A decade ago, Polly and Keith Nelson began touring towns touted as ideal for retirees. As the teacher and school psychologist approached their final working years, they explored communities in the Carolinas and Arizona. The couple, who've long resided in a Philadelphia suburb, thought they'd appreciate a more temperate climate.

But four years ago, they bought a condominium in Minneapolis. The city offered something they could find nowhere else.

A grandchild.

"Peter was born, and six months later we had the condo," said Keith Nelson, now 68, whose grandkids (yes, there's another one now) call him "K-Pop."

The Nelsons purchased a unit at RiverWest — in the same building where their daughter and son-in-law are raising their family. The Pennsylvania couple come to Minneapolis for monthlong visits three or four times a year.

"It's convenient for us to all have our own space, but it's nice to be able to go back and forth without getting out of your slippers," said their son-in-law Eric Laska, 37, who lives five floors above the Nelsons.

New multi-story, multi-family buildings springing up in the Twin Cities offer a fresh way for different generations of a family to be close. They can share an address without sharing a bathroom, by living in separate units of the same building.

"Nationally, about 20 percent of Americans live in multi-family dwellings," said Tom Musil, real estate professor at the University of St. Thomas. "There wasn't much of that kind of property in the Twin Cities in the past, but it's a segment of the housing industry that's seeing a big shift. In the past 10 years, there have been thousands of these units added with new construction or conversions."

Condo living is appealing to both downsizing baby boomers and young professionals who are building their first nests. It's kismet when what they're looking for intersects in the same building, with easy access to one another as the big drawing card.

"Living with extended family has significant positives. It tightens the family unit, cements the bonds," said Susan Newman, a social psychologist and author of 15 books on family topics, including "Under One Roof Again: All Grown Up and (Re)Learning to Live Together Happily."

"It gives children a sense of security and continuity. It gives their parents support; they have someone they trust in the fallback position. It gives the eldest members of the family great pleasure and meaning."

'Here for the milestones'

The Nelsons have embraced the civic culture of their new city during their extended visits. They've become enthusiastic fans of the Twins, the Lynx and the Guthrie, regular volunteers at a nearby shelter and visitors to a church within walking distance.

But the stimulating culture of downtown Minneapolis pales in comparison with the satisfaction of grandparenting.

"We wanted to know them, and we wanted them to know us. We want to be involved in their lives," said Keith Nelson. "We're here for the milestones — when Ruby took her first steps, when Peter began to talk. I notice the little things more than I did as a parent, when I was going off to work every day. I have more time and fewer distractions."

"They get to interact in the routines of our real life," said their daughter, Melissa Laska, 38, an associate professor at the University of Minnesota. "In the morning, they might come up and have coffee and give an extra hand while we're getting out the door. We're folded into each other's lives in an easy way."

A growing number of Americans are choosing to reside with extended-family members. A study by the Pew Research Center tracked a small but significant recent jump in such households, with the number of Americans living in multi-generational homes rising from 51.5 million in 2009 to 56.8 million in 2012, a number that represents 18.1 percent of the population.

Such familiarity may not work for every family, but consumer strategist Kate Muhl, who tracks trends for CEB Iconoculture, finds family engagement has high value across the generations.

"People realize that quantity time is super-valuable; there's no replacement for it. For young families with dual-income parents, there's a real practical need to have trusted people around to keep things going. And grandparents have a great investment in the lives of these children."

Today's young adults often come from small families and have experienced tighter relationships with their parents than had been the norm in previous generations. Those connections may prime some families to consider living arrangements that make them neighbors.

The grandparents upstairs

When 2-year-old Rozlyn Gerber grows up and remembers going to visit her grandparents, it will not involve going over the river and through the woods.

She will remember getting into an elevator.

The toddler lives with her parents in their condominium on the second floor of the Carlyle, a building that overlooks the Mississippi riverfront in downtown Minneapolis. Her maternal grandparents own a unit on the 36th floor.

"You know that sitcom 'Everybody Loves Raymond,' with those in-laws barging in all the time? It's nothing like that," said Rozlyn's mother, Betsy Kuller, 32, an analyst at Best Buy. "They're very respectful of our space, and that's why it works."

Kuller's parents left their traditional home in the western suburbs in 2008 to join the first wave of owners when the Carlyle opened. When Kuller and her husband, Ben Gerber, began their search for a condo, they walked through a unit in the building to gain a point of reference, but decided they needed to look no further.

"We were thrilled they chose it; it's kind of a compliment," said Susie Kuller, 63. "Being in a large building with multiple units makes this work. We don't often bump into each other. If I have something to drop off but they're at work, I have a key but I don't go in."

Often when adult children and their parents initiate a move to live closer or even together, it's to fulfill caregiving needs of the elder generation. But that's not a factor for most baby-boomer grandparents moving into multi-family buildings. In good health and active, they don't need help — instead, they're eager to provide it.

"My husband works two-thirds time, and I volunteer twice a week, so we have time to have fun with our family, said Susie Kuller. "We both still have one living parent, so needing to be looked after is not on our minds."

One of the benefits that people who live in the same building mention is how the arrangement allows them to initiate spontaneous time together.

"We get to see each other without any pressure — no one has to be the host," said Betsy Kuller. "I can run out on a short errand without having to line up sitting; they love to have her come up."

Families who consider living in the same building may think they are part of a new housing trend, but in fact, it's a return to the way that families lived for centuries.

"Middle-class white families are catching up to what's been working for multicultural communities forever. For newcomers, the way to get a foothold has always been with the support that family can provide," said consumer strategist Muhl. "There's a lot to learn from them."

Kevyn Burger is a Minneapolis-based freelance writer and newscaster at BringMeTheNews.com.