The architect Philip Johnson’s Glass House, a rectangular glass-and-steel residence set on a grassy shelf above a wooded bluff in New Canaan, Conn., has epitomized a certain East Coast ideal of midcentury elegance since its completion in 1949. Before becoming an architect at age 37, Johnson ran the architecture department at MoMA, and the spare, luminous building, which he inhabited for over half a century, embodies the Modernist International Style that he helped define in a landmark exhibition at the museum in 1932. The home also established Johnson himself as the paragon of a specific type of New York architect: erudite, absolutist in his refinement and formidable in his influence wielding, shaping careers, institutions and public opinion like few others in his field.

But since the National Trust of Historic Preservation opened the Glass House to the public as a museum in 2007, visitors have discovered there’s more to the place than its namesake centerpiece. By the time Johnson died in 2005, the five acres he’d bought in 1946 had grown tenfold to encompass 14 structures, including experimental follies, a subterranean painting gallery and three wooden homes from earlier periods, including a shingled 18th-century dwelling that Johnson and his partner, the curator David Whitney, would use as a refuge in hot weather. For the past 15 years, however, a pivotal part of the estate has remained semi-concealed: Johnson’s guesthouse, known as the Brick House and situated just 80 feet from the site’s main attraction, has been closed to the public because of water damage. Now, after an extensive restoration and in time for the Glass House’s 75th anniversary, the building has finally been unveiled.

Johnson considered the 1,728-square-foot Glass House and its 860-square-foot brick companion, which was built at the same time, two parts of a single home — one alluringly crystalline, the other introverted and opaque. He wrapped the smaller building entirely in iron-spotted red brick and positioned it facing the main house at a slight angle, with a gravel pathway crossing the courtyard between them. The structures are also linked below ground: Along with a bedroom, study, storage room and bathroom, the Brick House contains the unsightly mechanical equipment that supplies the Glass House with electricity and heat, enabling the larger building to maintain its aesthetic purity. Tellingly, Johnson placed the Brick House’s only windows — three big mahogany-framed portholes — on the building’s back side, facing away from his glass retreat. “I didn’t see why the guests should have a window looking out toward my house,” he said in an unpublished 1991 interview for the National Trust. “They can look their own way out to the hill.” But he and Whitney also often slept in the building when they didn’t have visitors.

The Brick House is stern, squat and solid, its front interrupted only by a tall, centered black pinewood door. Even Johnson admitted it wasn’t much to look at, calling it “perfectly plain.” But if the exterior is unassuming, Johnson created an unexpected landscape of color, texture and fantastical detail inside. At one end of the bright entrance hall, which runs parallel to the front of the house, a door gives way to the building’s showpiece: a dim, sand-hued bedroom that is at once monastic, womblike and glamorous. Johnson — who never shied away from, as he put it in the 1991 interview, “deliberately copying whatever I felt like” — modeled it after a domed parlor in the early 19th-century London home of the English architect John Soane. Soane described the layered design of that room as “a succession of fanciful effects,’’ and Johnson deployed his own series of clever tricks. First, he built an off-white plaster pavilion inside the 10-by-26-foot room. A row of vaults seem to be supported by 14 superslim columns but are, in fact, suspended from the ceiling and give the room the sheltered quality of a cloister.

To heighten the sense of intimacy — “This was a bedroom; why not get cuddly?” Johnson said — the walls are hidden behind panels clad in cotton from the Venice-based textile house Fortuny with a pink, aquamarine and gold feather motif. (One of several donations made during the restoration by the manufacturers Johnson worked with, the fabric was replicated by Fortuny to replace the original, which had darkened with age.) Johnson’s third fanciful idea, developed with the lighting designer Richard Kelly, was to tuck lights between the ceiling and the canopy, producing an indirect glow that accentuates the only artwork in the room, a gridlike abstract metal sculpture by the Jewish Egyptian-born artist Ibram Lassaw that hangs above a low, spartan bed. (Johnson hated headboards.) A plush champagne-colored carpet completes the enveloping effect.

With the voluptuous, theatrical bedroom — part of a 1953 redesign of the Brick House interior — Johnson broke with the purist tenets and rigid lines of his mentor, the German-born Modernist Ludwig Mies van der Rohe, whose own glass-encased structure, Farnsworth House, inspired the Glass House. (Located in Plano, Ill., and completed in 1951, Farnsworth House was designed before but built after Johnson’s project.) Famously prolific and stylistically fickle, Johnson used the Brick House to try out ideas he could revisit in other commissions. The bedroom’s Fortuny fabric reappeared in his interior for the Four Seasons restaurant in New York, completed in 1958, and the canopy’s arches were the seed of his classicist infatuation, which culminated at the Beck House, an opulent private home in Dallas finished in 1964. Johnson’s liberal historical referencing on his own property — he also incorporated nods to the Baroque and Romantic periods — anticipated his embrace of postmodernism in the 1970s and ’80s in projects such as the AT&T Building in Manhattan, where he reprised the Brick House’s circular windows.

The rest of the Brick House’s interior can’t compete with the bedroom’s atmospheric power. The adjacent study has been restored to a 1980s iteration with gleeful blue-and-pastel pink cotton-upholstered wool Feltri chairs by Gaetano Pesce, a dense purple carpet and a section of Johnson’s library that confirms his troubling fascination with Fascism — he openly admired Hitler’s regime before publicly distancing himself from Nazism in 1940. The skylit bathroom next door, with its Greek frieze, black marble walls and floor and polished brass fixtures, borders on camp. And the entrance hall is simple — a gallery-like space currently home to Johnson and Whitney’s prized set of etchings by the painter Brice Marden. (Over the years, Johnson donated some of the artworks he kept in the building, including a Paul Klee drawing that he bought from the painter in 1929 for $75, to MoMA; others were sold after his and Whitney’s deaths, which occurred just five months apart in 2005.)

In designing the Brick House bedroom, “Johnson wanted to show how modern architecture could be warm, and not just sensual but sexy,” says the architecture critic Paul Goldberger, 73, the chair of the Glass House’s advisory council. The room’s reopening will surely encourage theorizing about what purpose the sensuality served. Some scholars have viewed the building as a queer space, decoding the interiors’ seductive artifice as the work of a gay man who was closeted for much of his life. But while the restored Brick House might symbolize Johnson’s compartmentalized identity, it also sheds light on a fundamental paradox of his masterpiece. A separate private domain was necessary not just to act out needs that society couldn’t accept but also because a pristine glass temple wasn’t always conducive to introspection or relaxation. The Brick House both completes an iconic American home and reminds visitors of its author’s humanity: Even a master of spectacle sometimes just wanted to hide.