Pennsylvania Avenue's new commercial center will be a clever concoction of old and new buildings -- an instant cityscape.

The just-approved project will cover the block between 10th and 11th streets, bounded by E Street on the north.The developers are David W. Evans and Richard S. Cohen, who prosaically call their structure 1001 Pennsylvania Ave. The architects are Hartman-Cox with Smith Segreti Tepper, who don't like either hyphens or commas between them.

Visually, architect George Hartman's brillant melange will go far to relieve the sense of Angst created by that big bully next door, the FBI building. It will give the bouncy Old Post Office across the avenue a pleasant opposite to have a conversation with.

Most of all, however, Hartman's design gives variety and sparkle to the choros line of buildings facing the Federal Triangle. The star on the line is, of course, the Willard Hotel. It is to be enhanced by a bevy of little Willards designed by architects Hardy, Holzman, Pfeiffer.

Economically, the 1001 Pennsylvania Ave. venture will uplift the dear old buildings it has so tenderly taken unto its bosom. By themselves the poor things would never have made it. In fact, these ramshackle bits of nostalgia -- they include the turn-of-the-century brick palazzo, now occupied by Dart Drug and the stately United States Storage Building -- survived only because the area has become economically so depressed that nobody bothered to tear them down even for a parking lot.

Now things have changed -- our architectural tastes, out attitude towards old buildings, our expectations of downtown, the prevailing life style.

The Pennsylvania Avenue plan, which was all pompous, wrong and stuck in the '60s, was re-thought in the '70s and seems to be taking off in a boom in the '80s. The boom might pull the rest of downtown along with it.

Metro is bringing people from the hinterland. The tourists are still corralled on the Mall, but in time food, drink and diversions other than strictly Smithsonian might tempt them to stampede across Constitution Avenue. In time, Metro might even dare to run a "culture bus" as far as the Phillips Collection at the risk of annoying the private bus companies who think competition is un-American.

At any rate, there are plenty of suburbanites and tourists ready to enjoy downtown if downtown were only more enjoyable. These people don't want more of the Tyson's Corner corn and White Flint chic. To say it again, the second generation of affluent Americans likes old buildings and home-baked bread. It has expectations of downtown other than chained restaurants and parking lot ambience.

I therefore think Hartman, Cox, Evans and Cohen are smart to offer a melange of old and new architecture, along with a melange of attractions -- arcades, an atrium, a street with only a few cars and plenty of kiosks, street vendors, push carts, displays, concerts, street-performers, small movie theaters, bars, restaurants, cafes and 30 to 40 small shops on two levels. Above this bustle will be offices, sharing the prestigious 1001 Pennsylvania Ave. address.

The commercial potential of the venture is further assured by the proximity of Woodward & Lothrop's department store and other stores. It becomes part of Washington's downtown retail core which has the possibility, if its merchants wake up, to become the most exciting and unique regional shopping center in this 2.5-million-person market.

In his design of the structure, Hartman responded to the exceptional architectural character of his site. To the west, there is the elaborately beaux arts facade of the Star Building, projecting out into the avenue. To the south is the assertive Romanesque Old Post Office. Next to it is the monumental facade of the IRS Building. On the site itself are a number of very good turn-of-the-century commerical structures, many of which will be retained.

Hartman's building evolves out of this context. The mass, scale and materials of this ensemble determine the mass, scale and materials of his building.

Hartman does this by preserving and restoring Dart Drug, the Storage Building and three other antiques and integrating them into one structure. It consists of three layers, one in front of the other. The first comes up to the height of the remaining old buildings. The second steps up and down to create the impression of a building complex rather than a monolith. And the third towers in the inner core of the block, rising to the permissible 160 feet.

The structure has a small interior atrium linked by pedestrian passageways to all of the surrounding streets.

Under the plan, 10th Street, which faces the FBI's fortress walls but also smiles with the large Storage Building arch, would become a "pedestrianized" street with one-way car traffic, kiosks and lots of human activity.

This kind of inclusive ensemble design is still relatively new. It was, as far as I remember, pioneered here in Washington by architect Arthur Cotton Moore with his Canal Square, which integrates an old warehouse and small townhouses with a new, modern building.

But it hs never been done on this scale and Hartman's solution is sure to inspire similar efforts by other architects and in other cities.

The ensemble design solution has been forced on architects and developers by the preservation vs. profit pincer. Preservationists, notably an enthusiastic group called "Don't Tear It Down," insisted on preserving some of our past along Pennsylvania Avenue. Developers say they can profit only on large spaces with all the modern conveniences. Preservation makes these conveniences more human.

But militant "don't tear it down" noises, like all extremes, can also be counterproductive. A block or two further west on Pennsylvania Avenue, preservation militants have the developers in court for allegedly not giving the Munsey Building a fair chance to escape the bulldozers. The Munsey, now gutted and forlorn, was pleasant but not overly distinguished. It is to be sacrificed for a large, mixed development, featuring a Marriott hotel, that would include the National Theater untouched, in its present form.

If the needless lawsuit drags on, the present developer may well tire of paying taxes and being unable to move while construction costs go up. If he pulls out, another developer will hardly be inclined to preserve the National Theater, when he can make more money on an office building.

Preservation zeal may thus destroy one of the most distinguished cultural landmarks we have in this city.

Complex planning matters should not be thrown on the mercy of the courts. They should be solved in creative trade-offs of the kind we will see at 1001 Pennsylvania Ave.