IN THE BUSINESS . . . The National Association of Realtors' housing affordability index hit 83.7 in April, meaning that a family earning the U.S. median income--$24,267--had 83.7 percent of the income necessary to buy the median-priced home, which last month was $68,900. The current level is the best in three years, according to the Realtors. The last time the index was 100, meaning that a median-income family could buy a median-priced house, was in December 1978 . . . Omni Construction Inc. has signed a $6.1 million contract for the Demonet Building project at the corner of Connecticut Ave. and M St. NW. The project involves construction of a 12-story office building and renovation of the 19th century Demonet Building, including restoration of the old building's exterior. The architect for the project is Skidmore, Owings & Merrill; the owner is Second British American Properties Inc.; the developer is Viking Property Group Inc. Completion is scheduled for next May . . . A Hispanic group, Pan American Associates, is planning a mixed-use retail and office complex on University Boulevard in Langley Park. Tentatively titled Central Plaza, the 95,000-square-foot development is being financed with industrial revenue bonds issued by Prince George's County . . . The Van Ness Station office building, developed by Prudential Insurance Co., has opened. The 200,000-square-foot structure has two floors of retail and five floors of office space, plus a Metrobus station and, of course, the Van Ness Metrorail stop beneath. It is at 4250 Connecticut Ave. NW . . . The buyers of mobile homes are younger, more affluent and better educated than they used to be, according to a survey by the Manufactured Housing Institute. It found that the median income of buyers of new mobile homes was $16,881, and that more than a quarter are white-collar workers, up from 20 percent in 1974. The buyers surveyed paid an average of $21,861 for their homes. FILE . . . Oliver T. Carr Jr., president of the Oliver T. Carr Co., has been appointed to a three-year term on the board of directors of Partners for Livable Places . . . Craig A. Mason has been named vice president and executive director of the Thrift Industry Accounting Committee of the U.S. League of Savings Institutions . . . Coldwell Banker Routh Robbins-Realtors has named Virginia Jones resident manager of the firm's Arlington sales office.

ON THE MOVE . . . Wang Laboratories has leased 10,000 square feet at 1019 19th St. NW. In addition to offices, Wang will open its first "retail" store on the first floor. Greenhoot Inc. handled the transaction. Wang also recently leased 22,000 square feet at 1300 N. 17th St. in Arlington, where Julien J. Studley is the exclusive agent . . . Coldwell Banker has been appointed exclusive leasing agent for 1718 Connecticut Ave., with 35,000 square feet of rentable retail and office space.