Living down the hall from a bosom buddy is the stuff of sitcoms. Consider Lucy and Ethel in “I Love Lucy,” Seinfeld and Kramer in “Seinfeld,” or most of the characters in “Friends.”

Everybody knows TV togetherness is good for a few laughs and convenient for scriptwriters. But even in today’s heated real estate market, some New Yorkers choose to move into the same building as their pals, creating enclaves where it is perfectly appropriate to show up for a get-together in pajamas and slippers.

The concept is firmly rooted in history: Immigrants have long rebuilt communities by populating buildings and neighborhoods with extended families.

Nevertheless, artificially creating a neighborhood in a single building is not always easy, frequently requiring a healthy dose of serendipity. A suitable unit must be available in the building; an applicant must win the blessing of the landlord or the co-op board; and friends must adhere to a set of social graces so as to live amicably in shouting distance of one another.

For the New Yorkers who make it work, the payoff can be a lot of fun. New York, after all, can be a lonely city, and having your crew an elevator ride away certainly takes the edge off.

For parents with small children, the arrangement provides for impromptu play dates and moral support. For retirees, living near friends can mitigate the isolation that can accompany aging. And for recent graduates, moving into a building with a friendly face can extend the college dorm years (assuming, of course, that is a good thing).

“We used to live with extended families. In the tenements there would be multiple generations living together. Now most people are in small apartments by themselves,” said Irene S. Levine, a clinical professor in the department of psychiatry at New York University Langone Medical Center who has a blog on friendship. “To have your group nearby gives you some self-confidence — it makes you feel more connected. It can enhance your life to have that sense of support.”

On a Saturday night last March, Catie Abrams realized that she might be able to live in the same building as her boyfriend, two of her college friends and their boyfriends. The three couples had gathered at the Hog Pit, a Flatiron restaurant and bar, to watch a college basketball game. Ms. Abrams and her boyfriend, William McKee, had just returned from a day spent traipsing through the rain looking at rentals.

After visiting a few buildings, they had happened upon 388 Bridge, a new luxury tower in Downtown Brooklyn named for its street address, with numerous rentals available. Ms. Abrams could hardly contain her excitement as she passed out floor plans to the group.

“I said, ‘You guys have to live in this building,’” recalled Ms. Abrams, who is 27 and a hedge fund analyst. “I think everybody was excited.”

All three couples were planning to move out of their Manhattan apartments by summer, and although the idea of Brooklyn was appealing — they could potentially get more space for the money — it was also unnerving. None of them had lived in Brooklyn before. Each worried that if the others did not follow, he or she could end up living in an unfamiliar borough without friends nearby.

“One of the issues that people my age have about moving to Brooklyn is that you think that the second you live there, you are moving to a foreign country and will never see anyone again,” said Woody Wright, 27, who grew up on East 58th Street and, at the time of the Hog Pit gathering, was planning to move in with his girlfriend, Britaania Poppie, who is 26 and works in finance.

Ms. Abrams’s enthusiasm proved infectious. By August, all three couples had moved into one-bedroom apartments at 388 Bridge, paying around $3,200 a month in rent for apartments on the 23rd, 24th and 25th floors.

Rather than gather at bars, they now visit one another’s apartments, making dinners together and watching sports at home. They frequently commute to their jobs together. (Two of the women, Chrissy Hunt, 26, and Ms. Poppie, work at the same bank.)

“It almost feels like college again, but a little bit more grown up,” said Mr. Wright, who works in advertising and who went to college with Ms. Hunt’s boyfriend, Sean Oliver, 27, who has a job in sales at a video technology company.

It is not uncommon for tenants to refer friends to buildings. Of the 234 units at 388 Bridge, 222 had been leased as of a few weeks ago, 11 through friend referrals. Some property management companies and developments, including 388 Bridge, encourage the practice, offering cash bonuses and other perks to tenants who make successful referrals.

Related Companies, which manages about 6,000 rental units in the city, receives calls weekly from prospective tenants who were referred by friends, and pays its tenants $1,000 for a successful referral.

About 10 percent of those who move into the 11,000 rental units owned and operated in the New York City metro area by Equity Residential come from friend referrals.

A friend’s endorsement goes a long way. “They are going to believe their friends before they believe the broker,” said Pat Publik, an associate broker for Halstead Property. “If their friend tells them it’s a great building, they are going to believe it.”

According to brokers and property managers, the practice is more common in newer buildings, which may have several units available at once. But some buildings develop a reputation for their fraternal atmosphere, like the Upper East Side complex Normandie Court, which has long carried the telling nickname Dormandie Court. Some neighborhoods are more referral territory than others. “Like the Murray Hill area. Everybody knows everybody there,” said Laurie Zucker, the vice chairwoman of Manhattan Skyline Management Corporation, which gives residents a $1,000 rent credit for a referral.

While some boards do not accept recommendation letters from current shareholders, others weigh a shareholder’s opinion heavily. “If the tenant shareholder is a good person known to me or other people on the board, that is a four-star recommendation,” said Steven R. Wagner, the president of the co-op board of Southgate in Midtown East, and a real estate lawyer.

In a tight real estate market, having a friend in the building means ears on the ground. When they came into the city from New Jersey, George and Terri K. Hicks frequently visited their longtime friends, Harvey and Lynne Schmelter-Davis, at their place on West 66th Street near Lincoln Center. “It became our meeting point for social outings,” said Ms. Hicks, 66, a retired sociology professor.

When the Schmelter-Davises heard that the unit next door was about to go on the market in 2012, they quickly spread the news. Mr. and Ms. Hicks bought the one-bedroom apartment after viewing it only once — and without ever embarking on a full-fledged apartment hunt. The arrangement also was a boon for the sellers, who avoided a broker’s fee. Ms. Hicks has had no second thoughts about her speedy decision and has decorated the space with midcentury modern furniture, painting the living room in various shades of orange.

“If I open up my bedroom window, I can wave at Lynne in the living room,” Ms. Hicks said. Like the Hickses, the Schmelter-Davises also have a home in New Jersey. “Our lives intersect all the time, but we don’t have to plan,” said Mr. Schmelter-Davis, who is 74 and a retired community college administrator.

Having a friend next door has other advantages. If Ms. Hicks leaves an important item behind in the city, her neighbors can take it with them to New Jersey. A friend living next door in a large city also provides a psychological benefit. “It’s very comforting to know that there is somebody there,” Ms. Hicks said. “If something happened, you could call them.”

The Hickses are not alone in their desire to have friends close at hand. Retiring in separate apartments in the same building is one such option, providing seniors with both companionship and autonomy.

“People are getting a lot more proactive about not spending their old age alone,” said Beth Baker, the author of “With a Little Help From Our Friends: Creating Community as We Grow Older” (Vanderbilt University Press, 2014). “But people want privacy, too. And this gives them both.”

A familiar face nearby can be especially meaningful when New York City is thousands of miles from home.

In October, Xiyan Qian, 26, a media planner from Shenzhen, China, moved to Jersey City from Roosevelt Island so she could live near her friend Yezhen Zheng, 28, a graduate student who is also from China. Now the two women live in neighboring buildings in the Newport neighborhood, sharing American and Chinese holidays together, as well as most of their free time.

Ms. Qian’s father is especially pleased that his daughter lives near friends, a practice that is common in China. “When you get married, you want to live close with your parents,” said Ms. Qian, who grew up just one building away from an uncle. “People always want you in the same building, in the same community.”

The Chinese have a saying, she added: One should live near enough to family that if you were to hand-deliver a bowl of soup, it would still be warm when you arrived. “You want to live that close to each other,” she said.

Soup deliveries aside, maintaining privacy and autonomy when you live on the same elevator bank as your social circle can prove challenging. If three friends call the same address home, can just two of them go out for a movie and dinner? If you don’t want to announce to the world that you have a hot date with so-and-so, how do you get around being seen together by your in-building buddy? To avoid awkward moments, ground rules are in order. One common mandate: no unannounced visits.

“If people aren’t in sync and they don’t establish boundaries, you can wind up in a situation like on ‘Seinfeld,’ with Kramer dropping in unexpectedly,” said Dr. Levine, the friendship blogger, referring to the character who makes frequent grand and unwelcome entrances.

After all, one of the hallmarks of New York City apartment living is the anonymous neighbor. New Yorkers can live alongside each other for decades without sharing more than a few words — and many prefer it that way. “I’ve been in my building for almost 30 years,” said Kathy Braddock, a managing director of the New York City office of William Raveis. “My neighbors are very nice, but we don’t say, ‘Let’s break bread.’ ”

Living close to friends can strain relationships in other ways. Getting to know a person too well can be a friendship’s undoing. “Do you want your best friend living downstairs from you? Maybe not,” Ms. Braddock said. If a friendship sours, the future could hold countless uncomfortable encounters in the mailroom.

But as much as New Yorkers treasure anonymity, many bond tightly with their neighbors over the years, turning apartment buildings into de facto neighborhoods. So taking your friends with you to a building may simply speed up an inevitable process.

“It’s kind of like we’re going back to an older style of living,” said Jason Turetsky, 29, who works for the Anti-Defamation League and rents a two-bedroom apartment with his wife, Robyn, in the Kalahari, a condo on West 116th Street in Harlem. In June, Mrs. Turetsky’s cousin, Richard Gold, and his wife, Micaela, bought a two-bedroom in the Kalahari. “Having them up here is even better than we could have imagined,” said Mr. Gold, 31.

Now that they live in the same building, their extended families often gravitate to Harlem, with the couples hosting Hanukkah and birthday parties for siblings, parents, aunts and uncles. Ruth and Yigal Gafni, longtime friends of Mrs. Turetsky’s parents, recently bought a two-bedroom in the Adeline, a new condo directly across the street. And Mr. Turetsky’s brother and his fiancée also live on West 116th Street.

“I like it, it makes me feel like I’m not in a city. I’m in a neighborhood,” said Mrs. Turetsky, 28, sitting on her cousin’s white leather couch in his spacious living room one recent evening. Mrs. Turetsky, a dietitian, was sharing hot cocoa with Mrs. Gold, 30, who is expecting her first child in March. “It feels comforting to have people around,” Mrs. Gold said.

This month, Mr. and Mrs. Turetsky plan to move into a three-bedroom apartment they bought in the Adeline. After living so close to the Golds, having to cross 116th Street to visit will seem like a trek.

“When they move across the street, I’m actually going to be sad,” said Mr. Gold, who works for the management consultants McKinsey & Company.

“I literally come up here in my pajamas,” Mrs. Turetsky said. “I would never go outside like that.”