These architects broke with the British Raj’s ornate Victorian Gothic and Indo-Saracenic styles for the latest international trend — described by a leading British architect as “the nudist movement in our profession” — while adapting it to the local environment. The choice could be seen as a form of resistance, said Mustansir Dalvi, professor at the Sir J. J. School of Art and Architecture, and as a backdrop of the freedom movement.

Movie houses, many built by American movie companies like MGM, glamorized the new aesthetic. A new technology, reinforced cement concrete, made all this construction quick and cheap. The apartments were snapped up by a rising urban elite “who aspired to be modern and were willing to live next to those who were not like themselves,” said Mr. Dalvi. “They sat next to each other in offices, on commutes, in cinema halls.”

He adds: “For Bombay , what Art Deco represents is cosmopolitanism.”

In a city increasingly dominated by gated communities and hodgepodge skylines, the Deco neighborhoods recall an age of openness and urban coherence. Strict bylaws ensured public spaces and amenities. Buildings had low compound walls. “The wonderful thing about the Art Deco era is that it gave us neighborhoods, not just single pieces,” Mr. Dalvi said.

From Mumbai, Art Deco spread to other cities. The style’s afterlife in India lasted into the late 1940s and early ’50s, and paved the way for modernism after independence in 1947. For decades, however, the contribution of the Art Deco era was overlooked. Architects worshiped high modernists like Louis Kahn and Le Corbusier while conservationists focused on ancient and colonial monuments.

For years, residents like Nayana Kathpalia, a member of the Oval Trust that supported the heritage campaign, were unaware of the historical or aesthetic value of their buildings. “We just thought it was a good place to live in and look at,” Ms. Kathpalia said.

The recent interest comes just as this layer of the city is vanishing. The Unesco tag now protects Marine Drive and Oval Maidan, but everywhere else old buildings are falling daily — and with them many memories.