NUS Corp., an energy and environmental consulting company, has leased two connecting buildings totaling 185,000 square feet at 910 Clopper Rd. in Gaithersburg. The 600-employe subsidiary of the Halliburton Corp. previously occupied space in six scattered locations in Montgomery County.

The pair of two-story buildings, designed by Donald N. Coupard and Associates of Rockville, are the first to be built at Tech Park 270, an office complex being developed by Mulligan/Griffin and Associates, Inc. When work is finished in five years, nine buildings will total 785,000 square feet, said John F. Griffin, a partner in the development firm. Construction of the third building is scheduled to begin this year, he said.

Mulligan/Griffin specializes in developing, leasing and managing customized facilities for corporate and government users. The firm is developing more than 1.5 million square feet in Montgomery and Fairfax Counties.

Other recent developments in commercial real estate in the Washington area:

* Union Oil Co. of California has leased more than 25,000 square feet at Washington Square, 1050 Connecticut Ave. NW, for use as its Washington headquarters.

* C and G Sales Co., Inc., a wholesale distributor of automobile parts, has leased a 48,000-square-foot warehouse with offices at 635 Taylor St. NE. The company will use the space as its corporate headquarters and distribution center.

* Syscon Corp. (formerly Systems Consultants, Inc.), a computer and electrical engineering firm, has leased four floors totaling 90,000 square feet at 1000 Thomas Jefferson St. NW in Georgetown. The six-story, 138,000-square-foot building was designed by Vlastimil Koubek amd built by Richmarr Corp.

* International Voluntary Services has leased 4,000 square feet in the Embassy Building, 1424 16th St. NW. The seven-story, 40,000-square-foot structure is under renovation.

* Israel and Raley, a law firm, has purchased an unrestored brick house at 1513 16th St. NW. The firm plans to renovate the 7,000-square-foot structure for use as law offices.