When Lisa Cini and her husband, kids, and rescue dog moved in with her parents and grandmother a few years ago, the Ohio-based architect pored over the design of her 94-year-old grandma’s bedroom “apartment.” An Alzheimer’s diagnosis made security and mobility important, but her ideas went beyond extra locks and grab-bars; she felt it was crucial that she have her own living room within the family home.

“It’s interesting, when we’re younger and full of life, when we’re just doing life so hard, we have to find time to sleep. But when we get old, when we’re slowing down so much, we have to work to find ways to do more life and less sleep,” Cini recalls in her book about living with four generations under one roof. Designing a separate living room gave her grandma a space to hang out, engage, and entertain visitors outside of her bedroom, a subtle but important distinction. “Her living room really helps her keep living life,” Cini observes.

Cini’s situation–four generations under one roof–was an unusual one, compared to the way most Americans have lived over the past century, when socioeconomic forces have made it normal for Americans to live as nuclear families, in contrast to the last few thousand years of human history.

But for complex reasons that still puzzle researchers, multigenerational households are now on the rise once more. As many as 41% of Americans buying a home are considering accommodating an elderly parent or an adult child, according to a survey conducted by John Burns Real Estate Consulting. Living with your parents (or your adult children) has plenty of potential benefits–everyone tends to save money, it can potentially benefit health outcomes, and you get to spend more time together.

The emphasis on physical and financial independence at every stage of adulthood has high incurred costs, though. The first is the massive accumulation of capital, from money to land to natural resources to labor, necessary to supply the cars, airports, fuel, roads, land, and housing for a country of 327 million people who want to live conspicuously apart.

The second is social isolation. The idea that it’s normal for each nuclear family to own a single-family home, connected to other people only by cars, is actually “radical,” as architect and cohousing development consultant Katie McCamant puts it. “It’s held up this great dream that not only Americans should strive so hard for, but the whole rest of the world looks to as a model now,” she says. “There’s been so much emphasis on independence and on privacy that we really designed community right out of our lives without knowing it.”

In short, for complex economic, social, and cultural reasons, what constitutes “normal” housing for seniors in America is changing. Culture may also play a role. “I think there’s a tighter connection just generationally between young adults and their parents,” says Chris Porter, an analyst at John Burns Real Estate Consulting who tracks housing trends. That closeness is influencing the senior housing market, as well as the way senior-focused housing is marketed. “We’re seeing the golf course as less of an amenity these days for senior housing,” says Porter, who has worked with several developers to redevelop golf courses as housing. “The real amenity for seniors is being near their kids and grandkids. I think that comes back to that connection between the boomers and their kids.”

Cini’s grandmother passed last year, and looking back, she has a few things she would do differently. Some have to do with small quality-of-life details: She wishes she would have added heated flooring to her grandma’s bathroom and a light under every stair. Others underline the challenges of designing for four generations with different expectations about technology. Even though she and her husband could control their Philips Hue lights from their phones, her dad missed the light switch. “It’s still about choice. I think we forget that that should be an option,” she says.

According to research by the AARP, almost 90% of seniors want to remain in their own homes as they age, also known as aging in place. That can be complex. For a multitude of reasons, living with your adult children, if you have them, isn’t always an option. Caregivers are increasingly hard to come by, and not all homes are designed for aging bodies.

There are other, subtle problems that aging in place can create, as the architect Katie McCamant points out. “What I hear a lot is when people first retire, they often say, ‘I’ve never been busier. I’m involved in all these clubs and these groups and doing this and that and volunteering there,’” she says. But all that activity usually depends entirely on the ability to drive. Take that away, and aging in place gets more complex. “You find out that my connection to all these things I’m so busy with is my car. And if I can’t drive, I’m totally cut off.”

Read the complete article on Fast Company here.