Zoning is a hot topic in the District lately.

The D.C. Zoning Commission is trying to figure out how much housing to force-feed into downtown.

The development community is trying to buy its way out of the requirement for downtown housing with dollars for units in depressed areas.

And housing advocates are calling for new government subsidies.

In testimony as to how complex the housing issue has become, consider that the Pennsylvania Avenue Developing Commission -- after 18 years and $100 million dollars -- has come up with only about 750 housing units.

But there may be an opportunity to create a significant amount of housing downtown with no government investment while at the same time completing a component of L'Enfant's grand plan for our capital.

As anyone who has navigated the city's ubiquitous triangular traffic islands knows, the plan of Washington is a clever 18th century game of juxtaposed geometries -- a grid overlaid and intersected by radial avenues. Along Independence Avenue, some rather boxy buildings respect that grid, but their backyards extend to one of those radial lines producing a collection of leftover triangular parcels.

L'Enfant's plan stressed the importance of two avenues radiating out from the Capitol. One, Pennsylvania Avenue, is well known. The other, Maryland Avenue, is not, largely because it is just a railroad line -- the "avenue" is missing.

The three-dimensional aspect of these leftover triangular parcels is that they exist at street level and the railroad lines are depressed about 20 feet in order to pass under the north/south streets. The opportunity, therefore, exists to create on these triangular and trapezoidal parcels, two- and three-story garages up from the industrial level of the tracks, which would then serve as a platform for a series of up to six apartment houses. As many as 1,500 housing units could be built, with the first-floor spaces of these buildings (where people do not generally like to live) being used for small-scale, low-rent-paying convenience businesses that are now being pounded out of downtown. The area would get some 24-hour life, while L'Enfant's avenue/vista running from the Capitol to the Jefferson Memorial would become a reality.

The land is clearly surplus. Federal office buildings have already been built, and the leftover triangular areas are in any case unsuitable to the large office floor plans desired by General Services Administration. They are, however, appropriate for a series of slender, linear apartment buildings.

The federal government could provide this surplus land at little or no cost to developers, who would then find it profitable to build housing and the missing avenue. The federal interest would be served by finishing one of the major strokes of the grand plan of the city not to mention by providing some new parking.

Defined at one end by the new site of the Museum of the American Indian, and at the other by the Portals project, which provides access to the Washington channel waterfront and the Tidal Basin, this new "avenue" could be partially vehicular or completely pedestrian, at times spanning the railroad like the Portals, and at others, for economy's sake, having a modest cantilever with screen planting.

Acoustically, the cantilever, the recess of the railroad and acoustical glazing could largely eliminate the presence of the rail lines for the new community. This has been achieved elsewhere -- for example, few people walking on Howard Street in Baltimore know that the same main railroad line rumbles below that major downtown avenue. In fact, despite the presense of rail lines, this downtown residential area would be quieter than most -- because it is adjacent to the air traffic restriction zone, the lucky residents would be free of the daily bombardment of noisy flights to and from National Airport.

As with any concept, there will be technical difficulties -- relocated roads, parcels that might not work out, etc. But the creation of new housing and the realization of a grand avenue would be worth the effort of overcoming them.

Arthur Cotton Moore is an architect.