IN THE BUSINESS . . . The value of the typical single-family home built since 1970 climbed to $70,900 in 1981, the government reported this week.

The median value of homes built in the last 13 years went up $3,600 from 1980 to 1981, according to a joint report from the Census Bureau and the Department of Housing and Urban Development. Those newer houses constitute about one-fourth of all homes in the country.

The median value of all single-family homes in the United States, regardless of when built, was $55,300, up from $51,300 in 1980. Median value means half of all homes were worth more and half less. The median gross rent of newer homes was $323, up $32 from the year before . . . Miller and Smith has announced plans for a 221-acre development at Darnestown and Dufief Mill roads in the Potomac area featuring 800 detached houses and town houses and a recreation center . . . Washington Realty Group has broken ground for a 49,000-square-foot medical office complex at 2112 F St. NW. Suites from 500 feet to entire floors will be available for purchase as condominiums, for lease, or for lease with option to purchase. Occupancy is planned for next August . . . John G. Georgelas and Sons Inc. of McLean has started construction on its Park Lane professional park on Old Court House Road in Vienna. Three buildings with a total rentable area of 93,000 square feet are planned . . . International Telephone and Telegraph's 42,000-acre Florida planned community, Palm Coast, has named Culmore Realty Co. of Baily's Crossroads and Fred L. McKee Realty Inc. of Bowie as brokers to represent the development in this area.

PERSONNEL FILE . . . The Federal National Mortgage Association has added three new directors to its government relations staff. They are William R. Maloni, director of congressional relations; David W. Roderer, director of legislative analysis, and William L. Warfield, director of industry relations . . . Scott R. Daugherty has been named vice president of investor marketing at the Federal Home Loan Mortgage Corp. . . . The National Corporation for Housing Partnerships has promoted Carol J. C. Mills and James L. Peerson Jr. to vice president . . . Merrill Lynch Realty has appointed George Middleton director of its new homes division for the Washington area, and has named George Bigus branch manager of its Springfield office . . . Spaulding & Slye has named Gail Scott assistant vice president and project manager in its Washington office . . . Steven A. Solomon has joined the leasing department of Julien J. Studley's Washington office, and David D. Agronsky has joined the sales department.

ON THE MOVE . . . Lanier Business Products Inc., will occupy a 45,000-square-foot building in Prudential Insurance Co. of America's Prosperity Business Campus in Fairfax. The space is for Lanier's Thought Processing and Electronic Office Systems divisions . . . The Federal National Mortgage Association has leased 48,000 square feet at 4250 Connecticut Ave. NW for expansion space. Julien J. Studley handled the transaction. In two other deals handled by Studley, Arthur D. Little Inc., a Massachusetts-based consulting firm, leased 12,000 square feet at 600 Maryland Ave. SW for a new Washington office, and the law firm of Mahn, Franklin & Goldberg has taken 12,500 square feet at 1718 Connecticut Ave. NW.