with financing provided by the First American Bank of Virginia.

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

* Construction has begun on a 42,500-square-foot addition to the Litton Bionetics Institute of Applied Biotechnology facility at 1330 Piccard Dr. in Rockville.

The $3.5 million project is scheduled for completion in February 1984.

* Electro-Nucleonics Inc., a medical systems manufacturer with corporate offices in Fairfield, N.J., recently opened its 43,000-square-foot diagnostic research and manufacturing facility at The Rivers Corporate Park in Columbia. The single-story building is located on seven acres at the Rivers Park.

ENI also operates a disc manufacturing and distribution center facility at the Oakland Ridge Industrial Park in Columbia and a research facility in Silver Spring.

* The American Petroleum Institute has leased approximately 130,000 square feet of space for its new headquarters at 1220 L St. NW. The trade association is relocating from 2101 L St. NW.

* Greenhorne & O'Mara Inc., an engineering and architectural firm, has moved into its new headquarters building at 9001 Edmonston Rd. in Greenbelt.

The firm is occupying four floors of the 80,000-square-foot building, with the remaining 15,000 square feet of space available for leasing.

* Teledyne Brown Engineering, an Atlanta-based defense contractor, has leased 8,052 square feet of space at 7926 Jones Branch Dr. in Tysons Corner.

* Centralfed Mortgage Co. of California is establishing eastern regional headquarters and has leased 3,712 square feet of space at the Metroplex at 8201 Corporate Dr. in Landover. The aggregate rental value of the five-year lease is about $250,000.

* Advanced Systems Development, a data processing consulting firm, has leased 1,412 square feet of space at 962 Wayne Ave. in Silver Spring. The aggregate rental value of the two-year lease is about $35,000.

* A research firm, The Naisbitt Group, has leased about 4,300 square feet of space at 1211 Connecticut Ave. NW.