Not since the Romans built their Colosseum almost 2,000 years ago have architects been able to design a stadium of convention hall that esthetically fits into a living city. And we probably find the Colosseum fitting only because it is a ruin among ruins.

Cities are woven, as it were, in an intricate pattern to meet the diverse needs of its citizens. Stadia and convention halls serve a mass of humanity. They are, almost by definition, massive blockbusters that tear up the city fabric.

Ideally, they should be huge, festive circus tents out beyond the city gates on the municipal meadow or placed in some other open setting where they can be viewed with some detachment.

But circumstances these days are batty, not ideal. Cities are not surrounded by country but by suburbs which are not sub at all but greedy fiefdoms. They keep every penny they get cut of the privilege of being located close to civilization.

So if the city is to have any financial benefits at all from conventions, it must suffer the esthetic disadvantages of housing them. The only convention hall I know of which manages to be both big and attractive is McCormick Place in Chicago. It does not overwhelm its surroundings but, placed a good, green distance apart from other buildings, looks almost modest in its lake side setting.

Washington's proposed Civic Center is sited not for looks but income - to attract big crowds and their money into the central business district. It is to be built along New York Avenue and H Street, between 11th and 9th streets, just southwest of Mount Vernon Square. This is only four blocks from Pennsylvania Avenue which is about to become the capital's and the nation's main street.

It will be easily accessible to Metro rail, which will have two transfer stations within a block of the building. Very little parking is planned within the proposed megastructure - for only 375 cars. But the planners argue that conventioneers are likely to come by Metro or taxi. Area residents, they say, who come to attend a special evening event - a rock concert or a flower show - will find some 6,000 parking spaces vacated by downtown office workers and shoppers.

Conventioneers - optimistrically estimated at 310,000 to 390,000 a year - can walk to countless restaurants and shops, existing and to come, to say nothing of the Mall, the center of America's culture. As yet there is nothing much immediately around the proposed center, but Woodward & Lothrop's soon-to-be-renovated North Building almost touches it.

One can also argue, as our municipal planners do that the deterioration immediately around the Civic Center site is not a liability but an opportunity. Something will sprout if the center materializes. The question is only what.

So far our downtown merchants have not shown much imagination or sophistication. There is a danger that they will offer the visitors from Humdrum City. U.S.A. the very humdrum architecture, food and merchandise they find at home. But then, we can always hope that they will start to realize that this is the capital of the Western world and that it should not be hard to attract the people who now run out to Tysons Corner and White Flint without losing their downtown customers. Nor is there much sense in forcing the conventioneers to further crowd Georgetown in search of quality and novelly.

The municipal planners and Chinese merchants are now trying to enhance the identity of Chinatown, immediately east of the convention hall site.

The main campus of the District of Columbia University is being built just north of Mount Vernon Square.

As to the proposed design for the Civic Center, we can be grateful to Sen. Patrick Leahy (D-Vt.) and his appropriations subcommittee. He would not approve the first, artless $116 million scheme. The Municipal Planning Office and the architects (Welton Becket of Los Angeles with two local firms) have therefore worked out a reduced, $98.7 million model. It still covers three large city blocks, to say nothing of a little green triangle, and it is still considerably larger than the Roman Colosseum (9.7 acres as against the Colosseum's 6 acres).

Reworking their first proposal gave the designers the opportunity to integrate their colossus a little better into its surroundings, to adapt it a little better to the fabric of the city.

This was done primarily by articulating the building's three main functions: the convention auditorium with its stage, the exhibition hall, and a large meeting room. They appear almost as separate buildings of different size and height to break up the bulky mass.

The architects further moved the truck entrance and hidden loading area to the New York Avenue side and gave the building a more pleasant facade with two entrances and a row of shops on the H Street side. New York Avenue is, after all, a heavy traffic road. H Street could become an interesting shopping street, depending on what happens opposite the center.

The main taxi driveway and entrance lobby is now along 9th Street, facing Chinatown. And all four corners of the complex have been snipped, so to speak, to create small plazas, two of them with fountains. The plan is to have a "water wall," a cascade as on Paley Plaza in Manhattan, at the New York Avenue and 11th Street corner.

As one of the economy measures, the facade of the building is to be in precast concrete rather than limestone. Much will depend on the color, quality and workmanship used. In rare instances, concrete ages well. More often it is the stuff that instant slums are made of.

Perhaps the most important features of the center's design are the shops and a restaurant on the corner of H and 9th Street. The restaurant might be changed from a fast food operation for the daytime convention crowd to a more elegant eatery that helps attract people downtown in the evening.

In other ways, too, flexibility is of utmost importance. And, to their credit, the designers have done well to make the huge hall a flexible multi-purpose building.

The Civic Center design promises no great work of architecture, and that is precisely its virtue. It does not pretend to be more than it is.