Condominium sales are heavy in the northeastern United States, Chicago and Detroit, according to a new U.S. Housing Markets survey. Condo sales in the Washington area are up, but not as strongly as the market for single-family homes.

Almost everywhere else, however, the condominium market is in decline, the report said. In the South, 79,000 condos were started last year, only slightly more than half of the 1983-84 average. At least 250,000 condos in the South reverted to rentals in the past two years, more than were built in those years.

But in the Northeast, condo starts totaled 78,000 last year, more than double the figure for 1983 and four times the 1981 total.

In the Washington area, 1985 was the first year that new condo sales equaled those for conversion units -- 3,150 each. The projection for this year is that 300 to 400 converted units will be sold and about 4,000 new units.

IN THE BUSINESS . . . Baier Properties Inc. is planning a $50 million, two-building office complex near the Capital Beltway and Rte. 50 in Fairfax County . . . International Developers Inc. has started construction on Belvedere West, a 17-story condominium in Rosslyn . . . The Washington development firm of Hoskinson & Davis Inc. has started a new company, Modern Property Management Inc. . . . St. Louis, for the second straight year in 1985, led the nation in the use of federal tax credits for rehabilitating historic buildings, according to the National Trust for Historic Preservation . . . Georgetown University Law Center and the D.C. Bar are sponsoring a seminar Tuesday, at 600 New Jersey Ave. NW, on current issues in zoning, planning and development regulation in the District . . . Washington-area builders are holding their annual fall convention this year in late October on St. Thomas in the Virgin Islands . . . Winn Group, developers of Lake Arbor, a residential community near the Capital Centre, has donated a one-acre site valued at $110,000 to the Prince George's County Chamber of Commerce for its new headquarters . . . Pat Stevens of the Shannon & Luchs realty firm's Fairfax office has been named the company's top sales agent for 1985. The firm sold 1,940 homes worth $246.3 million in March, up 45 percent over the same month in 1985 . . . Construction contracts in the District totaled $219.4 million in the first three months of the year, up 18 percent from the $186.5 million figure in the first quarter of 1985, according to the F. W. Dodge Division of McGraw-Hill Information Systems Co. But the March 1985 figure of $79.6 million dropped to $69 million . . . Julien J. Studley Inc. reports that 1,067,391 square feet of office space was leased in the District in January and February, with 542,601 of it in new buildings. During the same period, 842,450 square feet was leased in suburban Maryland and 996,495 in Northern Virginia . . . Loyola College Business Center has broken ground for a new office park in Columbia . . . Potomac Investment Associates has selected seven companies to build at Avenal in Potomac, the planned community revolving around a new Tournament Players Club golf course. The seven are C-I/Mitchell & Best Co., Samuel P. Pardoe, Rocky Gorge Properties Inc., Porten Sullivan Corp., Saddlebrook Development Corp., Martens Edward Development Co. Inc. and G. B. Builders & Developers Inc., which will build 750 homes on lots ranging from 10,000 square feet to two acres . . . Erkiletian Construction Corp. has started construction on Park Center IV, a 14-story, 240,000-square-foot office building at King Street and the Shirley Highway interchange. The new building is the fourth of six planned for the 1.3-million-square-foot complex . . . Weaver Bros. Inc. has opened its first Northern Virginia commercial sales and leasing office in Chantilly in western Fairfax County . . . NVProperties says it has plans to build a total of 1,774 rental units at four locations in Fairfax and one in Montgomery County . . . Pennsylvania Avenue Development Corp., which is overseeing the redevelopment of the avenue, has sold the Market Square site between Seventh and Ninth streets NW for $26.14 million to Avenue Associates Limited Partnership, a group that includes principals from Western Development Corp. of Georgetown. The partnership won the right in November 1984 to build a masive office-retail-housing complex on the site. Construction is expected to start later this year . . . Harvey Construction Co. is building a $4.2 million headquarters, refrigerated warehouse and distribution center for Washington Beef Co. at the Capital City Business and Industrial Park at W Street and Brentwood Road NE . . . Fifteen Reston home builders sold 250 homes worth $35 million in the first three months of the year . . . Sign of the times: Rouse & Associates is providing on-site child care for 72 kids at the Fair Oaks Corporate Center . . . The Loudoun County Board of Supervisors has approved the rezoning of a 275-acre tract along Rte. 7 that will permit Evans Co. to build about 4.8 million square feet of office space, starting next fall . . . First Housing and Construction Corp. and Curry Development Inc. are rehabilitating the boarded, 118-unit Southern Court apartment complex at Southern Avenue and Chesapeake Street SE . . . Allied Van Lines has started a new program, called Money on Relocation Expense, in which it will pay home sellers or buyers $100 for for every $10,000 of a real estate transaction if the individuals move their belongings with Allied and use the firm's national real estate referral network . . . The $20 million, 222-unit Regency Condominium project in McLean, developed by K&M Properties and built by Donohoe Construction Co., has been completed.

PERSONNEL FILE . . . Legum & Norman Inc., a community association and rental property manager, has named John A. Rhodes and Fredda F. Ruppenthal as senior vice presidents and Ahmed H. Motawie and Ellen S. Linder as vice presidents . . . Nelson J. Cooney has been named president of the Brick Institute of America in Reston . . . C. Neal Fleming Jr. has been named president and chief operating officer of Development Resources Inc. and Candice C. Fazakerley chief executive officer . . . John T. Caldwell has joined Spaulding & Slye's Washington office as vice president . . . Artery Organization, a Chevy Chase real estate development and financial services firm, has promoted Michael A. McGinn to senior vice president, director of property acquisition . . . Brian Tucker has been named vice president of leasing for Evans Co. . . . John Simon, director of housing for a New York Teamsters local, has been elected president of the National Housing Conference.