The nation's capital is broke and booming.

The must-discussed fiscal deficit is sure to bring "pain and suffering," in Mayor Marion Barry's words, as services and vital subsidies for the needy will be cut.

The little discussed building boom, however, will soon yield the city additional tax income. With a little luck and more determined leadership than Mayor Barry is exercising now, the prsent building boom could, in a few yelars, pay for many of this city's social needs. It could give us the cosmopolitan bustle of London and the cultural charm of Paris. It could make Washington the world's most pleasant city to live in and visit.

In contrast to other U.S. cities, where intense building activity is confined to certain districts, you see scaffolding and mortar mixers all over the District of Columbia.

New luxury condominiums, fashionable shopping arcades and offices are invading Georgetown, particularly the old industrial slum south of M Street, at such a rate that Georgetown's excitable citizen leaders will soon have nothing more to get excited about.

The biggest of these new developments, a three-level, super-chic shopping center, with garages below and townhouses on top, is rising out of the ground between M Street and the C&O Canal at Wisconsin Avenue. The first two segments of this "Georgetown park" complex, two remodeled old warehouses on the south side of the Canal, are already open for business. One is Conran's, a European "design" store which offers highbrow household furnishings at middlebrow prices, along with deafening radio music. The other is a row of expensive condominium apartments.

At Friendship Heights, the ring road that should ease the traffic congestion is about to become a reality. The end of West End construction is in sight Van Ness Center, on upper Connecticut Avenue, is sprouting foreign embassies and agencies that should add to downtown diversity.

The federal government is getting ready to build a $106 million Defense Intelligence colossus in Anacostia Bolling and a $204 million Government Printing Office on Brentwood Road in the northeast.

The tempo of "gentrification," the renovation of old, mostly Victorial slum houses by young professionals, is still accelerating. The first "urban pioneers" have crossed the Anacostia River. Another contingent is stripping paint in Fort Totten. Something or other, it seems, is being built -- or at least stapled together -- on every vacant lot in the city. Planners call this "infill."

But the most exciting and least expected building activity is in what planners call the old "central business district" the delinquent commercial area between the Capitol and the White House.

Mercifully, this untidy part of our town has, to date, been hidden from the tourists on the Mall by the seemingly impenetrable masonry wall known as the Federal Triangle. Soon, courtesy of the Pennsylvania Avenue Development Corporation, the other side of the triangle will not only be respectable, but also positively delightful. The feds, with the help of Harry Weese, the architect of our Metro stations, are therefore looking for ways to make all that bureaucratic masonry pleasant to walk through.

North of the about-to-be-reborn Pennsylvania Avenue is the single largest area of undeveloped land -- or perhaps I should say underdeveloped land -- in the Washington metropolitan area. Its development is the key to Washington's future greatness, if any.

The opportunity is particularly exciting and promising because this area is a gold mine of lovely, late Victorian commercial architecture buried, as it were, under ugly store fronts and neglect. It is like an archeological site waiting for a Schliemann.

Everything the city has attempted to do in this area in the past -- the insipid "streets for people" in front of th Martin Luther King Library, the hideous balloon lanterns in front of the Ford Theater, the idiotic half-mall on F Street, to say nothing of the Union Station Visitors Center -- turned out to be a disaster. These boondoggles did not attract more business or tourists but scared all but the last stalwarts away.

But now that all the suburban commercial areas are nearly built up, this negelected business heart of the center city is suddenly in great demand.

There is, to begin with, all that space. While the new, hitherto more fashionable downtown commerical areas -- Farragut Square, West End, Georgetown, etc. -- have only 1.3 million square feet of potential commerical space left, the hitherto unfashionable old business district still has 5 million square feet to be developed.

There are all these empty second, third and fourth floors over the dingy ground floor shops and restaurants. There are hundreds of hopelessly deteriorated and/or deformed four- or five-story buildings where zoning permits 10 stories. There are innumerable vacant lots held out of the market by speculators and used as untidy parking lots.

What is more, Metro, which has eight subway stations in the area, has suddenly made this undesirable space highly desirable. Littered, unlovely downtown is suddenly in the hub of the metropolitan region, quickly, cheaply and, if they learn to cope with farecards, conveniently accessible to almost 3 million affluent people.

Besides, the completion of the Convention Center and of tourists attractions in the rebuilt Old Post Office and along Pennsylvania Avenue, promise to draw large numbers of visitors into the area. An increasing number of free-spending foreigners is expected.

It is entirely possible that the city will bungle this incredible opportunity. There are, in fact, discouraging signs that it is about to, that the "sleepy, lil' old Southern town" mentality still prevails in city hall.

So far, virtually all the new construction downtown is for office buildings.

And that is about as healthy for a growing downtown as an exclusive diet of potato chips is for a growing child.

But the mayor and his planners seem to be so preoccupied with the hazards of the fiscal crisis that they don't take time to worry about the dangers of an unguided downtown building boom.