Four years ago, when the idea of buying condominiums was fairly new to many area residents, the city housing department approved the conversion of only 613 apartments in the District of Columbia, many of them in Adams Morgan, a neighborhood that was rapidly growing in popularity.

But the condominium conversion movement has grown to such an extent that during the first months of this years, more than 5,000 housing units located in many different areas of the city have been approved for conversion, according to statistics released yesterday by the D.C. Rental Accommodations Office (RAO).

The Study released yesterday was the first phase of a comprehensive look at changing patterns in the city's rental housing market being conducted by the rent administrator's office.A city rental official said yesterday that figures on actual conversions that have taken place in the last four years are not years are not yet available. But the increase the number certificates permitting conversions holds both good and bad news for the city according to Bowles C. Ford, rent administrator.

"While the conversions provide substantial benefits in terms of the tax base and the housing industry." Ford said in a statement, "the social damage is significant as the city loses rental units for those citizens whose limited resources lock them into the rental market." The District of Columbia has between 170,000 and 180,000 rental apartments and houses, according to RAO figures.

Evelyn Onwuachi, a spokeswoman for the City Wide Housing Coalition, said: "We're losing more and more (rental) units every days. We have to do something to preserve the housing that's here. We need airtight controls - a moratorium on conversions. We have to stop displacement until we can figure out how we're going to be able to stay in the city."

The conversions - or at least applications for them - have progressed steadily since 1974, the RAO study found, as demand for single-family housing has increased and, as many apartment building owners contend, rent control has made it increasingly difficult to get an adequate return on their investment.

John T. O'Neill, executive vice president of the Apartment and Office that he believes that the new statistics on conversions are misleading. He noted that getting a certificate of eligibility is the first step in a long process leading to actual conversion, and that many owners who get certificates do not intend to convert immediately. O'Neill said statistics he obtained from the city housing department show that 985 rental apartments and houses were actually converted to condomniums in 1977 - well below the 2.323 certificates granted last year.

Apartment owners are "being forced by City Council action to get certificates of eligibility," O'Neill said. "It's becoming more difficult to get one, what with the nonsense of the council fooling around with the rental rates every month," he added.

Indeed, the condominium conversion movement has taken place despite restrictions limiting conversions to buildings with high rents only. The council has twice this year passed emergency legislation raising the rent levels at which buildings are eligible for conversion, although the levels approved were not nearly as high as tenant organizers were hoping. The most recent legislation approved by the Council would, for example, mean that a one-bedroom apartment renting for less than $267 a month could not be converted.

Mayor Walter E. Washington also has proposed several measures that he said would protest renters. One would raise rent levels even further - to $300 for a one bedroom - in order for a building to qualify for conversion. The mayor also has proposed that landlords would reapply for certificates to convert if they have not converted within six months.

James Burns, director of research for the RAO, found in his study that in 1974 the main conversion movement was in Adams-Morgan, with some units converted in Ward 3. In 1975, when there were 776 conversions approved, the main movement was across the Dupont Circle area towards Shaw, with a "tiny but important movement" northwest of Capitol Hill and some low-cost conversions in Southeast Washington.

By 1976, when there were 1,083 certificates granted for conversion, the movement had spread through Adams-Morgan toward Mount Pleasant, continuing towards Shaw. Northwest of Capitol Hill, condominium conversion interest became heavier, and continued in Ward 3 and in Southeast.

Last year, when 2,323 units were approved for conversion, the pattern was generally the same as in 1976, only heavier, Burns found.

And during the first part of this year, the same pattern continued, getting wider and deeper, converging towards Shaw on the west from Foggy Bottom and Mount Pleasant and on the east from Capitol Hill. Burns believes tha the pattern of evictions closely follows the movement of condominium conversions. However, as a condominium specialist for the housing department noted, some of the buildings approved for conversion had been vacant for years.