The final and highest piece of structural steel has been installed at TransPotomac Plaza, a five-building office complex on the Alexandria waterfront. Developed in conjunction with Dutch financing institutions by Savage/Fogarty Cos. Inc., the structures range in size from 33,000 to 110,000 square feet with total rentable space of about 305,000 square feet.

Some space in the complex at 1001, 1033, 1055, 1111 and 1199 N. Fairfax St. will be ready for occupancy beginning this winter; the project is scheduled to be completed by May, 1983. Leggat McCall and Werner Co., Inc., is the leasing agent.

Other recent developments in commercial real estate in the Washington area:

* IBM's Federal Systems Division, a data processing marketing group, has leased a three-story building with 84,000 square feet at 704 Quince Orchard Road in Gaithersburg.

* Time-Out Family Amusement Centers, Inc., has leased a 22,000-square-foot building at Fairfax Executive Park. The company will use the facility at 3157 Pender Drive in Fairfax as its corporate headquarters.

* Mallorey Office Supply Co. has leased 23,754 square feet at 3055 V St. NE.

* Kiles and McDermott, a law firm, has leased 2,500 square feet at the Canal Square Building, 1054 31st St. NW.

* Mujica and Berlin, an investment consulting firm, has leased 3,800 square feet at 2828 Pennsylvania Ave. NW.

* The Fred Ezra Co. has been named exclusive leasing agent for the Madison National Bank Building at 2833 M St. NW. Two floors totaling 6,600 square feet square feet are available.

* Mudge Rose Guthrie and Alexander, a law firm, has leased 17,028 square feet at the McGregor Building, 2121 K St. NW.

* Northern Telecom, a communications firm, has leased 3,712 square feet at Capital Gallery, 600 Maryland Ave. SW. Sperry Univac's air traffic control systems division has leased 1,347 square feet at the same address.

* Legal Cousel for the Elderly, a law firm, has leased 3,439 square feet at 1331 H St. NW.

* Three firms have signed leases at the Maryland Trade Center, 7500 Greenway Drive in Greenbelt: Lawyers Herbert Reichelt, Paul Nussbaum and Josef Brown have leased 5,400 square feet; SDA Federal Systems, Inc., a subsidiary of Software Design Associates, Inc., has leased 2,170 square feet; and Xanadu Limited/Jasper's Restaurant has leased 277 square feet.

* Financial Planning Center has leased 2,054 square feet at the Gould Building, 5900 Princess Garden Parkway in Lanham. The firm, which is based in Connecticut, will use the space as a training facility.

* Van American Network, a transportation management company, has leased 1,802 square feet at the Lexington Building, 11900 Parklawn Drive in Rockville. The firm, which provides passenger vans for corporate and commuter groups, will use the office as its corporate headquarters.

* Four firms have leased space at the Concord Building, 10605 Concord St. in Kensington: Hodge, Hart and Ewing, Inc., a general insurance company, has leased 3,354 square feet; Benjamin/Clark Associates, a fire safety consulting firm, has leased 1,471 square feet; Space Savers Systems, Inc., a distributor of filing and storage equipment, has leased 739 square feet; Expo Systems, Inc., a manufacturer of modular exhibits, has leased 1,496 square feet.

* 1217 E Street Associates, a real estate investment partnership, has signed a 50-year ground lease for an office building at 1217 E St. NW. The group plans to renovate the 20,000-square-foot structure and sublease the space.