Like two siblings who tease each other relentlessly, New York City and London are more alike than they prefer to admit. A conference sponsored by the British Council for Offices last month in Manhattan demonstrated how these cities often copy each other’s best practices for competing in today’s global economy—and architecture, it turns out, is increasingly important to both.

“For a long time, high quality design was seen as an inefficiency,” observed Mark Wigley, dean of the Columbia University Graduate School of Architecture. “Today it’s seen as leverage.”

New York’s real estate developers have come to realize the value of design both in commercial and residential buildings: architecture helps them earn higher rents and sales. Yet Wigley highlighted an irony in this trend. Although New York clients are hiring high-profile architects from both sides of the Atlantic—among the Brits are Nicholas Grimshaw, Richard Rogers, and Norman Foster—“none of them are doing their best work here yet.”

But when it comes to green buildings, although London had an early edge on sustainable architecture, PlaNYC promises to level the playing field. “Anyone who is not building green today is not looking forward,” said John E. Zuccotti, chairman of Brookfield Properties. Noting that rising energy costs will make green buildings more valuable in the future, Zuccotti added that constructing anything less than green “is not smart business.”

Sustainable development could also hold the key to the long-term competitiveness of both New York and London—particularly as Chinese cities begin to challenge them for the position of the world’s economic capital. In an age of megalopolises, these sibling cities must begin to think of themselves as mid-sized, which could offer new advantages compared to their more polluted and harried rivals.