The Arlington County Board gave unanimous approval yesterday to a 19-story office building in Rosslyn that would be the last major development in the 25-year evolution of the area from a scene of blight and pawnshops to a lucrative high-rise mini-city.

"We've completed one whole phase of Rosslyn," said Board Chairwoman Mary Margaret Whipple. "The old Rosslyn is gone and the new Rosslyn is there . . . . That feels good."

The $50 million building, to be developed by the Kaempfer Co., will occupy one of the last two vacant parcels of significant size in Rosslyn.

The Kaempfer site is on a corner lot wedged between North 19th Street and the I-66 right-of-way.

Only one other large Rosslyn site is still vacant -- a parcel at the corner of North Kent Street and Wilson Boulevard that the county has designated for hotel or residential construction and the developer wants to use for a third office building to match the USA Today building and its twin tower.

Whipple said that the board's approval of the Kaempfer building yesterday "completes" the area. But, she added, "What we may look forward to in the future is some redevelopment of existing properties, not new projects of buildings on vacant blocks."

In exchange for the board's approval of the building and related reduced-parking and increased-height allowances, the Kaempfer Co. will convert two other Rosslyn parcels it owns into miniparks to help offset the area's "concrete canyon" effect.

The firm, which has created two other small parks in Rosslyn, swapped the building rights it would have had at the park sites to get extra density for the triangle-shaped office building designed by the architecture firm of internationally renowned I.M. Pei.

Locally, the firm designed such buildings as the John F. Kennedy Center for the Performing Arts and the East Wing of the National Gallery of Art.

One of the Kaempfer parks would be at the corner of North Moore and 19th streets, a site now used for a parking lot.

The second park is to be created on surplus I-66 right-of-way next to the office building site, which Kaempfer purchased from the state.

Joseph Kaempfer, the firm's president, said it would cost more than $1.1 million to landscape and buy sculpture for the first park.

The second site, which would face the park that the state is to build on a deck over I-66 at Rosslyn Circle, will remain as open space.

Board members said they believe that the extra height and slightly reduced parking they approved for the new building is justified because of the parks and the building's unusual design.

Only a small portion of the building will be 19 stories tall.

"Arlington has very few buildings of genuine architectural distinction and merit. This one does," said resident Kathy Freshley, who has been active in community arts groups. "It will make a major aesthetic contribution to Rosslyn . . . . [It will make Rosslyn] look like a city instead of a crewcut."

Kaempfer is planning to build an eight-level garage that can accommodate 447 vehicles; five of those levels will be above ground.