Omni Construction Inc. announced it has begun construction of Clarendon Square, across from the Clarendon Metro station at 3011 Wilson Blvd. in Arlington.

The building, expected to be completed by early 1987, will contain 300,000 square feet of space in seven levels of office space and will have three levels of underground parking.

Phillips Development Corp. of Bethesda is developing the project on behalf of Clarendon Square Associates.

Smith Segreti McMahon Harned Architects of Washington is the architect; GHT Chartered of Arlington is the mechanical and electrical engineer; and Allison, McCormac & Nickolaus of Rockville is the structural engineer.

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

*Performance Engineering Corp., a computer system design company in Merrifield, has signed a lease for space in Fairfax Executive Park at 3959 Pender Drive in Fairfax.

*HTB Inc., an architecture and engineering firm based in Oklahoma City, has leased office space in the National Press Building, now 80 percent occupied.

*Greenwell Goetz Architects announced the completion of offices it designed for three Washington law firms: Weiner, McCaffrey, Brodsky and Kaplan, 1350 New York Ave. NW; Freedman, Levy, Kroll and Simonds, Washington Square, 1050 Connecticut Ave. NW; and Leff and Mason, 1020 19th St. NW.

*Gates, Hudson and Associates Inc. has announced the opening of Diane's Carriage House, the first shop at Pear Tree Village, a "colonial motif" shopping center located at Cooper Road and Rte. 1 in Mt. Vernon.