The Bar Association of D.C. will sponsor a housing ownership seminar -- covering condominiums and cooperatives and open to lawyers, tenants, community workers and the public -- from 9 a.m. to 1:30 p.m. May 17 at Marie Reed Community Learning Center, 2200 Champlain St. NW. For information, call 638-1500.

The Oliver T. Carr Co. has started construction of the final phase of its $75 million International Square office-retail complex in midtown. The newest building at 18th and I streets NW will have access to an entrance to the Farragut West Metro station. Equitable Life Assurance Society and George Hyman Construction Co. are partners with the Carr firm in the 1.3 million-square-foot project.

Montgomery College and the Montgomery County Board of Realtors is sponsoring a real estate career program today from 11 a.m. to 2 p.m. in the fine arts building on the college's Rockville campus. For information, call 587-2246.

The Braedon Companies has been named Washington affiliate of the Office Network Inc., which works on national corporate expansions and relocations.

Town & Country Properties has joined the Homerica broker-to-broker referral system to help service the 275,000 homebuyers transferred annually from one community to another.

Franklin Paulson of Weaver Bros. Inc. and James A. Barker of H.G. Smithy Co. have been accepted for membership in the Society of Industrial Realtors.

Broker Sidney Spiegel has moved his real estate office to 2401 Blueridge Ave., Wheaton.

Black's Guide, publisher of an annual compilation on the office space market in the Washington-Baltimore area, has moved its regional office to the new Air Rights Building in Bethesda. Michelle Liliestedt is the firm's new research director.

The Northern Virginia Board of Realtors reported 1,432 residential sales were handled through the multiple listing service in the first four months of 1980 for a total of $126 million, down considerably from the 2,530 sales for $199 million in the same period in 1979.

The D.C. Builders Association will hold a "house for sale" program May 20 at 8 p.m. at the Touchdown Club. Panelists will include Sarah Oleskar, Richard Barrow, Sue Ives and Joseph Robert Jr.

Wintergreen, the mountain and valley condominum dealer in Charlottesville, Va., reports it had total sales of $6.1 million in the first quarter of 1980, 153 percent higher than the sale total for that period last year.

Thorsen Real Estate and Constructin Co. has opened offices at 113 S. St. Asaph St., Alexandria.

David Robinson and Clark Massie, owners of Realty World/New Town Realtors, recently opened a branch office in the City of Fairfax. It is the 56th Realty World franchise office in this region.

The Carey Winston Co. has been named management and leasing agent for the office building at 1325 G St. NW, now owned by Prudential Life Insurance Co.