A surprising evolution has occurred in recent years in the world of real estate development and architectural design.

Conventional wisdom used to hold that whenever aesthetics was at issue in commercial buildings, architects proposed while developers opposed. Because building owners and investors were interested only in the financial bottom line, designers believed themselves to be restrained from considering such architectural "upgrades" as polished brass hardware, solid-core doors, granite and marble finishes, and, of course, monumental atrium spaces.

But the market and mood have changed. Often building owners do as much proposing as architects, although the rationale nevertheless remains economic. Today's clients may take the lead in advocating "image" architecture based on what they perceive as tenant expectations in a tenants' market.

In Washington, where office-building tenants seem to have plenty of rent money (especially law firms billing hundreds of dollars an hour), the new conventional wisdom says that only first-class buildings can compete. The cut-and-dried, super-efficient, functionalist building of the past just won't do.

The proliferation of atriums symbolizes this evolution. So many projects, both completed and planned, now have interior atria that they can hardly be thought of as innovative. The atrium has become conventional, even indispensable in the eyes of some, almost like a dress standard for conducting business downtown. Atriums have become decorous, which Webster's defines as something "suitable or proper," in "good taste."

Take note of a few of the many recent buildings that make the atrium seem almost commonplace.

*Washington's Hyatt Regency, at 400 New Jersey Ave. NW, humble by comparison with other Hyatts, has an atrium shaped by an undecorated steel-frame-and-glass roof. Contrary to expectations, it slopes down, not up, as it spans from the front facade back to the enclosing wall of the enveloping U-shaped building. The main activity level is depressed a full story below street level, obliging visitors to descend immediately upon entering the atrium. The below-grade lobby and sloping atrium roof allow hotel rooms inside the U to have exterior windows looking out, over and onto, rather than through, the atrium space.

*The Vista International Hotel, at 1400 M St. NW, occupies a very narrow mid-block site, yet contains a surprisingly tall entrance atrium surrounded on all but the street side by rooms. Rising the full height of the building, the intensely decorated atrium space wraps around a central stack of circulation, restaurant and service elements.

*At 525 New Jersey Ave. NW, one block north of the Hyatt Regency, is the new Sheraton Grand Hotel. Like the Vista, it is an in-fill hotel on a deep lot, but the proportions of its atrium are relatively long, low and narrow. Its grid of pyramidal skylights spans between the mass of hotel rooms on the south side and the abutting office building on the north. The atrium, with fountains, plants, miscellaneous finishes, furniture, and variations in floor level, is a linear and less orderly extension of the long, processional entrance lobby.

*The atrium-as-lobby phenomenon in office buildings can be seen at 1201 Pennsylvania Ave. NW. Soaring from basement level to a trussed, skylit roof, its trapezoidal atrium sits directly behind the Pennsylvania Avenue facade and is enclosed by offices. Entering the building, there is direct street-level access to a bar/restaurant or to the elevator core via a bridge. A staircase descends to a sunken lower level with a fountain and restaurant. Although tables occupy this depressed level, most diners probably prefer sitting elsewhere, either to see the avenue or to feel some sense of intimacy without being exhibited to passers-by a few feet overhead.

*The Daon building at 1300 New York Ave. NW offers an ambitious atrium lobby of unmitigated pomp. The huge building, with two separate elevator cores in the east and west halves accessible through the atrium, has an irregular, trapezoidal footprint. But inside, a small circular entry rotunda behind the arched entrace in the curving New York Avenue facade guides the visitor adroitly to the squared atrium aligned with the District's street grid. A marble inlay floor leads to a gigantic, tiered south wall composed of cascading waterfalls, plants, and windows framed in steel, like the roof overhead. A panoply of solid geometries -- cylindrical columns, pyramidal trellises, globular light fixtures -- punctuate this opulent space dedicated to passage.

*No less impressive in its own way is the atrium lobby space at 1615 L St. NW, a mid-block office building that seems deceptively modest when viewed only from the exterior. Unlike the Daon atrium, this one is irregular in its geometry, complexly varying in plan geometry, in height and in floor level. Most remarkable are its unexpected size and the incredible (and unquestionably costly) display of decorative, multicolored, precision inlaid marble that patterns thousands of square feet of wall and floor surfaces. The atrium's light comes not from the roof (offices sit over it) but from the west wall facing the abutting alley. The intricate stonework -- deep salmon, beige and black -- combine with greenish glass, glass block, chrome trim and cove lighting to create a solemn, amost funereal atmosphere.

*Red Lion Row presents a most unusual atrium space at 2000 Pennsylvania Ave. NW, where a recently built, modern office building of concrete, glass and metal has been inserted in the middle of the block behind a string of preserved, 19th-century brick rowhouse facades. A three-story high, 30-foot wide, continuously skylit residual space between the backs of the rowhouses and the new building runs the length of the block. The linear atrium, lined by shops in both new and old buildings, seems visually schizophrenic. Its south side, slick and modern, contrasts sharply with its north side formed by the leftover, irregularly shaped rowhouse shells.

*Washington Square, on the west side of Connecticut Avenue at L Street NW, represents an unusual transformation of the atrium idea. Here, octagonal atriums appear on the corners of the building as if exuding from the interior and awaiting additional building masses to enclose them. These crystalline towers of space, rising the full height of the building and accommodating eating, viewing and a Metro subway entrance, are Washington Square's most unforgettable exterior feature, having become Connecticut Avenue landmarks.

Atrium buildings never really stand alone. Although each is conceived, financed, designed, constructed and leased as if it were one of a kind, each nevertheless becomes part of a greater fabric, contributing another interior public space to a city characterized by significant exterior public spaces. Indeed, it is symbolic that so many of the District's atrium projects are called "Square," a term usually associated with exterior urban spaces.

If the atrium-making trend continues, Washington could someday become a city where automobiles dominate the streetscape as people thread their way from one building to another, from one atrium "square" to another, perhaps through underground tunnels or overhead covered bridges. Fortunately, the capital's climate and L'Enfant's plan are too hospitable to move everything indoors.