When Sonya Hoots and her husband moved into an 18-story luxury condominium building in Crystal City a little more than a year ago, they believed the lot next door would soon hold a sister condominium building.

Today, there is a hole almost as big as a football field next door, and the Hoots are among scores of area residents fighting a zoning request by the Charles E. Smith Co. that would allow the developer to build mostly offices--not residential units--on the Crystal Gateway site. "We would prefer to have residences next door to us," said Hoots. "There is enough office buildings already in Crystal City."

For officials of the Smith Company, Arlington's biggest development firm, the question is one of economics. Sales have been slow on the condos, with marble baths and pricetags up to $400,000. Offices might be more marketable.

"We found that the market is there, but it's smaller than we ever thought," said Joseph A. Klea Jr., vice president of Smith's condominium department. "There just aren't that many people with that kind of money."

But area residents and county planners say the dispute goes to the very heart of neighborhood preservation. Unless Arlington can hold the line against proposals like Smith's, they say, the rapidly developing county could find itself with more Rosslyn-style office jungles that close up every evening when office workers go home.

"Rosslyn's just really not much of a place to live, and there's really not much we can do about that now," said Robert E. Brosnan, a county planner. "We're trying to plan Crystal City so we wouldn't have those problems."

Control of development is a potent political issue in Arlington, which has seen a forest of office and residential buildings sprout since the 1960s. Many more office and commercial buildings are on the drawing board, and county economic development officials are projecting that the recent leasing of a large office building in the Courthouse area will soon prompt a new office boom.

Democratic-backed County Board member Mary Margaret Whipple predicted yesterday that the board will reject the Smith proposal. The county planning staff and a County Planning Commission advisory committee have recommended against approval of the plan.

"It's up to the County Board to keep sight of what we want in the long run for the county," said Whipple, whose upset victory over Board Chairman Stephen H. Detwiler last fall was viewed in part as an antidevelopment backlash. "We have to be wary of the developers' short-term profits, as opposed to a long-term business interest in the county."

The Smith Company's original plans for the north Crystal City site called for the construction of five buildings, two of them residential and the rest office, commercial and hotel space. With four of the five buildings completed or begun, Smith came to the county late last year requesting permission to convert 60 percent of the space in one of the crescent-shaped residential buildings, or some 200,000 square feet, to offices.

The new Smith proposal calls for building the deleted residential units as part of the neighboring Crystal Park project, allowing the developer to build smaller, less expensive units that might sell more readily in today's depressed market.

"We came up with a plan in order to finish the site," said Klea. "Based on sales, we don't believe we could build another 174 residential units of the same size and sell them."

The proposal brought an immediate outcry from residents of Crystal Gateway south, whose ranks include wealthy business leaders and congressmen lured by the project's dual swimming pools, sauna facilities, marble bathtubs and pink quartz exterior.

Many worried about the loss of what they had hoped would be a neighborhood atmosphere, while others griped that an office building next door would jeopardize their property values.

"There was a concern that it would just turn into concrete and towers every evening," said Arnold M. Greenspan, an engineering consultant who heads the condominium's ad hoc board of directors.

The Planning Commission had been expected to consider the matter last week, but the meeting was canceled when the commission could not get a quorum.

The developer and condominium association are now asking for a 30-to-60-day deferral to resolve their differences. graphics/1 photo: The Crystal Gateway condominiums stands at left, and the disputed tract is in the foreground. By Ja,es A. Parcell--TWP graphics/2 map: Crystal Gateway