Contel ASC is moving its executive offices and its newly acquired international services division, formerly Comsat International Communications Inc., to the Decoverly office park in Rockville at the end of this month.

"Due to the expansion of our company, we needed more space," said Dennis Mesko, vice president of human resources for Contel ASC. "Decoverly had the proximity to our Research Boulevard offices, the price, and it was available within the time frame we needed."

Contel ASC, whose parent company is Contel Corp., the nation's third-largest independent phone company, is based in Rockville and provides transmission services integrating voice, data, facsimile and video to large businesses and government agencies.

Contel is leasing 67,000 square feet of Mulligan/Griffin's new $12 million, 77,747-square-foot building at 15200 Omega Dr. The three-story building is leased to Contel and Mentor Graphics Corp., a Beaverton, Ore.-based company that manufactures and services electronic design automation systems. Mentor is leasing 10,000 square feet; the remaining 800 square feet will house a delicatessen.

The building was designed by Donald N. Coupard and Associates of Rockville and features heavy-duty electrical and mechanical systems to accommodate computer rooms and large conference, training and secure/classified areas. The building's amenities include open-space planning and floor-to-ceiling glass in all building corners and central areas.

The 42-acre Decoverly office park is located in Montgomery County's "R&D Village," within the Shady Grove/I-270 corridor. The complex is planned for 800,000 square feet in seven buildings ranging in size from 75,000 to 270,000 square feet.

The building leased by Contel is the third building in the park to be completed. Construction of the park's fourth building, 77,000 square feet in three stories, began earlier this month and will be completed in late 1988. The remaining three buildings are planned for development in the next two to three years.

The park already is home to the National Association of Securities Dealers Inc. and GTE Government Systems, which occupy a four-story, 110,000-square-foot building and a two-story, 60,000-square-foot building, respectively. The office park has a four-acre lake, jogging and bike trails and picnic areas.

In other developments in commercial realty:

Comprehensive Technologies International, a government contractor, leased 20,000 square feet at Avion Park, 14500 Avion Parkway, Chantilly, from Trammell Crow Co. The Fred Ezra Co. was the leasing agent.

People for the American Way, a nonprofit lobbying group, leased 19,221 square feet at 2000 M St. NW. from Juster Associates. Long & Foster was the leasing agent.