Robert Adam, the 18th-century English architect, was a man of superabundant gifts.

Having transformed a phenomenal number of country house interiors from medieval darkness to fashionable gaiety and light, he's best known as the quintessential decorator. Despite the hint of superciliousness the label holds today, there's nothing wrong with it. Adam's decorations were total works of art.

From his vast London offices, where it is said more than 2,000 people were employed, he directed what one contemporary aptly described as a "regiment of artificers" -- draftsmen, painters, sculptors, ornamental carvers, cabinetmakers, carpet weavers, upholsterers, plasterers and so on. His attention to detail remains legendary. In any given room he would design walls, ceilings, floors, columns, pilasters, fireplaces, bookshelves, picture frames, chairs, tables, mirrors, candelabra ... everything.

Almost always he would design the very room, its fundamental shape and form; usually he conceived the entire floor plan, relating room to room in ways at once novel and pleasing; more frequently than he's generally given credit for, Adam designed the whole building from ground up; then, too, he sometimes designed the grounds for these country estates. He was, in short, a complete and splendid architect.

All of this is in evidence in the exhibition "Robert Adam and Kedleston: The Making of a Neo-Classical Masterpiece," currently on view at the Octagon Museum. Kedleston Hall is the ancestral home of the Curzon family. A number of objects from it, including one Adam drawing, were included in "The Treasure Houses of Britain" two years ago at the National Gallery of Art.

The Octagon show is focused upon the building itself -- it is the next best thing to going there. Comprising beautiful watercolors and drawings from the Adam workshop, as well as drawings by other architects (Matthew Brettingham II, James Paine and Samuel Wyatt) and a few supplementary photographs of the building as it looks today -- and it looks terrific, very much as it did two centuries ago when Adam completed his work -- the exhibition provides insight into the world of Adam's clients as well as into the totality of his architectural enterprise.

Nathaniel Curzon, the first Lord Scarsdale, caused Kedleston Hall to be built. Like Lord Burlington before him, who in the 1720s had commissioned the design of Chiswick House in admiration for the 16th-century Villa Rotonda by Palladio, Curzon was "imbued with the culture of the Mediterranean" (writes Leslie Harris in the catalogue) and wanted to settle a piece of Renaissance Italy and ancient Rome in Derbyshire.

Curzon did not do things by halves. When he inherited the property in 1758, as one can see in a 1710 painting of the estate, it encompassed a rather undistinguished grand house, a small Norman chapel and a compact little village, all resting in pleasant disorder on the crest of a knoll. Curzon's idea was simply to sweep the old away (except for the chapel) in favor of the new. By 1759 his first architects, Brettingham and Paine, had conceived, doubtless with the client's close advice, a tremendous four-winged building for the site, based (of course) upon another of Palladio's designs.

Construction began almost immediately, and this, perhaps, accounts for Kedleston's contrasting faces. The north front, its pedimented centerpiece and symmetrical wings designed mainly by Brettingham and/or Paine (but with a few telling alterations by Adam), is an impressive neoclassical exercise -- strong, simple, clear. Harris judges it to be "the grandest Palladian fac ade in Britain and with few rivals anywhere in the world." The south front, designed by Adam in 1760, by which time he was thoroughly in charge of the project, is a vastly more muscular, more original undertaking, with projecting domed rotunda, sweeping double staircase and powerful Corinthian columns combined with elegant surface decorations.

Any doubts concerning Adam's ability as an architect can be dismissed with this fac ade and similarly inventive buildings (a "fishing room," a "grotto or rock room," a "view tower") he designed to be scattered about his picturesque, informal landscape setting. They embody his stated intention to bring "movement" to architecture, which he defined as "the rise and fall, the advance and recess with other diversity of form, in the different parts of a building."

Obviously, to Adam, interiors were equal if not chief among a building's parts. To him, as to contemporaneous or slightly older French and German rococo masters, the articulation and decoration of interior volumes and surfaces was integral to the architectural task. Adam's refinement and delicacy, of course, contrast strongly with the ebullience and sensuousness of the continental rococo, but the underlying aims are similar. Adam's concern with the wholeness of an architectural space was his great obsession.

The wonder is -- or so it seems today -- that so many of the beautiful decorative schemes we see in this exhibition (and so many thousands of others) actually were executed, and, as is shown in photographs, to the most minute detail. It's a scale of costliness kings no longer can afford, and a level of taste and achievement to which very few, if any, of today's richest of the rich could even hope to aspire.

Kedleston was one of Adam's first major architectural commissions. Born in Scotland in 1728 and advantageously reared to the profession by his architect father and older brother John, Adam had returned to England from his four-year study sojourn in Italy in 1758. Brilliant and tirelessly self-promoting, he must haved bowled Curzon over with his enthusiasm and his direct knowledge of Renaissance and classical architecture. His lordship's confidence clearly was not misplaced.

One of this show's reasons for being is to help raise $3 million, through the Royal Oak Foundation, the American affiliate of the British National Trust, in order to help save Kedleston Hall and its contents. Supported by several private firms and foundations, the exhibition continues through Jan. 5. Admission is $2. The Octagon Museum, 1799 New York Ave. NW, is an ideal setting for the show: Its interiors, as do those of many federal period American buildings, clearly reflect Robert Adam's influence.

Curator Gervase Jackson-Stops, architectural adviser to the British National Trust, will lecture on Adam and Kedleston Hall Dec. 3 at 8 p.m. at the Corcoran Gallery Auditorium.