Commercial leasing in Fairfax County is advancing at "a relatively brisk pace" and developers are continuing to launch new projects there, according to a recent study by the Fairfax County Economic Development Authority.

The report says that .75 million square feet were leased in the last half of 1981. Tysons Corner had the largest portion of recently-leased space, with 259,000 square feet. Fairfax West and McLean both leased more than 100,000 square feet.

The majority of these recent transactions have been concluded by firms that are engaged in local expansion, such as USAir, Melpar-E Systems, Prime Computer, Space Applications and Sesa Honeywell. Companies new to the Fairfax County office space market are the Westinghouse Standard Products Division and Piedmont Airlines.

In January there were 24 office buildings under construction in the county, compared to 14 buildings in June, 1981. Lease rates quoted in the study range from $13 to $18 per square foot.

Planned projects total seven million square feet, with 3.2 million of that amount scheduled for possible completion by late 1983. Tysons Corner leads with 2.6 million square feet; Reston expects 1.3 million, the second-largest amount, and Springfield is third, with 613,000 square feet. Projected lease rates vary from $14 to $18.50 per square foot.

The county report is the fourth in a series, compiled with information obtained from owners, developers and brokers in the local office space market. The economic development authority stresses that the figures in the study do not include office space custom-built for specified tenants.

Developments in real estate in the Washington area:

* Construction was recently completed on the headquarters for the International Association of Chiefs of Police. The two-story, 20,000-square-foot facility is on Firstfield Road in Gaithersburg.

Construction was started recently on a 120,000-square-foot office building at 1550 Wilson Blvd. in Rosslyn, with completion expected in the spring of next year.

* At Rosslyn Plaza North, Smithfield Foods Inc. has leased 1,787 square feet of space; Southern Pacific Communications has leased 4,000 square feet; Delta Airlines, 2,000 square feet, and Whittaker Corp., 19,650 square feet.

* Construction has begun on a 22,000-square-foot addition to the offices of Litton Computer Services, a division of Litton Industries located on Michael Faraday Drive in Reston. The addition, which is scheduled for completion by this fall, will more than double the size of the company's computer headquarters.