Development of the first phase of the 55-acre Greenbelt Golden Triangle may began this year.

Kenneth H. Michael, a Cheverly realtor who is working with the owners of the tract as a developer, said that the site is bounded by the Beltway and Kenilworth Avenue on the east and west and by Greenbelt Road on the south. He said plans call for 450,000 square feet of office space, a major motel and a medical complex to be built over the next five years on the commercially zoned tract.

Capitol Cadillac, long a landmark auto dealership on 22d St. NW. just north of M Street, has a contract to purchase a 6.5-acre tract in the Greenbelt triangle. A spokesman for the auto firm said that plans to build a new showcase building on the Greenbelt site are still tentative, however.

Meanwhile, developer Michael said he is also negotiating for a banquet and mini-convention center to be built in the triangle. Also, he said he own firm, Kenneth H. Michael Cos., is drawing plans for a five-story office building on the site. His Cheverly-based firm specializes in commercial and industrial sales and development.

The Greenbelt triangle property, now undeveloped land, is owned by a group of Philadelphia investors headed by Milton Selig.