Even with its glistening emerald-green glass, the boxy 1960s-era Zalco Building in downtown Silver Spring is hardly noticed by many passersby, let alone thought of as a historic structure.

The very idea makes John Cranston, the building’s engineer, chuckle. “I don’t think George Washington slept here or anything,” he said.

But to Clare Lise Kelly, a historic-preservation planner for Montgomery County, and to other architectural experts, the office building at Georgia Avenue and Cameron Street is a shining example of International style. It’s time, they say, for it and other “mid-century modern” buildings and homes — those with sleek, boxy designs from the 1950s and 1960s — to be considered for historic preservation.

“The challenge is always preserving the recent past,” Kelly said. “It’s easy to look at things from 100 years ago and see them as historic. . . . If we don’t act now to assess resources from this time period, they’ll be gone, and then it’s too late to say, ‘That apartment complex was really special.’ ”

Popular interest in mid-century architecture and interior design has surged in the past few years with the success of the “Mad Men” TV series. But architectural preservationists began paying closer attention over the past decade as more modernist buildings passed the 50-year mark, traditionally the minimum age for consideration as historic.

Preserving some of that past, experts say, is especially important now, as areas such as White Flint in Montgomery and Tysons Corner in Fairfax County continue to transform from sprawling suburbs into more urban nodes of high-rise buildings clustered around transit stations. Many of the newer developments, they say, feature more traditional architecture because they are designed to re-create the feel of the more walkable, old-fashioned Main Street.

Kelly said she realizes that some people consider modernist buildings too young — and, in some cases, too plain or ugly — to warrant protection. It’s not about age or looks, she said. It’s about preserving critical pieces of architectural history from the post-World War II population and building boom that transformed suburbs such as Montgomery from rural bedroom communities into dense subdivisions and commercial districts.

County planners have begun building their first database of mid-century modern buildings and subdivisions. The “Montgomery Modern” architectural survey, Kelly said, will ensure that the most significant structures survive.

Montgomery is especially fertile ground for mid-century modern architecture. In the county, as many new homes and buildings took root during the first decade after World War II as had been built in its entire prewar history, Kelly said.

The buildings generally lack classical details such as columns and ornamentation, and often have construction materials — concrete, glass panels, steel frames — embedded in the design. Mid-century modern houses typically have open spaces, a casual feel and extra-large windows to mesh living spaces with nature.

“For Montgomery County, the late 1940s, 1950s and into the 1960s was one of the most important periods of its development and its history,” said Richard Longstreth, director of the graduate program in historic preservation at George Washington University.

Current county plans to redevelop some areas into densely populated high-rise communities need to make provisions for preserving the more open housing built earlier, Longstreth said. Replicating it today would be too expensive.

“We can’t afford to build the tract houses that were built in the 1950s,” he said. “Construction costs are too high. Land values are too high. . . . A lot of these neighborhoods need to be seen as irreplaceable assets to a community.”

Montgomery’s efforts don’t mean that every ranch house or 1950s-era office building will be saved forever, Kelly said. Planners will focus on those designed by noteworthy architects, those who have won major architectural awards and designs that best exemplify modernism, she said.

Structures in the database will be further evaluated when long-term plans for their areas come up for review. If granted a local historic designation — a decision ultimately made by the County Council — they would have to undergo a special review as part of any proposals to tear them down or change them significantly.

Montgomery appears to be at the forefront of trying to fully document mid-century modern architecture. The District’s historic preservation office published a sample of significant mid-century modern buildings in 2008, but no citywide analysis has been done, said Rebecca Miller, executive director of the D.C. Preservation League. Prince George’s County hasn’t done such a survey because most of the county’s postwar architecture was more traditional, said Howard Berger, supervisor of the Prince George’s planning department’s historic preservation section.

Much of Arlington County was developed around World War II, and its architecture is primarily Colonial. However, the county is updating its historic resources survey to include more modern homes and building styles, said Cynthia Liccese-Torres, Arlington’s acting historic preservation coordinator. Fairfax lists 15 structures and subdivisions from the late 1940s through 1960s on its inventory of historic sites, though it’s difficult to discern which made the list for modernist architecture.

Montgomery’s database includes Weller’s Dry Cleaners at 8237 Fenton St. in downtown Silver Spring (1960), a prime example of Googie architecture’s space-age motifs. Others include the Geico insurance company at 5260 Western Ave. in Friendship Heights (1959), the White Oak Professional Center at Lockwood Drive and New Hampshire Avenue (1965) and the Glenmont Forest apartment complex at Georgia Avenue and Randolph Road. Formerly known as Americana Glenmont, most of the complex was built in 1961.

Subdivisions include Hammond Wood (1950) and Rock Creek Woods (1958) — Silver Spring neighborhoods that have homes designed by well-known modernist architect Charles Goodman — and Carderock Springs in Bethesda (1962). Those subdivisions are on the National Register of Historic Places. However, they don’t have county historic status, which protects buildings from tear-downs or significant changes.

Whether some property owners will want that protection is another matter.

Anthony G. LaBarbera, managing director of Guardian Realty Investors, whose subsidiary owns the Zalco Building, said the company is trying to determine how any local historical designation would affect future redevelopment plans.

“In general, building owners get concerned . . . about what you can do with your property,” LaBarbera said. “It’s a limitation. That’s the question.”

Michael Shapiro, a Realtor with Long & Foster who specializes in mid-century modern homes and runs the Modern Capital Web site, said mid-century houses appeal to people looking for a more casual, open feel, often at more moderate prices.

“It's not the traditional homey Colonial all decked out for Christmas,” Shapiro said. “But I like the clean lines. It’s simple and minimalist. People don’t want the fussiness. Why do you need wainscoting and crown molding?”

Even so, Kelly and other architectural historians say that they know others will need some convincing. Kelly predicted people soon will appreciate the historical significance of mid-century modern like they now cherish the Victorian homes and art deco buildings once considered outdated and ugly.

Steve Smith looked a bit befuddled when asked what he thought of the Spring Street Center office building at 1400 Spring St., across the street from the park where his son played one recent afternoon. The building is one that Kelly described as a “mid-century modern gem” with “beautiful turquoise spandrel panels made of porcelain enamel.”

Smith sized up the panels and the plain box shape and concluded with a light laugh: “It’s okay, but it also looks like it could have been built in the Soviet Union. It’s no Frank Lloyd Wright, let’s just say that.”