Ten months after the Arlington County Board rejected a proposed office building on 2.2 acres of prime land in the heart of Rosslyn, the site remains a parking lot.

Westfield Realty Co., owner of the land at Wilson Boulevard and North Kent Street, across the street from the USA Today building, had planned to construct a 30-story, $40 million office tower on what is the last major piece of developable land in Rosslyn. It would have been the third office tower in the Arland Towers complex, which has become a local landmark because of the two curved silver-colored office buildings there.

One of those buildings houses the newspaper's national headquarters, and the second is expected to be ready for occupancy this summer.

But the County Board last March unanimously rejected the developer's plan for a third office tower, pointing out that the other two towers were given extra density and height in return for Westfield's promise in 1978 to build a hotel at the third site. The hotel building would have been 80 feet shorter than the office towers.

Westfield contended that it could not interest a hotel operator in the site but had potential office tenants. The development firm also argued that a third office tower would yield the county an extra $1 million in taxes annually and allow for the completion of the "Rosslyn Loop Road." The road would be a minibeltway around the area that would allow Rosslyn workers to avoid streets heavily traveled by commuters to the District.

Since then, the county and Westfield officials say they have tried to interest hotels in the site but have been unsuccessful because neither the hotels nor Westfield wants to assume the financial risks involved in constructing such a project.

Meanwhile, the land, assessed at $4.1 million, remains a parking lot. Although the company is paying almost $40,000 annually in taxes on the lot, Westfield official Stanley Westreich said the firm has "no plans" for it and still prefers an office building at the site.

"Certainly the county hopes they will build something in accordance with the original plan," said County Board Chairman John G. Milliken, adding that he doubts the lot will sit there vacant indefinitely because of the changing economy.

Westreich said the firm does not feel any pressure to come up with another proposal by February 1986, when the county plans to begin constructing the loop road.