Carroll County has received a $500,000 loan from the Maryland Department of Economic and Community Development to construct a 30,000-square-foot building on a three-acre site at the county's Air Business Center Industrial Park. The loan is to help the county attract new industry.

The all-brick structure will be built by the McCormick Construction Co., a wholly owned subsidiary of McCormick Properties. Construction will begin early next year and is scheduled for completion in June 1983.

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

Medallion Industries Inc., a 15-year-old residential and commercial heating and air conditioning company, has constructed a new 16,000-square-foot multi-purpose facilty at 2502 Urbana Pike in Ijamsville, Md. The new brick-and-block building will be the firm's Frederick branch office and contain a business office, service department, sales and engineering offices and a warehouse.

The company's corporate offices are in Bowie with branch offices in Columbia, and Springfield and Sterling, Va.

Site preparation has begun on the Prince George's County campus of the Capitol Institute of Technology. The new facility, set on 52 acres near the old Beltsville Speedway in Beltsville, will triple the size of Capitol's Kensington facility.

Gov. Harry Hughes had signed a $1 million bond bill in 1980 to help fund the new campus construction, and in 1981, Capitol Tech matched the state grant.

Capitol Tech's enrollment has tripled over the past eight years, necessitating the move to a larger facility. Groundbreaking ceremonies are set for April 1983.

The Merrifield Center at Old Lee Highway and Route 29-211 in Merrifield, Va., opened for occupancy earlier this month. The seven-floor office building contains 44,750 square feet of space and the single-level retail and professional complex contains 12,120 square feet of space.