The key to a healthy metropolitan area is a healthy downtown heart.

That was the concept on which Metro was built, and it is working. Washington's center city recovery is amazing. Even the somewhat seedy business district is being redeveloped.

The trouble is that nothing but offices are being built north of Pennsylvania Avenue. And neighborhoods that consist exclusively of office buildings are dull and devitalizing, rather than revitalizing.

As we all know, office districts go dark at night and on weekends. They attract only lunch business. Most of Washington's 600,000 office workers go home to the suburbs, not because they necessarily like to live there, but because there is not enough affordable housing for them in the city.

Office buildings, furthermore, are poor tax money makers for the city.

A 200,000 square foot office building, according to figures from City Hall, pays $228,000 a year in taxes.

A 200,000 square foot department store pays $1.5 million, or six times as much. A department store, furthermore, attracts other stores, restaurants and people in the mood to spend money.

A 200,000 square foot hotel pays $1.2 million, creates the kind of jobs this city is most in need of and is also a catalyst for municipal tax revenue. The sales tax of a good restaurant alone yields the city an average annual $1,000 per seat.

More housing downtown would bring still more taxpayers. There is easily enough space north of Pennsylvania Avenue and south of the Shaw area for 10,000 condominium apartments for 30,000 people.

If the revenues from all this potential are added up, they come to at least $46 million a year.

Obviously, the city should discourage the downtown office building zeal and encourage more retail stores, hotels, apartment buildings, restaurants, culture and entertainment downtown.

According to Washington city planners inside and outside the District Building, there is not much doubt that there is a market for such diversity.

While income in the city has gone up by 4.2 percent since 1972, retail sales in the city have declined by 7.5 percent. That means people who live in the city are taking some of their business to the suburbs.

Yet the mayor and his planning office have no firm plan for downtown diversity. To the contrary. Earlier this year the mayor supported a zoning regulation that would have allowed big hotels, such as the Washington Hilton, to expand in lily-white residential areas rather than build hotels close to the convention center downtown. Thanks to citizen outrage, this absurbity failed. But now the city's planners propose to give special inducements -- in the form of a greater building height than present zoning allows -- to attract new hotels along Massachusetts Avenue east of Mount Vernon Square.

Why? We don't need hotels along Massachusetts Avenue east of Mount Vernon Square. This, again, is a residential area. Again people would be driven out, most likely to the suburbs.

If the city makes it impossible for new hotels to go anywhere else, they will go where they ought to go. There is no need to mollycoddle them.

Furthermore, it is absolute nonsense under present circumstances to give bribes and inducements to lure anyone downtown. New York City has found it does not need special give-aways to get what is good for everone. We in Washington have already paid our bribe, bonus and inducement to the developers. It is called Metro, and we are paying $7 billion for it. For that much money we can expect a vibrant, attractive downtown.

With office construction in the District of Columbia having gone up 56.74 percent in the past five years, the city can afford to control and direct downtown development. It is time to overcome subservience to greedy speculators. This city, as the Pennsylvania Avenue Development Corporation (PADC) is demonstrating, is now in a position to deal with enlightened business and to demand quality.

As the new corporation chairman, Max N. Berry, put it, Pennsylvania Avenue is striving to become our Champs Elysees or Via Veneto. What is more, it seems to be getting there faster than anyone dared to hope. Perhaps too fast. Some PADC staff planners tend to resist legitimate citizen demands for preservation and amenity in the name of expediency.

There are now nine major projects under construction. But they are not just shoddy K Street-type office boxes that are easy to build and make money on. They are carefully designed by well-known architects, chosen in competition. Nearly 20 percent of the new construction is for shops. And when the avenue is completed, it is to have 1,400 hotel rooms and at least 1,500 residential units.

There will be four new squares or parks. The two opposite the District Building are nearing completion. One, Pershing Park, will have a pond and ice skating rink and lots of trees. The other is for playing hopscotch on a marble rendition of L'Enfants city plan. Both, I think, will be nifty.

There will be arcades of passages lined with shops in the middle of most new blocks, a feature that makes strolling through old Prague so delightful and that is especially welcome in the Washington climate.

So will be the 700 trees that will shortly be planted along the western part of the avenue now under construction. In their shadow are to be 300 benches. New, specially designed light fixtures are soon to be installed. And under the planned street program, designed by Joseph R. Passoneau, even the new Canadian embassy, to be built opposite the National Gallery, is required to have stores and/or other public uses on the street level.

If W. Anderson Barnes, the Pennsylvania Avenue Development Corporation director, can tell the Canadians how to build their embassy, why can't the mayor of Washington tell downtown developers to build some apartment houses and retail establishments?

What we need is a central business district development plan of the sophisticated kind that will attract a higher class of developers than we have on K Street and show them how to make money.

As of now, the only first class developer in the downtown business district north of Pennsylvania Avenue is Oliver Carr, who is redoing the block opposite the Old Treasury Building and the crucial area around the Metro Center station.

Carr understands what is needed. During his presidency, the Board of Trade Commissioned a study on how to improve the quality of downtown, which will shortly be published. The business community is beginning to learn, it seems, that Washington's downtown business district can regain its position as Washington's regional shopping center only if it is made more attractive and exciting than the suburban centers and commercial strips.

Carr's Metro Center developments may include a new building for the Hecht's department store, which is considering moving out of its old building on 7th Street NW.

The present thinking in City Hall seems to be that this is just fine, that the triangle formed by Woodward & Lothrop, Garfinckels and the newly located Hecht's would become downtown's retail core. There is no thought about replacing Hecht's.

There is room, and surely a market for five or six department stores. Bloomingdale's is already thinking of a downtown store. Marshall Field's, Nieman Marcus and others should be approached with self-confidence and imagination.

Imagination on paper alone won't do. What the city needs is an energetiic and savvy central business district redevelopment board headed by a top-notch director -- an Anderson Barnes, or an Edward J. Logue (the renewer of Boston) or a Baron Haussmann.