In 2012, Lennar Corp, a $4bn US-based home-building company, launched a new product line called "The Next Gen." The line boasted floor plans that offered "a home within a home," perfect to accommodate a second generation.

The company, like many others, was looking to take advantage of the growth in multigenerational living in the US, a trend that's likely to grow as youth unemployment rises and more adult children move back home after college.

In theory, this is not just a great marketing and sales opportunity, but also a chance to make households both more sustainable and more resilient. As the number of residents per home increases, their ecological footprints shrink. And adding more income earners – or family members to care for children, grandchildren, parents or grandparents – to a household can potentially boost its resilience.

But a closer look at Lennar's current models for sale makes it clear that these gigantic homes – ranging from 3,101 to 4,058 square feet – are far to large to ever be deemed sustainable (even when adding an extra generation into the mix). They're also too big for any compact, walkable city design.

The good news is that a more organic effort to increase housing density and enable multiple generations to live together appears to be underway, especially in the Pacific Northwest. Specifically, this includes the development of "detached accessory dwelling units" or DADUs, in which a small home is added to the yard of a primary dwelling. (An in-law apartment is an example of what is called an AADU, or Attached ADU).

Two homes, one lot

These DADUs aren't as efficient with energy or materials as AADUs, explains Jordan Palmeri, a science and policy analyst with the Oregon environmental quality department's green building program. But they provide a much more flexible housing option that can evolve as communities evolve.

Currently, just 5.6% of US households are multi-generational, according to the Census Bureau. The odds are good that this percentage will grow in the future, as the era of economic growth ends, but in the meantime, existing DADUs could be easily filled with other tenants, creating income for homeowners and smaller, more affordable places for renters to live.

In Portland, for example, nearly 80% of ADUs are primary residences. At an average of 665 square feet, they don't qualify for "tiny house" status, but are definitely on the sustainable side for a couple. ADUs' average number of square feet per occupant is 488 square feet, just 59% of the 827-square-foot average of a primary home. As Palmeri puts it: "Larger homes are typically much more underoccupied than ADUs." Having a smaller home also means having less space for stuff, which means that total consumer product purchases also should shrink accordingly, to additional ecological benefit.

"ADUs are not a perfect silver bullet," Palmeri says, "but they are a good way to help make the transition and increase density" of single-family neighborhoods. That's not insignificant: In Seattle, for example, single-family neighborhoods – filled with 7m single-family homes – make up 65% of all zoned land. In fact, Alan Durning, executive director of the Seattle-based sustainability think tank Sightline Institute, says opening up single-family neighborhoods to more residents is "the only path of urban development that can lead to affordability, and climate and energy security" for the Pacific Northwest. That could also be said about many other parts of the United States.

Hurdles ahead

But unfortunately, right now there are plenty of hurdles in accelerating ADU development, from arbitrary rules to financing challenges. There's also the fact that few ADUs are included in the construction of new residential housing, meaning that most of these ADUs need to be initiated by existing homeowners

Some cities, such as Vancouver, Canada, have streamlined the process of adding ADUs to the housing mix. And some cities, such as Coquitlam, Canada, have taken this a step further, requiring new residential properties to be designed so that owners can easily convert part of their home into an AADU.

As humanity moves towards sustainability (proactively or otherwise) one thing that will surely change is how and where we live. If we wait too long to make the transition, perhaps we'll end up taking a page out of the books of slum dwellers and tent city residents on how to make basic informal shelters to keep us warm and dry.

But if we're proactive, perhaps the future will look like a denser version of Vancouver or Portland, with ADUs speckled throughout neighborhoods, grandparents once again helping to look after of grandchildren, grandchildren helping to care for grandma and grandpa, and more resilient and robust local economies as a result of the multigenerational neighborhoods.

Erik Assadourian is a senior fellow at Worldwatch Institute and co-director of State of the World 2013: Is Sustainability Still Possible?