One cannot but admire the conviction and skill displayed by the Philadelphia firm of John Blatteau Associates in its several architectural projects for Riggs National Bank here. On the other hand, it is also impossible not to wonder, "What century are these people from?"

Blatteau Associates, in business as a collaborative enterprise since 1983, has firmly established a position on the conservative fringe of architectural discourse. Its principal designers -- Blatteau, Stephen Bonitatibus, Arthur Miller and Diane Hecht -- preach and practice a notably pure form of classic revival architecture.

In Riggs, they would appear to have found their ideal client. It is a company that decided, five years ago, to reassert a traditional architectural image, and it put the Blatteau firm in charge. In each of the Riggs projects, as Blatteau has written, the intention has been to ensure that "all the architectural and decorative elements express the sense of dignity, security, stability and permanence traditionally associated with banking."

Three major projects have been completed -- the remodeling of the interior of the Lincoln branch bank, at 17th and H streets NW (1985); the new executive offices and restoration of the impressive turn-of-the-century interior at the main office, on Pennsylvania Avenue NW across from the Treasury Building (1987); and the inside-and-out restoration of the 1922 Farmers and Mechanics branch at Wisconsin Avenue and M Street NW in Georgetown (1989). Two thoroughgoing redesigns are under construction, at the Bethesda branch on Wisconsin Avenue and the Northwest branch on Columbia Road NW in Adams-Morgan.

This is spectacular and exemplary work, in a way, although it strains credulity if one takes the all-embracing architectural theory too closely to heart. The restorations are exquisitely done. More remarkably still, the new pieces, from an entablature to an entire facade or room, are designed in the spirit of the old and executed with the same kind of attention to detail.

At the Georgetown branch, for example, the old classical facade has simply been dusted off, as it were, for another half-century's good use. Such recyclings are always pleasing to see. Besides being ecologically correct, they're psychologically mending: How much better to save this modest landmark, with its golden dome and cute little cupola, than to tear it down or wound it irreparably with a spurious new face. (The gilding of dome and cupola, I learned to my surprise, was a 1961 innovation, thoughtfully preserved today. Purism has its limits.) The old interior, by contrast, had been pretty much obliterated by insensitive alterations over time, and has been restored to its original dimensions and sparkle. There's some sleight of hand involved -- the job combines old with new so deftly one cannot quite be sure which is which. If there are signs of the new, they're small ones: The relief ornament on the handsome white oak teller screen would appear to have been cast from a mold and then glued to the surface. Basically, though, the banking hall is once again a splendid chamber appropriate to its enclosure.

It is best, of course, to visit the completed projects to get the full effect, but one should not miss the complementary experience of the exhibition "Four Washington Banks," devoted to four of Blatteau's Riggs projects; it is on view through July 30 at the National Building Museum.

Besides photographs, the show contains fascinating supplementary materials such as a set of working drawings for the Georgetown project and a similar set by Washington architect A.P. Clark for the 1922 addition to the main Riggs building on Pennsylvania Avenue. It tells a lot about the Blatteau enterprise that the new drawings are every bit as meticulously wrought as the old, as if 70 years had not passed and the computer had never been invented. Tucked among the many images on one inside page, there is the following neatly lettered message -- "NOTE: PAINT ALL JAMBS BEFORE INSTALLING GLASS!!"

A visit to the exhibition is necessary, too, in order to see how dramatically (and drastically) the Northwest branch will change in the coming months, for there are photographs here of what it did look like to compare with renderings of the new design. As curator David Chase points out in a wall label, the sharp black-on-white clarity of these drawings is in itself an act of homage to neoclassical architects of old. The design, of course, follows suit: Friederich Schinkel himself could have designed this upright little building, with its symmetrical facade and carefully modulated interior space.

It is a fine design, even a beautiful one, but it does give one pause. The replaced facade, described by the architects as a "bland modernist structure," was a nice little asymmetrical abstraction of a 1950s type. Its virtues of modesty, economy and a sweet Main Street allure are not at all those of the new building, nor the ones the client wants to project. Riggs, after all, is the company that commissioned one of the most sickeningly pretentious set of ads -- the "most important bank in the most important city" series -- ever to run on local television.

Zeal, as well as skill, is a Blatteau trademark. One senses this especially in the first of the firm's Riggs projects, the remodeling of the banking hall and offices in the Lincoln branch. One walks through the dark, and indeed quite bland, portals of this '50s downtown office building and enters a simulacrum of the 18th century.

This sort of time warp is familiar enough to any person who visits a typical Washington law office or, more to the point, Blatteau's first commission in Washington, the Benjamin Franklin dining room at the State Department, completed in 1984 as part of classic revival stage set there. Though by no means so splendid, the Lincoln branch interior, too, is something of an ardent, cloying museum piece.

My argument is more with the theory than the practice -- Blatteau's insertions into the cityscape, whether for Riggs National Bank or the federal government, are in general beautifully accomplished, and they're fundamentally minimal. They'll give pleasure to many and, because of their appropriate scale, will hurt nobody at all. But still, they stubbornly embody a static, even a stultifying, world view. Fortunately, there are other options.