Increases in real estate prices and rentals will taper off in 1982 after the meteoric rises of recent years, according to Landauer Associates Inc.

"While we have probably not yet reached a peak, it seems clear that moderation in rents and prices will inevitably prevail," said John White, chairman of the New York-based corporate real estate consulting firm. He added that "the next five years will prove to be less profitable than the last five years."

White said that with the supply of attractive properties growing and the recession holding back demand, "Sellers no longer will have the independence they have so enjoyed over the past 10 years. Buyers have taken the offensive and urged on sellers all types of concessions as conditions of purchase."

"Buyers and sellers will be on a more equal footing in 1982," White concluded. "The buyer will reason that reduced inflation, high money costs, an uncertain economy and the prospect of an ample supply by the mid-1980s are all sound reasons for price moderation."

Developments in real estate in the Washington area:

* U.S. Synthetic Fuels Corp., a government-owned concern, has leased 67,000 square feet of space in the McGregor Building at 2121 K St. NW.

* The new Loudoun County headquarters of Systematics General Corp. was recently completed by Spaulding & Slye Construction. The company expects full occupancy of the 49,184-square-foot building by the end of February. The Industrial Development Authority of Loudoun County financed the construction.

* Computer Consoles Inc. has leased 10,165 square feet of space at 962 Wayne Ave., Silver Spring. The company provides turn-key systems to the Bell System and is currently involved in entering the office automation business.

* Utility Data Institute Inc., a nonprofit information services company which provides data base services to gas and electric utilities industries, has leased 4,693 square feet of space at 2011 I St. NW.