By Sunday it will be over. Fairlington Villages - 30 years ago a rental complex swarming with young couples and their children and more recently home to some 900 senior citizens - will lose the last of its tenants.

The few remaining apartments at Fairlington will be converted into condominiums by this winter and, if experience serves, the transformation from rental apartments to condominiums also will change the kind of resident who lives there. The "new Fairlington is likely to be a young government worker with a relatively high salary and no children.

Fairlington was "taken away from the old renters and given to the young," said a 73-year-old widower helplessly as he recently moved from the apartment he had occupied for 26 years.

The very young are eager to get them. On Memorial Day, buyers were lined up at 7:30 a.m. awaiting the opening of the sales office at 10. Two buyers ended up flipping a coin and drawing straws to determine who got to buy one unit that won't be finished until winter.

The popularity of the townhouses, which now cost an average of $51,000, illustrates the acute shortage of relatively moderate income housing close to Washington.

The conversion of Fairlington, straddling the Arlington-Alexandria line, began in 1972. It was one of the first complexes to be restored for sale as condominiums in the Washington area, and its success prompted other conversions.

Over the past six years some 20,000 housing units that were once for rent have been converted to condominiums in the Washington area. However, there are still more than 500,000 rental-units here.

The conversion statistics do not include developments, like the massive Parkfairfax complex just across the Alexandria line from Fairlington Villages that are in the process of being converted.

Fairlington's brick and stone homes, built by the government during World War II to house its workers, were primary sources of moderate income rental housing in Northern Virginia for more than 30 years. Its 3,400 town houses and apartments, divided into North and South Fairlington, are scattered over a broad expanse of 300 acres on gently sloping hills.

Fairlington now is owned by CBI Chicago Cridge and Iron Works Fairmac, which began the $100 million conversion to condominiums more than five years ago.

Why? "We were faced with properties that were 35 years old that needed massive rehabilitation. We felt a first class restoration was in order and we decided it would serve a housing market that there was a crying need for inside the Beltway," explained Walter Hodges, president of CBI Fairmac.

Hodges said the owner of a deteriorating rental apartment acomplex decides to convert when he realizes he's got to do "Something drastic."

"It happens when you're renting at market level and your operating costs for your 25-to 30-year-old buildings increase more rapidly than rent," Hodges said. "It gets to the point where you're no longer just painting and cutting the grass. Your boiler rooms and utility systems are starting to break down."

When CBI Fairmac announced plans to convert Fairlington, there was a brief flurry of resistance, but the conversion went on.

McLean Gardens, a rental complex in Northwest Washington also owned by CBI Fairmac was a different story. The company's plans to change McLean Gardens, first to condominiums and later to an elaborate diplomatic enclave of apartments, shops, boutiques, embassies and chanceries, were met with strong and immediate opposition from the tenants association.

James McCabe, president of the tenants association at McLean Gardens, said the residents have so far prevented conversion because "we've successfully talled them, and caused an economic bind so that they couldn't make the profit. By the time they cleared the legal hurdles we put in their way they no longer could do the conversion at the price they had proposed."

He said that McLean Gardens did not lend itself as readily to condominium conversion as did Fairlington, and because Fairlington is in Virginia, it was not subject the same rent controls and conversion moratoriums as McLean Gardens, he said.

The company left standing the "shell" of the homes, but replaced "every pipe every wire - it was the largest total restoration in the country," Hodges said. When the first were sold, the units averaged $31,000, he said, but the average price now is $20,000 higher. Slightly more than 400 units or about 60 per cent of the condominiums in the first three villages were bought by former tenants, according to figures from CBI Fairmac officials.

Typical of the "new" Fairlingtonian is Howard Kinitsky, 27, a work study program specialist at Gallaudet College and his wife, Diane, 25, a stewardess for Eastern Airlines.

The Kinitskys moved to the metropolitan area from New York last March, first living in a Crystal City apartment where the rent was $350 a month until hey discovered they could afford a $47,000 home at Fairlington.

"I guess I'd sum up how we feel this way. This isn't our eventual dream house, but it's a lot better than living in an apartment," Kinitsky said, after taking a reporter on a quick tour of his living and dining rooms, kitchen, bedrooms, and the den and office in the basement.

Linda Yahn, 33, and the mother of twin boys bought her South Fairlington town house four years ago. One of its attractions, she said is that she doesn't have to worry about the lawn or the pool.

"I'm basically lazy," said Mrs. Yahn, a research assistant for a writer. "I'm not a gardener . . . On the weekend, I'd much rather my husband take care of the boys than take care of the yard."

She says she likes Fairlington's oldness, its trees, schools and its closeness to Georgetown. I couldn't stand living 45 minutes from the city," she said. "We bougth it as a starter house, but now we aren't very anxious to move."

One of the Fairlington tenants who refused to buy a condominium is Vernon Foster, 70, who moved into a Fairlington apartment when Dwight Eisenhower was President and the block was 75 per cent Republican. He has moved to high-rise apartment across the highway from Fairlington, but he returned to his old apartment recently to move his marigolds from a side yard to a flower pot on the balcony of new apartment.

"I'm scared of condominiums." Foster said "I might not have enough control over the condominium fee, and I'm afraid that some day it might approach my rent."

His new home, Foster said is "dark, restricting," compared to his old Fairlington apartment. "It was like living in the country with windows on three sides with my own room in the cellar," he said.

Another former tenant remembers seeing "one little old lady who must have come by here 15 times before she moved," each time carrying a couple of boxes.

"These people are in their 70s and 80s," he added. "I don't know where they're going to be able to go now."

In North Fairlington, which has more singles and more apartment-style condominiums than South Fairlington, a third of the families make $25,000 a year or more, according to a demographic study. In South, the figure is 41 per cent. About 10 U.S. congressmen and about 10 black families live at Fairlington, according to Tommy Grimes, sales director.