A 29-acre tract of industrial land at Backlick Road and the Beltway in Alexandria has been purchase for $3 million by Robinson Terminal Warehouse Corp. Formely occupied by Boise Cascade, the land was purchased from the Leachman family, which manufactures and sells vacation houses. Nathan Isikoff of the Carey Winston Co. neogitated the transaction for both parties.

Entertainer Mark Russell has acquired a two-storey condominium residence in the new Worland Town building at 2828 Wisconcin Ave. NW for $130,000. . . . Mark P. Riedy, new executive vice president of the Mortgage Bankers Association, said he has purchased a contemporary house in Great Falls, where price appreciation has been strong in recent years. . . . George and Ada DeFranceaux have purchase a condominium apartment in the Foxhall, 4200 Masachusetts Ave NW, for $133,550. The mortgage banker and his wife lived at 2029 Connecticut before that fine old rental building was converted to condominium ownership.

And speaking of housing: "The needs are so great we must make use of every resource we've got. . . the luxury we could earlier enjoy of neglecting the existing stock (of housing) is no longer an option open to us." Nathaniel H. Rogg, former executive of the National Association of (new) Home Builders, said that this week at the White House Conference on Balanced Natinal Growth and Economic Development. Rogg now is a consultant to an association of savings and loan associations. S&Ls, which have become a lot more interested in in-town lending and rehabilitation in recent years, make more loans for existing houses than they do for new ones.

A year ago pre-selling began at the Rotonda, the 1,200-unit complex of condominium apartments at Tysons Corner. As of this week, 13 units are occupied in the recently completed first building and eight others have gone to settlement. The 239-unit first building is sold out and the second building, now under way, is reported to be 40 percent sold. Prices range from about $40,000 to $91,500.

An office building at 1100 Vermont Ave. NW was sold for $2.2 million by attorney-investors H. Max Ammerman and others. The federal government is the dominant tenant. . . A small office building at 2461 Wisconsin Ave. NW was sold for $525,000 by Mr. and Mrs. A. H. Eckles to Carl H. Richmond Jr.

Appraiser Alfred W. Jarchow reports that the median price of existing homes sold in Montgomery County in December was $64,150. About 20 percent of the sales involved town houses and condominium apartments. Also, Jarchow figures show an average price increase in existing house sales of 8.1 percent in 1977, which was slightly less than the 8.5 percent increase in 1976, 8.8 per cent in 1975 and 8.6 in 1974. The record upturn was 18 percent in 1972.