Ellie Haile, 7, right, helps to decorate the Christmas tree at the family house of Kimberly Taylor who opened her home to housemate Mitzi Hellmer. (Astrid Riecken/For The Washington Post)

When Kimberly Taylor separated from her husband three years ago, she and her daughter, Amelia, stayed in the marital home — a four-bedroom house in Woodbridge, Va., with a finished basement and a huge back yard. It was a lot of square-footage and a financial strain. But rather than downsize, Taylor decided to fill it with someone she could relate to: another single mom.

She placed an ad in Craigslist — “Mother w/awesome child seeks same” — and was inundated with responses.

After requesting 10 references per applicant, and spending time with them, she settled on another separated mother with a young son. Living together, the moms and kids bonded. For them, putting together the halves of two broken homes created a whole home.

“It’s fun,” said Taylor, 46, a medical-journal publisher whose daughter is now 7. “We’re usually bonding over different stages of divorce, going through similar things.”

She loved it so much — the “sister-wives” aspect of it, the commiserating, the camaraderie — that when the original renter reunited with her husband, she took in a new single mom, this one with a daughter about Amelia’s age.

A few alternative-minded single mothers embraced house-sharing in the 1960s and ’70s. But as the number of single mothers in the United States has soared and the cost of living has shot up, more are choosing such arrangements.

There are nearly 11 million single parents in the United States today, up from 3.3 million in 1970. Most — around 8.6 million — are mothers. About 4 in 10 babies born in the United States are born to unmarried mothers, up from about 1 in 10 in 1970. The median income for families led by single mothers is one third of that headed by married couples, according to U.S. Census data.

So it makes sense that some would want to live together.

“People are having trouble making ends meet, and it’s cheaper to do it with two. But a bigger factor that we’ve noticed is just loneliness,” said Carmel Sullivan-Boss, who runs the Web site ­CoAbode, which matches single moms. Currently about 120,000 women across the United States are registered to share housing, she said. “I don’t think we’re really built that way, to live on our own.”

Throughout history in traditional societies, women have banded together to help one another with child-rearing. In modern times, too, living with another mother provides some of what married life offers — a helping hand and another adult to talk to. A person who can hold the ladder while you change the kitchen lightbulb. A person who can hang out with the kids while you take a bath or go on a date.

When it works, such arrangements can benefit the children as much as the mothers. They gain playmates as well as the support of another adult.

“The research is pretty clear that having another adult in the home is very helpful to kids’ well-being,” said Stephanie Coontz, research director at the Council on Contemporary Families. A second adult can balance out the first one, “counter their bad moods, supplement their weaknesses. If you develop a good working relationship, you can do what a good marital relationship does — model how you negotiate things, and it’s not just the adult-child relationship all the time.”

Kimberly Taylor, 46, right, and her housemate Mitzi Hellmer, 34, left, who have lived together with their kids for the past 6 months, prepare breakfast at Taylor's family home. (Astrid Riecken/For The Washington Post)

But as with any family dynamic, there can be pitfalls. Tension can arise if one parent is less financially stable, enforces stricter rules or has different ideas of cleanliness. The children, too, may have conflicting needs.

A 42-year-old woman in Waldorf, Md., who is currently seeking another single mom to live with tried it once and ran into trouble.

“She had her mother living there, which wasn’t supposed to be part of the deal,” said the woman, who didn’t want her name used because her family doesn’t yet know that she is leaving her husband. “Her mother had two male friends who didn’t live there but were there all day, and these people are extra adults influencing your kids. Her child was not even 1 yet, but my kids were 4 and 6, and they were watching something with bad language, and I said, ‘Can we change the channel?’ You could hear a pin drop — they were like, ‘Well, how does she decide what we get to watch on television?’ ”

But even when parenting values are incompatible, leaving isn’t so simple. Research shows that moving in and out of too many different situations can be detrimental to children, said Philip Cohen, a sociology professor at the University of Maryland.

“You wouldn’t want to change the arrangement more often than necessary,” he said. “The concern would be that children don’t deal very well with relationship ambiguity. Children do need to know who they can count on and they need stability and security.”

Fern Allen, 37, of Portland, Ore., has been on both sides of the equation. When she was growing up, her mother, who was single, moved in with another single mom. After Allen had a baby at 21, she lived in several co-mom situations until her son was 8.

“As I got older it became harder,” she said. “You want to build a life with someone. I definitely did at some point say, ‘Yeah, I’ve had enough of this.’ ”

“I’ve got resourcefulness and openness to community in a way I wouldn’t have if I was raised in a close-knit family, but at the same time, I think I live with a lot of the insecurities,” she said.

The impermanence of living with another single mom gave Taylor pause after the original mother and son moved out.

“My daughter, when he left, was in tears, and he was, too. She always introduced him as her brother. So when he left, she cried and cried and I said, ‘We will stop and never do this again. I don’t want to see you in this kind of pain.’ She said, ‘Mommy, I would rather have a pain and have him here.’ ”

As her daughter and her housemate’s daughter, Ellie, 7, climbed a cherry tree in the front yard last weekend, Taylor made crepes for everyone, and Mitzi Hellmer, her housemate, set the table. This kind of leisurely morning, lounging around in T-shirts and sweats, is a luxury for single moms, Hellmer said.

“One can sleep in while the other makes breakfast,” she said. “It’s not a verbalized thing. We just do it.”

The girls ran inside and down to the plastic roller coaster in the basement.

“I was having a hard time when I first moved here; I was struggling,” said Hellmer, 34. “You’re expected to pay rent in this area for two people, and you can’t do it, and you can’t live with just anyone. You trust other mothers.”

Hellmer and Ellie had to adjust to Taylor’s household rules, such as taking their shoes off before walking across the white carpet. And the shared house is not a place she could bring a date. “I wouldn’t even ask,” she said. But when she did go out on a date recently, she was able to stay out as late as she liked, knowing Ellie was safe at home with a responsible adult.

The girls hit it off, though they fight like any siblings. “We got BFF bracelets,” Ellie said, sliding onto a chair at the kitchen table, where a plateful of crepes awaited.

But Taylor is once again on the hunt for just the right single-mom housemate. A recently minted immigration lawyer, Hellmer is moving out, to be closer to work. For them, and their daughters, the parting is bittersweet.

“It’s going to be different, getting home and no one being there,” Hellmer said as the girls finished their crepes and ran off to the back yard. “It’s going to take some getting used to.”