Recent commerical real estate developments in the Washington region:

* American Security Bank recently placed a $150 million mortgage on the Washington Square project under construction at Connecticut Avenue and L Street NW.

* An apartment building at 1514 17th St. NW was sold recently for $3.5 million by Mr. and Mrs J. L. Zeldon to Copley Plaza Cooperative Inc.

* The Pennsylvania Avenue Development Corp. recently paid $1.5 million to R. A. Fields and others for properties at 608-14 Indiana Ave. NW

* American Security Bank and others sold the former Magic Pan restaurant building at 1819 L St. NW for $2.2 million to 1819 L Street Associates.

* Oliver T. Carr Jr. and others sold an apartment building at 2030 F St. NW for $3.5 million to the Letterman House Tenants Association.

* Interstate Rental Services, an industrial uniform company, bought 3.6 acres on Gerwig Lane in Columbia's Guilford Industrial Park from the High Point Sprinkler Co. for $170,000. Interstate plans to build a 40,000-square-foot facility there.

* Close Up Foundation has leased 43,500 square feet in 1 Crystal Gateway and General Electric Co. has leased 22,500 square feet.

* The Hechinger Corp. plans to build a store, its 32nd in the mid-Atlantic region and its fourth in Baltimore, next to the Security Square Mall there.

* Manufacturers Hanover Corp., the second-largest mortgage corporation in the country, plans to lease space in the Maryland Trade Center in Greenbelt.

* The Aircraft Owners and Pilots Association will break ground Dec. 9 at the Frederick (Md.) Municipal Airport for a headquarters building.