CIT-Alcatel Inc. and CITCOM Systems Inc. last week announced plans for a new corporate headquarters at the Dulles International Center, a 60-acre office and research park located at the intersection of Route 23 and McLearen Road in Fairfax County.

The two affiliated telecommunications companies expect to move into a 90,000-square-foot building -- the first of a planned three-building, 18-acre complex -- next January. Construction is expected to begin soon.

Earl Hall, a vice president for the two firms, said they have signed a 45-year lease for the property.

CIT-Alcatel Inc., which produces telephone switching equipment, and CITCOM Systems Inc., which markets data transmission products, will employ about 400 people in the new complex.

Other recent developments in commercial realty in the Washington area:

* DANAC Real Estate Investment Corp. reports that constrution has begun on the fourth and final building of its DANAC Office Park I, a 216,000-square-foot office complex located on Gaither Road near Shady Grove Road in Rockville. The building is scheduled to open later this spring.

* Telesystems Inc., a subsidiary of Comsat Technology Products Inc., has leased 90,000 square feet in a new office building at 2700 Prosperity Ave. in Fairfax. The building, designed by the developer, Prudential Insurance Co., for Telesystems, is the fourth building in Fairfax's Prosperity Business Campus to hold some of the telecommunications firm's operations.

* OMNI Construction Inc. said it has completed construction on the 10-story, 135,000-square-foot office building at 8484 Georgia Ave. two months ahead of schedule. The project, located at the southwest corner of Georgia and Wayne avenues in Silver Spring, was begun in December 1983. The L. W. Moore Development Co. of Silver Spring developed the building. Ward/Hall Associates of Fairfax was the architect.

* The District government has signed a lease for 56,000 square feet at 1111 E St. NW.

* AYZ Services Inc. has signed a five-year lease for 45,255 square feet at 8241 Sandy Ct. in Jessup.

* Crick Valley Partners has bought 36,000 square feet of warehouse property at 16021 Industrial Dr. in Gaithersburg for $1.8 million.

* The General Services Administration has leased 29,085 square feet at 555 4th St. NW.

* Mantech International Corp. has signed a lease for 15,620 square feet at 2341 Jefferson Davis Hwy.

* Balmar Inc. has leased 15,556 square feet at 8501 Muscatello Ct. in Gaithersburg.

* Advantage International has leased 14,099 square feet at 1025 Thomas Jefferson St. NW.

* Grumman Data Corp. has leased 12,721 square feet at 6833 Beverly Rd. in McLean.