Back when bus trips from the suburbs to the Capitol were 15 cents and parking wasn't a problem at National Airport, Old Town Alexandria was a spunky trade and tourist district concentrated in the six blocks west of the Potomac River waterfront.

Thirty years later, in the 1970s, Old Town became almost a new Georgetown: a ritzy district with designer boutiques, exclusive restaurants and specialty shops. Still, it remained the same compact size.

Now Old Town -- the historic core of Alexandria that most define as the downtown commercial area -- is in the midst of a development explosion, which for the first time could move the focus of business and consumer activity away from the waterfront and toward the two-year-old King Street Metro stop.

In the past 12 months, developers have proposed new office buildings, apartments, restaurants, cinemas and hotels worth, all told, $300 million in the Metro station area. In addition, 11 restaurants are scheduled to open within a few minutes' walk of Metro before the end of 1987.

The just-completed red brick office buildings and the influx of hard-hatted construction workers are a sign that Old Town, an 18th century port that claims George Washington and Robert E. Lee as home town boys, is being reshaped in size and texture.

"Our design is to have a concentration of activity around the King Street Metro: outside cafes, movie theaters, plazas, a hotel," said Mayor James P. Moran Jr.

The mayor said that no longer are visitors from the District who drive in on the George Washington Parkway going to turn left automatically and head to the shops and restaurants in the six blocks nearest the river.

"With two ends of King Street having a good deal of pedestrian circulation, the rest will fill in," Moran said. And as the 15 blocks of King Street between the Metro station and the waterfront become one lively strip, Old Town, as many know it, will double in size.

Sheldon Lynn, the city's planning director, says he believes that the Metro station, which opened in December 1983, increasingly will bring tourists who will browse, shop and eat on upper King Street, a place that four years ago was full of used car lots and dilapidated houses.

Lynn said that along with bringing "more variety, more galleries, more restaurants, more revenue and more activity" to Alexandria, "the expanded notion of Old Town" will likely bring the problems already seen on the waterfront: crowded parking, traffic and noise.

The Oliver T. Carr company almost single-handedly is shaping the renovation with more than $250 million worth of planned development, including the 845,000-square-foot King Street Station office-retail complex. Carr said his firm is doing more construction in Alexandria than in any other suburb because of the "scale of the town, the shops, the entertainment."

Some residents and officials say, however, that precisely what is most attractive about the city -- its low-rise buildings and small-town atmosphere -- may be destroyed by those developers it is now enticing.

Said Dayton Cook, the city's transportation and environmental director: "Too much of a good thing is bad."

Cook said the bulk of the new construction lies within the Old and Historic District, which has been expanded many times since its creation in 1946 and now runs from one block short of the Metro station to the Potomac, and from the George Washington Parkway to Jones Point.

Because a review board attempts to ensure that no building in the preservation district is taller than seven stories or clashes with the 18th and 19th century motif, Cook said he is concerned that the district might become unwieldy and perhaps even lose a bit of charm as more red brick Colonial-style buildings are erected:

"The more you try to preserve a bigger area," he said, "the more likely you are to dilute the original character."

But City Manager Vola Lawson said the city could easily expand the number of people reviewing the architecture of the district, as the area itself expands. Along with boosting the tax revenue, Lawson said, the development will add "vitality to the city."

In 1983, Lawson noted, the city invested $3.5 million in improvements in the Metro station area, including cobblestone bricks and Colonial lanterns -- touches that now are reaping benefits.

Moran said, however, that along with drastically upgrading the upper King Street area and eliminating loiterers from street corners, there is a concern that the development many be pricing some established small businesses out of the market.

Sammy Terlitzky, who has run his kosher delicatessen at 1324 King St. since 1974, said his assessments and taxes have risen significantly but that he is counting on the new offices and the 142 new national associations in the area to beef up business.

"I don't think we'll have any problem," he said. "I know there are a lot of new restaurants going up, but I try to never let it bother me."

Terlitzky said other small business persons trying to locate on King Street now would probably find it too expensive. Even when he took out his mortgage 11 years ago, he said, the price was just about what he could afford, and "the area was a hangout for derelicts and drunks. It was awful. I didn't want my family to be around after 5 p.m., so that's when we closed."

City Assessor David J. Chitlik said that the land around the King Street Metro station is now the most expensive in the city, surpassing even that on the waterfront, and it has at least doubled in value since Metrorail service began.

A lot in the 1900 block of Duke Street that was assessed at $30 a square foot in 1982 was sold this year for $65.72 a square foot, Chitlik said.

And as Metro ridership increases -- up already from a combined weekday total of 2,000 boardings and alightings at the station in 1983 to more than 5,000 in 1985 -- and as automobile and bus traffic worsens in the area, the land within walking distance of the King Street station will be even more valuable and vital to the city's economy.

"This is it. The new business district," said G. Barton Middleton, executive vice president of the Alexandria Chamber of Commerce, as he unfolded a new promotion poster of the city.

Instead of the traditional photos of Gadsby's Tavern, where George Washington is said to have quenched his thirst, or the Torpedo Factory, a renovated arts and office complex on the waterfront, the poster showed a new view of Old Town: one that originated at the Metro stop.