For months the unfinished building has dominated the Rockville skyline -- 15 stories of concrete -- and now finally, the Montgomery County Executive Office Building is about to come to life.

By this evening, 106 Transportation Department employes will have finished packing their files, typewriters and office plants; by Tuesday they'll be in their new officers. And as other county workers join them in the new building in coming months, the City of Rockville will be watching closely.

Some 3,500 people will work in that building, a new circuit courthouse being completed next to it and adjacent buildings. City fathers hope their presence and buying power, coupled with Metro service due in 1983, will carry to success the city's latest redevelopment plan and purge Rockville's long-standing reputation for suburban blight.

A federally financed renewal scheme of the 1960s promised to create a vibrant, aesthetically pleasing central district for Rockville, the second largest city in Maryland. Though other aspects of the plan succeeded, this one failed, and downtown remained a place where cars were more common than people.

Now the city is trying again, this time relying heavily on private money. But some developers and civic groups wonder if old questions of access and financing might defeat this new strategy too.

Eighteen months ago, the city formally adopted a plan by the urban design firm Arthur Cotton Moore/Associates. Its sketches show a pleasing mix of hotel, dinner theatre, farmers' market, high-rise apartment houses and crowd-drawing gimmicks such as rubber-wheeled trolley cars and a solar-powered fountain.

Many pedestrian areas would be reopened to vehicles. Roads leading into Rockville would be upgraded and widened to accommodate new levels of traffic. At the same time, parking spaces in the county ofice center have been limited to encourage carpooling and use of public transportation.

Like its predecessor, the Moore plan begins at Rockville's historic Red Brick Courthouse, topped with a tower and dating to 1891. But it is far more ambitious in scale, covering 438 acres to the old plan's 46.

A plaque in the adjacent Courthouse Square proclaims the spot is already "Rockville Town Center." But in fact, it fills only during warm lunch hours and concerts that the city stages. Most of the time its fountain with concrete sculpture is dry.

Opening onto the square is another white elephant, the shopping mall known as "The Commons at Courthouse Square." Nine years after its opening, the mall is half empty and is notable for boarded windows and leaking roofs.

The privately owned mall was intended to draw the people to "amenities" that the city and federal dollars built around it.It didn't. Inconvenient road access, inept management, lack of a major department store and competition from nearby Montgomery and White Flint malls are most commonly blamed. Business is so bad, in fact, that tenants have repeatedly been threatened with shut-off of power and water because the mall's owner, Rockville Redevelopment Associates, has been behind in payments, according to Douglas Horne, Rockville community development director.

Moore's plan does not directly deal with the future of the mall, which Horne argues will eventually become valuable property due to its proximity to the Metro station and development that the new plan will spawn. Rockville's task now is to get building under way.

Last week, the city committed $1.5 million of its own money to improve Courthouse Square. A block-long arcade, street furniture, landscaping and other amenities will be put up.

The city's attention now focuses on two plots it owns in the city center itself. One, just west of the mall, is slated for the hotel, farmers' market and convention center (though Horne points out that the Moore plan is flexible about what goes where). The second, just to the north across Middle Lane, is zoned for offices.

Horne said the city had received proposals from three separate development groups for this land. One group withdrew after four months of talks, a second devised a plan linked to taking a share in the shopping mall and is now stalled by differences with the mall's prime financier. Talks are just beginning with the third group.

Privately owned land to the north of the courthouse would be developed in conjunction with the plan, the city hopes, as government workers, Metro and the initial shops and restaurants lead to further development. Officials estimate the plan will take 15 years to complete.

But some developers feel that some of the factors that ruined The Commons will work against the latest plan too.

"There's no reason for people to go there to shop," said one developer familiar with the plan who asked not to be identified. He argued that developers, shoppers and the stores themselves favor sites closer to Washington and more easily reached. There are still plenty of these around, he said.

Interest in the plan had been sluggish, he said, suggesting it might succeed if done on a smaller scale. Rehabilitating the mall would be a good first step.

Civic groups, meanwhile, have centered their concern on the problem of traffic congestion. "If the plan is ever implemented, you might as well leave your car at home," complained Michael Gordon, president of the Rockville Civic Federation. He suggested that "the city was more concerned with building a commercial tax base" than improving the quality of life. "The plan is a dream and will never be completed," Gordon predicted flatly.

Rockville officials counter that the plan is already going ahead. And they point out that the new county office building will be full by July, with Executive Charles Gilchrist and his staff among the last to arrive.

In a game of bureaucratic musical chairs, the County Council, meanwhile, will remain in the current county office building, expanding into much of the space formerly occupied by Gilchrist.

Next fall, clerks and judges at the county's circuit court will move across the street to the new Judicial Center, a smaller version of the new Executive building. Their old offices, meanwhile, will be occupied by personnel from the state district court, now located on Shady Grove Road.

County workers have mixed feelings about the move. Many offices in the new building have no windows, one employe complained. But the Transportation Department's deputy director Anthony Kanz, who is coordinating his office's move this weekend, said most people favor it.

"When you go from a vinyl asbestos floor to a carpeted floor," he said, "everybody feels it's a step up."