The University of the District of Columbia has received a one-year extension on its existing lease at the Pepco Building at 929 E St. NW. The building is used by the university's school of business administration. The value of the one-year lease is approximately $2.75 million.

The Pepco Building is currently being rennovated at a cost of about $3.5 million. The building will receive a 20,000-square-foot addition as well as new elevators and heating, ventilation and air conditioning systems. Turner Construction Co. is completing the renovation project.

The building was constructed in 1935 for the Potomac Electric Power Co. by the George Hyman Construction Co. and has been known as the Pepco Buliding since that time.

Recent developments in commercial leasing in the Washington metropolitan area:

* IBM Corp.'s national federal marketing division has subleased 33,849 square feet of space at Rockledge Executive Plaza II in Bethesda. Rockledge Executive Plaza II is one of two, six-story buildings located on nine acres, bordered by Old Georgetown Road, I-270 and the Capital Beltway. The space is being subleased from Suburban Bancorp, which leased all of the building in 1981.

* The law firm Beckett, Cromwell & Myers has subleased 12,643 square feet of space at the Rockledge Executive Plaza II.

* The administrative consulting firm Prospect Associates has leased 11,600 square feet of space at the STSC Headquarters Building in Rockville.

* Lumberman's Mutual Casualty Co. has leased over 3,200 square feet of space at One Flint Hill, 10530 Rosenhaven St., Fairfax.

* Ziegler Corp., a development company, has leased over 3,000 square feet of space at 8200 Greensboro Dr. in McLean.

* Computer Sciences Corp. has leased over 11,000 square feet of space at 11150 Main St., Fairfax.

* Datatronix has leased approximately 10,000 square feet of space at 2100 Reston Ave., Reston. The aggregate rental value of the long-term lease is over $750,000.

* Computers Plus has opened its second store and first downtown outlet at 1729 DeSales St. NW. Computers Plus opened its first store in 1977 at 6120 Franconia Rd. in Alexandria.

* Primedia Inc., a public relations consulting firm, has relocated its headquarters from Silver Spring to 4330 East West Hwy., Suite 900, in Bethesda.

* Off-The-Rax, a chain of women's apparel stores owned by the Raxton Corp. of Boston, Mass., opened its 15th Washington store at Prince Georges Plaza in Hyattsville.

* Ritz Camera Centers Inc. of Beltsville has opened two new stores in Florida. The company now operates 101 Ritz Camera stores throughout the country.

* The National Association of Elementary School principals has relocted its national headquarters from 1801 North Moore St. in Arlington to 1920 Association Dr. in Reston.