Sustainability cannot happen at the global scale — that is far too vast to be knowable or controllable. The smallest scale at which such problems can be meaningfully resolved is the city. Cities are capable of addressing the many urban architectural, social, economic, political and other imbalances besetting the modern world and simultaneously.

American cities account for nearly 90 percent of total U.S. economic output, and 85 percent of U.S. jobs. Science and business can make a lot of things possible. Due to their increasing economic importance, cities are the engines of economic growth of the future.

In 2010, the McKinsey Global Institute (MGI) released its study “Urban World: mapping the economic power of cities,” which identified the 600 cities on the planet that collectively will generate 60 percent of the world's GDP growth between now and 2025. While the megacities are the biggest attention grabbers, it’s the “middleweights” of the developing world — cities from 200,000 to 10 million — that will generate the bulk of this growth.

The view of MGI is that there is, in theory, no limit set by technology or infrastructure to how big or how fast cities can grow — but only if business and government leaders are able to manage the increased complexity that comes with bigger city size.

By the year 2050, 80 percent of the world’s population will live in cities, while 75 percent of our existing building stock will still be in use. Writing at McKinsey and Company’s “What Matters” site, Richard Dobbs and Jaana Remes suggest that cities have precious little to stop them from growing bigger than ever before — except the governments that run them. According to research from MGI, there is no limiting factor — in technology or infrastructure — to how big or fast a city can grow. The wild card: Whether government officials and business leaders can manage that increased complexity.

Green is good for the economy. Cities are natural sites for the emerging “green” economy. Many cities accelerate their sustainability agenda through green building, neighborhood revitalization, existing building upgrades and sustainable infrastructure development. These strategies are complemented by alternative transportation choices, restoration of the natural environment, food security, resilience and green jobs.

While the transformation of buildings and communities is happening, it needs to occur on a larger scale and be based on a broader set of next generation urban issues critical for a healthy environment and better quality of life.

Last year, Seattle City Council resolved to adopt guiding principles for strengthening and growing Seattle's economy, designed to create jobs. These reforms embraced community by putting a roundtable of stakeholders together to come up with new code that included backyard cottages, multifamily development and street level retail. The group was not a "blue ribbon commission," nor did it have a staff. Instead, it met regularly and worked with city officials to brainstorm next generation urban solutions. The goal: Help Seattle residents live closer to where they work. The starting place: Simplify the city's Land Use Code.

The roundtable assumed home-based businesses are good for the community. As incubators for new ideas, they create jobs. The new code allows property owners to pursue “home occupations" in any structure, as long as the impact to surrounding properties is minimized, but it did not end there. There is a trend toward re-establishing the corner store in residential neighborhoods. The new code allows small corner stores and for vending carts in "Lowrise 2 and 3" multifamily zones and light rail station area overlay districts. The code goes much further, allowing for Accessory Dwelling Units (ADUs), backyard cottages on "through lots" (lots that front two streets).

Laneway housing, called “backyard cottages” are becoming popular almost a century after people stopped building them. Toward the end of the 19th century urban areas began running out of room, and building small housing units behind the original home became popular. City governments encouraged homeowners to build cottages in their backyard to accommodate another family. The concept of laneway housing has come full circle.

Changes in City Land Use Code are the elephant in the room to some. There's a move to multiunit housing as well as smaller structures, such as backyard cottages. The prefab market is positioning itself for major growth here. New Hampshire-based Bensonwood Homes builds high-end homes in sections. Blu Homes trucks modular homes, origami-like, across the country to create homes more than twice that width.

The cost is all over the map. Santa Monica, Calif.’s LivingHomes charges more than $1 million. Virginia-based Nationwide Homes sells tiny Eco-Cottages that start at 250 square feet for $37,000, and Tennessee’s Clayton Homes sells a stylish, two-bedroom, 1,023 square foot iHouse for $100,000. That is pricier than many conventionally built production homes.

Some earn top ratings from the U.S. Green Building Council or qualify as "net zero.” "The folks doing modular now are going way beyond code," says green-building expert David Johnston, co-author of Toward a Zero Energy Home. He says those building old-fashioned, inefficient prefabs are struggling to survive. Green prefabs have multiple advantages. Among them, they waste fewer materials.

It’s easy to see how “middleweight” cities will be at the forefront of future growth as they encourage change and introduce new “old” styles of housing into the urban vernacular.