Extended life span, increased immigration, a weak economy and a tight housing market have contributed to a resurgence in multigenerational households.

For some time now, builders and remodelers have helped families accommodate these changes. They have done so by constructing extensions and "granny flats" or converting basements, attics, garages and great rooms into living areas for aging parents or "boomerang" young adult children.

As demand grows, some builders and architects are taking a new, more holistic approach to designing multigenerational housing for the 21st century.

Analysis of U.S. Census data by the Pew Research Center, released in July, found that the number of Americans living in multigenerational households doubled between 1980 and 2012, to a record 57 million — nearly 1 in 5 residents. For the first time, the number of young adults (ages 25 to 34) living in multigenerational households surpassed that of adults 85 and older, while between 2010 and 2012 the share of adults ages 65 to 84 living in multigenerational households declined slightly.

The most recent Home Design Trends Survey from the American Institute of Architects reflects these changes. The findings reveal an increased interest in "homes that can accommodate multiple generations" as well as "special features of homes [that] focus on improving accessibility around the home," including "ramps and elevators, first-floor master bedrooms, on-grade home entry and easy-to-use features."

In addition, respondents to the survey reported a notable increased interest in au pair or in-law suites (39 percent in 2014 vs. 26 percent in 2013), a result of dual-income couples preferring to maintain child care and/or adult care services at home.

An "ethnographically informed" list of 14 top current home design trends prepared for Builder magazine includes dual-use homes, noting, "Multigenerational living has become part of the 'next' culture. Families are staying together longer and the coupling of families becomes economic as well as cultural."

A new discussion paper from the Urban Land Institute (ULI) on multigenerational housing and intergenerational communities presents a more in-depth picture of this trend, describing a number of different scenarios in which the generations within families share housing to assist each other not only economically but also with child or adult care, household duties and home maintenance.

Although the greatest growth of late has been in the numbers of young adults (mainly males) living with parents or relatives, ULI members who provided input into the discussion state that the highest demand in multigenerational housing is still for single-level models with a separate (attached or unattached), accessible living unit, preferably with a bedroom, closet, bath, kitchenette or food bar, and a separate exterior entrance, that could accommodate a person or persons of any age.

Until recently, such accommodations were out of reach to nearly everyone except customer homebuyers. But that's changing.

Introduced in 2012, Lennar's "Next Gen" model takes this concept one step further by incorporating "a home with a home." This first-floor living quarter also includes a great room connected with the main living areas of the home, with additional bed and bathrooms on a second story to give everyone in the family their "space" and privacy.

Australian-based TR Homes offers a similar model they call "3G." Designed on a single level in three wings, it includes an independent living space — separated from the bedroom areas by the shared living areas — with a separate exterior entrance, en suite, living area and optional kitchenette and laundry facility.

But what of the urban dweller who cannot enjoy such spacious accommodations? ULI members mention that some families are linking townhomes, duplexes or triplexes to create shared but separate living quarters.

A recent article on Urbanful reports on innovative designs by architects in Philadelphia and San Francisco. One doubles the footprint of a typical row house to accommodate a single living unit with a street-level entrance; the other "flexes" a condo into two separate spaces, a studio and a two-bedroom unit.

Enterprising owners can rent out the unused units until need arises to house an au pair or aging parent. Perhaps we are seeing the beginning of a new trend, the evolution of multigenerational housing into lifespan housing?