If there had been a national preoccupation with historic preservation in 1915, a turreted, red brick "castle house" at 1787 Massachusetts Ave. NW might have been saved by a don't-tear-it-down campaign.

In that event, it is highly unlikely that the National Trust for Historic Preservation would have had the opportunity to buy, reconstitute and occupy the Beaux Art structure that is its new home. The Trust's new headquarters, finished on the site of the old Belden Noble mansion in 1917, comes with a bronze plaque noting that it is a national historic landmark.

Apparently, no one rose up to protest the demolition of the Belden Noble structure, which might have callously been described as a monstrosity mansion, U.S.A. style. After it was torn down, Stanley McCormick (whose father invented the reaping machine) created a truly luxurious apartment building on the site -- for $350,000.

The Trust purchased the building recently for $1.3 million from the neighboring Brookings Institution. An estimated $2.5 million has been spent on rehabilitation and improvements.

The original architect was Jules Henri de Sibour, whose choice of Beaux Art design was traceable both to his birth in France and his attendance (after prep school at St. Paul's in New Hampshire and then Yale University) at the Ecole des Beaux Arts in Paris.

The five-story, steel and concrete apartment house was faced with limestone, trimmed with pediments and metal railings and fronted with a modest, curving corner that dresses up the northeast corner of Massachusetts Avenue and 18th Street, just east of Dupont Circle.

When it was opened, 1787 (that's its name now after previously being called the McCormick and then the Andrew Mellon building) was the residence of six families and about 40 servants. Each apartment had 11,000 square feet of space. That's about seven times the size of the average American house today.

What the rent was might be anyone's guess. But it could have been even more in non-inflated dollars than people pay today to rent flats in the such developments as the Watergate, the Irene or Van Ness.

"Originally, the entrance floor contained two apartments, with their kitchens in the basement below (used now by the Trust for its library and legal offices)," noted writer Anne Woodward in a recent issue of Historic Preservation magazine. "The apartments on the four upper floors were nearly identical and each used a full floor of the building. The occupants could receive in three foyers and entertain in a 21-by-24-foot salon, a 24-by-45-foot living room and an adjoining 24-by-35-foot dining room.

"There were six bedrooms and servants worked from a series of service rooms.

In a unique layout, the servants' living quarters (five rooms) were located to the inside, on eight mezzanine levels adjacent to the five major floor levels and facing into a central light court."

It remained an apartment building until 1940. During that period, its residents included Andrew W. Mellon, Sumner Welles, Katherine Judge, Alanson B. Houghton, Pearl Mesta, Robert Woods Bliss and others whose names might be familiar after all these years.

The 1787 building took on particular class when it was the residence of industrialist Mellon, who also served as Treasury secretary. While living in his top-floor apartment, Mellon completed plans for the National Gallery of Art, abetted by his neighbor below, the legendary art dealer Joseph Duveen.

Mellon was charmed by Duveen's collection of famous paintings and finally bought them for many millions. He donated them when the National Gallery was organized in 1937.

During World War II, the apartment building at 18th Street and Massachusetts Avenue was converted into offices for the British government. The American Council on Education owned the building during the 1950s and 1960s and used it as a headquarters. Brookings acquired it in 1970 and leased out the space.

One of the tenants was the American Institute of Architects, which camped out there while its new headquarters was being erected at 18th Street and New York Avenue NW.

The 1787 building is taking on a new role now in the hands of the National Trust for Historic Preservation. The Trust is the only national, non-profit organization chartered by Congress to encourage public participation in the preservation of sites, buildings and objects significant in American history and culture.

The Trust had been looking for an appropriate site for its Washington office and wanted also to demonstrate adaptive reuse of older space. Its project architect, James Vipond, says that the Charles H. Tompkins Co. did a fine job as the general contractor for the new work.

The building has central air conditioning for the first time, but the Trust has retained the original hot water and steam radiators, which are hooked up to a gas-fired boiler that was "so great it warranted revamping," Vipond said.

Nick Pappas, of the Washington firm of David N. Yerkes & Associates, which planned the restoration, said the building is a good example of 17th century French design. The original blue color of the marquee trim has been restored, he added.

Pappas says that the Trust wanted to show support of energy conservation and to that end installed a small, auxiliary solar panel collector system on the roof to heat some of the water used in the building. "We talked about doing a windmill on the roof, but discarded that idea," Pappas recalled.

The purchase and reconstitution were paid for in part by the Department of Interior and partly through the fund-raising efforts of the Trust and the private sector. Washington mortgage banker and real estate firm executive O. Mallory Walker, who played a major role in finding money for the project, was lauded by Trust President James Biddle at a recent reception at the headquarters.

Biddle, who has headed the Trust since 1968, intends to resign and return to Philadelphia.A committee has been organized to select a successor.

In addition to the Trust, which occupies the basement and first three floors, the building is inhabited by the Carnegie Foundation, the Asia Society and the law firm of Iron & Sears. Leases average $14.50 a square foot, which is comparable to prime new office space in the downtown.

The National Trust continues to operate Decatur House at 748 Jackson Pl. NW, where some of its staff formerly worked, at a museum. A regional office of the Trust is in an adjoining wing of Decatur House on H Street NW. Other Trust offices are in San Francisco, Boston, Chicago, Oklahoma City and Charleston, S.C.