Negotiations are in the final stage for selling the 800-unit Columbia Plaza luxury apartment building to two, large Canadian-based real estate firms for $50 million.

The purchasers, Cadillac Fairview of Toronto and Daon Inc., which has an office in Newport Beach, Calif., reportedly still are debating whether to convert the Foggy Bottom building to cooperative or condominium ownership. A certificate of eligibility for conversion had been obtained earlier from the D.C. government.

The sellers of the property, which has one of the District's highest assessed valuations, are D.F. (Nick) Antonelli, Kingdon Gould Jr., Dr. Laszlo Tauber, Waren Montouri and Edward Mernone. All are identified with real estate investing here.

Antonelli confirmed that the parties are now in almost total agreement on details. He also pointed out that the sellers are retaining the near-7-acre site, the 1,500-car parking garage and 30,000 square feet of commercial space in the five-building complex at 2400 Virginia Ave. NW. He said the Coldwell Banker realty firm represented both buyers and sellers in the negotiations, under way for weeks.

Completed about 10 years ago, the Columbia Plaza apartment have enjoyed 100 percent occupancy over the years because of prime location near the Watergate and FHA-imposed rent ceilings.

Columbia Plaza was built on land that had been purchased from the former Redevelopment Land Agency for more than $6 million and financed with an FHA-insured loan for $22.5 million. FHA controls kept the rents at a level that now range from $300 for an efficiency to $600 for three bedrooms.

The apartments have had a long waiting list of prospective tenants. Max Cleland, current head of the VA, now resides there. David and Julie Edsenhower, and a number of congressmen and White House officials have been tenants there.

In 1963, the land transaction was the subject of an inquiry by the House District subcommittee.

While final details of the sale were being ironed out, the Columbia Plaza tenant association has been working with attorney Benny L. Kass, in anticipation of the expected sale, to set up a possible purchase arrangement.

District law requires that organized tenants have 90 days to meet any purchase price if a buyer or owner contemplates conversion to condo or coop ownership. CAPTION: Picture, The 800-unit Columbia Plaza luxury apartment building: Tenants will try to match the $50 million sale price. By Douglas Chevalier-The Washington Post