PARIS — Hard by the noisy highway, overlooking a cemetery and a former garbage dump, La Tour Bois-le-Prêtre glimmers on a spring morning. Sheathed in a fresh cloak of glass balconies and corrugated aluminum panels, it rises on the edge of this city amid a landscape of decaying cement-and-brick housing blocks.


This half-century-old tower used to be one of those blocks. Its makeover, by a creative team of local architects — Frédéric Druot, Anne Lacaton and Jean-Philippe Vassal — is a case study in architectural ingenuity and civic rejuvenation. It’s a challenge to urban innovators too. Instead of replacing the old tower with an entirely new building, the designers saw what was worthwhile about the existing architecture and added to it.


Retrofitting, it’s called. Preservationists in America have argued for a long time about the benefits of reusing obsolete structures. Since some 80 percent of what’s been built in the United States has been constructed during the last 50 years, reuse seems like the inevitable wave of the future. The practice is not common when it comes to large public housing projects. But there have been a few successful attempts. This one is the latest.


Poor neighborhoods on the outskirts of Paris and in the city’s inner-ring suburbs are, as in many cities, dominated by these much-maligned projects from the 1960s and ’70s.  Not long ago I visited Sevran, one of the poorest Paris suburbs, where the rioting that spread across France in 2005 started. Unemployment now hovers around 40 percent among the young there. Violence has gone up in the last couple of years. There was a shooting not long ago in a kindergarten.


 Sevran is full of housing towers. French policy, similar to the American approach that has reshaped the inner cities of Chicago, Pittsburgh, Louisville and elsewhere, favors demolishing these projects and moving out tenants. Several towers have come down in Sevran, replaced by community gardens, sports fields, some new housing and a new school. More towers stand empty, awaiting destruction.


President Nicolas Sarkozy has proposed a vast extension of the Paris subway system that would link the city center with dozens of alienated suburbs like Sevran, along with new exurban commercial districts. Employment and growth depend on improved access to public transit.


Stéphane Gatignon, Sevran’s mayor, told me: “Urban renovation alone can’t solve our problems of unemployment and drugs. But it at least gives us the opportunity to live with more dignity.” Architecture has its natural and obvious limits, in other words. But it is powerful as well.


So it is with La Tour Bois-le-Prêtre, which sits on the farthest edge of the 17th arrondissement, a mixed district with persistent pockets of poverty, where a Métro extension would also go. The tower was a natural candidate for the French wrecking ball after decades of neglect and decay, but tenants didn’t want to lose their homes. So an unusual question arose: might the building become a candidate for a different approach?


A competition was organized by Paris Habitat, the Paris Office for Public Housing, in 2005 to renovate the building. The challenge: to repair the tower’s crumbling infrastructure, upgrade its common spaces and its exterior, and — this was the most radical part — add more light and square footage to dark, cramped apartments, without changing the footprint of the building, which couldn’t be extended.


Oh, yes, and to spend less money for all this than the cost of tearing the building down and then rebuilding.


Designed by Raymond Lopez and opened in 1961, the 16-story prefabricated concrete tower had already undergone an ugly, claustrophobic update during the 1980s. Various competitors proposed leaving the shell basically intact but gutting the interior. Ms. Lacaton, Mr. Vassal and Mr. Druot won the competition with a novel approach.