At Winconsin Avenue and M Street, new urban chic is being poured into old buildings.

The old buildings are famous Georgetown landmarks-Rive Gauche, City Tavern and Clyde's as well as the old Car Barn across the C&O Canal.

The new urban chic that is to fill the recently dug construction hole between the canal and M Street consists of an unusual blend of shopping center and townhouses, flavored with luxury and sugared with instant architectural charm.

The $45-million project is called "Georgetown Park." It is scheduled to open in two years. And it is part of a sudden flowering in the ailing heart of our old cities.

It all started around 15 years ago, when landscape architect Lawrence Halprin had the idea of transforming the picturesque, Victorian Ghirardelli chocolate factory in San Francisco into an exciting, new kind of market place.

The idea spread. In most every American city, it turned out, there was some cherished old pile of brick or granite-a warehouse, factory, school building, or even city hall-that could be turned into a lively emporium offering out-of-the-ordinary merchandise, food entertainment and relaxation in varying proportions.

What makes these remodeled old buildings so attractive is that they have all the endearing qualities most modern buildings lack. They have interesting architectural features and convey a sense of luxuriant solidity, delight and sometimes wit. They have "atmosphere."

A local example is Canal Square in Georgetown which skillfully weaves a strong modern structure into the texture of old Georgetown houses and warehouses. Canal Square architect Arthur Cotton Moore is now converting the court and lower floors of the Old Post Office on Pennsylvania Avenue into a kind of Ghirardelli Square. The upper stories will house the National Endowment for the Arts.

The biggest and most lucrative of these restoration jobs is Fanheuil Hall Market Place in Boston. Both in terms of its architecture and its merchandising, the market found exactly the right balance between faithful historic restoration and fresh, modern ideas that suit our modern lifestyle.

Fanheuil Market gives substance to the hope that the recovery of old cities.

Georgetown, to be sure, may already have recovered not wisely, but too well. Like other historic places in America, it owes its charm to two factors. First, it was bypassed and neglected at the time when "progress" devasted much of the American cityscape. Secondly, when the charm was discovered, it could be translated into cash, so that the town remains economically solvent without resorting to the kind of conventional progress, like the proposed convention center in Charleston, S.C., that would destroy the charm.

What we now need is a calm, judicious balance between continuous revival (it can't be a dead museum) and over-scaled over-exertion that will harm that delicate organism.

In contrast to some of my Georgetown friends, I have never felt that intensive mixed development along the waterfront would endanger anything but the obsolete industrial slum.

The recent buildings south of M Street have, I believe, confirmed this belief. They are beginning to bring life to a delinquent area and may soon syphon off the excessive blue-jean traffic on M Street and Wisconsin Ave. NW. The height of these buildings, which almost led to sedition and bloodshed a year or so ago, is no longer controversial. It is easy to forget what is downhill and out of sight.

The ultra-chic Georgetown Park, on the other hand, is hardly out of sight, even if it is hidden behind Rive Gauche and Clyde's, and covered with Georgetown-sized residential buildings. The camouflage doesn't make it any smaller.

The venture will add 155 condominium townhouses and apartments, 121 stores and restaurants (a total of 210,000 square feet of retail space) and 500 parking spaces to the busiest intersection in Georgetown. White Flint, the shopping center in Maryland, by comparison, has 105 stores and 750,000 square feet of retail space. All of Georgetown presently has 360 stores.

Yet, no one in Georgetown has raised a public protest. No one seems as upset by the prospect of horrendous traffic jams as the Georgetown merchants were a year or two ago about a few street vendors on the Wisconsin Avenue sidewalks.

The difference, I am sure, is the snob appeal of Georgetown Park. It is the difference between a street vendor and a Garfinckel's sales person.

People will enter the three-level shopping mall from M Street through a gateway punched into the preserved facade of an old house between Publick House and Clyde's.

The interior of the mall will be designed in a kind of neo-Victorian style. In addition to the ubiquitous skylights and hanging plants, we are promised cast iron and brass rails, brass street lanterns, chandeliers, pressed tin ceilings, an "antique" elevator, and other bric-a-brac. I assume the Muzak will only pipe Vienna Waltzes.

Much of the mall will be inspired by Fanheuil Hall Market innovations.We are promised, for example, food stalls in the food market area with its bakeries, butcher shops and groceries.

The fashion corner will include a Garffinckel's store along M Street, as well as Britches and Ann Taylor.

A European home accessories firm, Conran's (known in Europe as "Habitat") will occupy the ground floor of the Old Car Barn along Grace Street. The Barn was originally built in the 1760s as a tobacco warehouse. The upper floors of the huge brick antique are being converted into condominium apartments.

The shopping/townhouse complex and the Car Barn are linked by two bridges across the Canal. The Canal is treated with much respect by Lockman Associates, the architects. The townpath countinues along the stone retaining wall, which is painstakingly preserved.

We will further enjoy the canal (if the Park Service ever fills it again) from a new colonnade as well as a terrace.

Below the three shopping levels are three parking levels. Above them is the village of townhouses. The complex may also include genuine historic Georgetown Market, a few yards further west, whose present developer seems to find it difficult to go it alone.

What saves this complex from overwhelming the Georgetown scale is the sloping topography. The slope enable architects to open their cavernous retail center directly to the Canal as well as to M Street and Wisconsin Avenue.

This alone should also assure the venture success for the developers. Whether it will also assure success for Georgetown, depends on two still open questions:

The first is whether Georgetown Park will be able to resist sliding from sophisticated chic to cheap kitsch?

The other is whether Georgetown will be able to cope with the new traffic.

But perhaps Jane Jacobs is right when she says that traffic is not all that important CAPTION: Illustration, Proposed Georgetown Park Shopping Mall; by Lauren Hurd