IT STILL TAKES a bit of imagination, but it is beginning to be possible to envision the Pennsylvania Avenue of the future, stretching in a lively and appealing way from the White House to the Capitol. Brick sidewalks, trees and shrubs, benches and newly designed lights are already in front of the new East Wing of the National Gallery of Art. Ground breaking has just taken place at 13th and E streets NW for the first privately financed office building in that area in 10 years, Art galleries and a unique jazz cafe are located in the 7th and E streets section: the only gallery in the city devoted exclusively to photography can be found on Indiana Avenue.

Those are a few of the things that have come to the area since the Pennsylvania Avenue Development Corporation got down to business a year ago. Chartered by Congress in 1972 to enliven Pennsylvania Avenue and to encourage downtown development, the corporation spent its early years in combat with local residents, planners and architects, members of Congress and federal officials. Some favored monuments and federal offices for that part of town, in keeping with the city's role as the nation's capital; others pressed for a modest dressing up, as with main streets in other cities. Finally, the PADC settled on something that may eventually please just about everyone: The area is to have a mixture of commercial, cultural and residential space, as well as small shops, renovated historic buildings, international and national offices - with special landscaping and lighting throughout.

Development of the area is exciting not only for what now can be seen, but also for what is still to come. By the end of this month, for example, the PADC will have a number of serious projects to renovate, including - would you believe? - the abandoned Willard Hotel, located at the corner of 14th and Pennsylvania. The corporation also expects several bids for the development of the block across from the Willard that includes the National Press Club. Plans for that block will determine the future of the National Theatre - the city's largest and oldest downtown legitimate playhouse. A shopping mall in the former Lansburgh's department store is being considered; art galleries are expected to move into the historic houses at 7th and D streets. The Canadian government will locate its chancery on the avenue, across from the National Gallery. What's more, residential housing, preservation of older and historic sites, and office buildings with activity that could enliven the area after working hours are all on the drawing boards.

All this activity serves not only to rebuild part of downtown but also to add to the allure of the area for new businesses, which, in increasing numbers, are moving here from outisde the area. Finally the Pennsylvania Avenue plan is beginning to fulfill the fondest dreams of its designers and supporters, and for this they deserve the city's thanks.