THE ORIGINAL DESIGN for the U.S. Capitol, which George Washington and Thomas Jefferson enthusiastically approved, was different than the history books tell us. It also implied a somewhat different form of government, reflecting the Federalist view of an Imperial Presidency.

Recent, as yet unpublished research has brought to light that the first sketches called for a dominant Congerence Room adjacent to a spacious office for the president, called the President's Apartment. The House and Senate chambers were relegated to secondary importance.

The round Conference Room, which a Boston newspaper described at the time as "an Audience Room," protruded to the western edge of what is now called Capitol Hill. It was topped by a temple. To the east of it, under a second, shallow dome, was a vestibule. The towering composition, 160 feet high, would have dominated the Mall and city, much as the Capitol does today.

The design was by William Thornton, the first architect of the Capitol, who was also a physician portrait painter and steamboat engineer. But the design was inspired, it now appears, by Maj. Pierre Charles L'Enfant, the friend and protege of George Washington. The president must have seen and heartily approved of L'Enfant's sketches for both the Capitol and the White House although there is no direct evidence of this. L'Enfant's architectural sketches are lost.

Thornton's official design, at any event, reflected the views of the Federalist Party, which called for a strongly centralized federal government led by an all-powerful president. As the Federalist influence receded, particularly after Washington's death in 1799, the Capitol design was correspondingly modified - "republicanized," one might say.

The temple-topped Conference Room had not yet been built when the British burned the Capitol in 1812. Instead, the House and Senate wings were connected by a temporary, wooden walkway. After the fire, Benjamin Latrobe took over as architect and plans were changed because political sentiment had changed. The Federalist Conference Room was omitted and the vestibule became what Jefferson called "a hall of the people . . . for impeachment and public ceremonies." The hall is now called the Rotunda.

Few other buildings today are as much "a translation of the spirit of an epoch space," as Mies van der Rohe put it. With ever-changing political ideas and artistic tastes, as well as shifting centers of power, the Capitol has grown into an almost unmanageable behemoth of contradictions: grandiosity and confusion, gaudy over-decoration and stern simplicity, boldness and mean inefficiency. With the nation, the nation's foremost building keeps changing - for better or worse. The Tell-Tale Stakes

THE DISCOVERER of the Federalist design for the Capitol is Bates. Lowry, one of the country's foremost architectural historians, professor at the University of Massachusetts in Boston and president of the Dunlap Society.

The Dunlap Society was recently created with a grant from the National Endowment for the Humanities to develop teaching materials - notably microfilm cards, slides and fact sheets - designed to promote understanding and scholarship of American art in schools and universities. There is a dearth of such material. It is easier, as Lowry said, "for a teacher to show students views of medieval churches in a remote part of Europe than to show them some of our country's most significant buildings."

Because of the lack of teaching materials, there is lack of qualified teachers. And because of the lack of qualified teachers, there is a deplorable lack of any but the most cursory mention of American building and pre-modern art in our high schools and colleges. Our cities show it.

Preparing a "visual biography" of the Capitol for the Dunlap Society, Lowry came across the original layout for the building's central section in the Library of Congress. It shows the foundation stakes, including the foundations for the circular Conference Room. Drawings of it had been known, but dismissed as alternative or preliminary designs. The foundation stakes indicate that the Federalist temple-topped tower was actually to be built.

The design of the Capitol was the result of an architectural competition advertised by the commissioners of the Federal City in the nation's newspapers in 1792. The winner was promised $500 and a city lot. The results were dismal. One designer proposed to house Congress in a giant weathervane. As so often happens in Washington, the commissioners met the dilemma by doing nothing for a while.

Then, three months after the competition deadline had passed, Dr. Thornton asked permission to submit an entry. The drawing of it that is published in the history books shows a sedate, carefully detailed Palladian structure of two identical wings for the House and Senate, linked by a domed center with a colonnaded porch. Jefferson, who was then secretary of state, was delighted. The design, he wrote, "captivated the eyes and judgment of all." It combined, he continued, "gradeur, simplicity and convenience."

This known and oft-published Thornton drawing shows only the east side of the building, however. Historians just assumed that the west side, fronting the Mall, would be identical. Thornton's West Front was thought to have been an alternative design. But if it was not part of the actual, approved design, Lowry asked himself, why are the stakes for the central portion so far out? He searched some more.

Another Thornton drawing turned up, a watercolor sketch, showing the architect's concept for the whole building. It is a rather whimsical confection - a temple with a prominent round protrusion, ringed with terraces and columns, in front of it and a domed pavilion on top. The central portion backing it is plan and square and has two wings that match the familiar East Front wings.

Lowry and his photographer wife, Isabell, made a montage of Thornton's East Front and the so-called "alternative design." The montage matches the watercolor sketch. It shows, Lowry asserts, the building Washington and Jefferson approved with such enthusiasm.

There is also a floor plan of this design. Thornton's description clearly labels the large, round space as the Conference Room and shows the spacious President's Apartment a few steps above the House and Senate chambers in the wings. The L'Enfant Connection

WHAT WAS contemplated here was obviously a predominance of joint House and Senate sessions with the president in charge, much as the king of France was in charge and occupied an office in the new National Assembly building which was to be built in Paris at the same time.

What was designed here became the prototype for virtually all state capitols in the United States and many parliaments throughout the world: a round, domed centerpiece, with wings on either side.

And what this scheme recalled in Lowry's mind was the simplified ground plan L'Enfant had drawn into his layout of the city and which his surveyor, Maj. Andrew Ellicott, had engraved. We have all seen this official plan of the Federal City. The floor plan on the L'Enfant layout matches the drawing of the stakes Lowry had found.

It stand to reason, then, that L'Enfant had made designs for the Capitol and the White House in some state of readiness. We know for a fact that he fully expected to be commissioned to design both buildings. We have reason to speculate that President Washington had seen and liked L'Enfant's building designs, because in one of his letters the president mentions the Capitol and asks for L'Enfant's "views of it." The "views" are lost.

L'Enfant, we should remember, was not only a city planner, but a talented and well-known architect. He designed many important buildings in Philadelphia and New York City. His predilection for an Imperial Presidency is evident in his design for the conversion of New York's City Hall into the Federal Hall for the Continental Congress in 1785. It included a kind of throne, a podium on which the president was seated under a rich canopy of crimson damask.

But why, then, was L'Enfant was not given the commission to design the two most important buildings in his plan? He focused his entire plan on the Capitol which he located on Jenkins Hill - "a pedestal waiting for a monument," as he called it - but yet was denied the opportunity to design the monument.

The answer is simple - he had been fired, peremptorily dismissed, before the planning of the city came to the point of building design. L'Enfant left Washington early in 1792. The competition for the Capitol and White House designs was announced several months later. It is entirely possible, however, that in expectation of the commission, L'Enfant had already started preliminary work on the two buildings. Hence the stakes. Hence a reference, before Thornton was chosen, to "men digging cellars" on the Capitol site. L'Enfant and His Enemies

WHY IS ALL THIS still shrouded in mystery, subject to speculation and detective work by art historians?

There are two reasons. One is that the documents concerning the founding, planning and early building of the capital have never been properly inventoried, according to Lowry. "A conscientious scholar may go through all the papers and never know that a key piece is missing - out being photographed or something."

Another, perhaps more important reason is that L'Enfant left his grand design and amid utter confusion and bitter intrigue. He took some of his papers with him and many of them were stolen or have otherwise disappeared. He never gave a clear account of his work and those who drove him into exile did not bother either.

But if L'Enfant's career came to an abrupt end, the intrigue from which the city emerged did not.

As construction of the Capitol slowly progressed, handicapped by feuding architects, constant shortages of funds and workmen, and British torches, the imperial aspects disappeared. House-Senate conferences are now held in small committee rooms rather than a dominant hall. The president, but for an occasional personal address, stays in his White House.

For all the muddle, there is a clear separation of powers and buildings. Which is what makes us the world's oldest democracy.