Image: Rental market © Dana Hoff, Beateworks, Corbis

In checking old census records, I discovered something interesting: My relatives in the 1800s and early 1900s often had boarders in their homes. It wasn't uncommon for working- and middle-class families to rent out rooms to help make ends meet.

And that trend may be making a comeback. The number of unrelated adults living in someone else's home jumped 12% between 2008 and 2010, to 7.3 million, the Census Bureau says. That includes roommates sharing rentals as well as adults renting out rooms in other people's homes.

A troubled economy and high unemployment are causing more people to delay starting households of their own, census experts say. The same causes are leading strapped homeowners to consider opening their doors to lodgers.

Stephen, one of my Facebook fans, recently rented out a room in his home for the first time.

"The decision was based purely on financial reasons," he wrote. "It was easier to bring someone into my home than to uproot an entire house and look for something smaller."

Taking in lodgers works out well for many. Michael Barton, 22, of Lincoln, Neb., rents bedrooms in his home to two other young guys.

Liz Weston

Liz Weston

"I get $1,050 a month in cash while my (mortgage) payment is only $710," Barton wrote on my Facebook page. "This arrangement is perfect as I pay nothing to live essentially as they cover my payment and utilities."

Kelly McCarron Sisk of Forest Lake, Minn., has rented out a room in her home for 15 years.

"They've always been good friends," Sisk said. "I've never rented to a stranger. I've never advertised; a friend has always come up to me and asked when they know someone is moving out."

Some people buy a home knowing that they'll have roommates to help pay the cost. That's what my friend Rachel did when she bought her first home in San Francisco.

"I was 20-something and single, but I knew a two-bedroom would be a better investment, so I made sure it was a place I could afford alone -- but could comfortably share with a roommate," Rachel wrote. "In my case, that meant comfortable public space (living room + eat-in kitchen), and an extra half-bath -- not enough to have us each get our own bath, but it made a huge difference to have just the second space."

Of course, not all rentals end well.

"After the last renter we had, we will never do this again!!!" one woman wrote of a boarder who played Second Life nonstop. "She wouldn't shower for three weeks at a time (and) she never left the house ONCE in three months of living with us!"

Another one-time landlord rented a room to a friend of her mother's, only to discover the woman had limpetlike tendencies.

"She felt that any time we had a family event, outing, going out to eat, it automatically included her . . . which no, not always," the woman wrote. "She felt entitled to eat any food in the house although we thought we had made it clear she had to provide her own."

The family might have overlooked the boarder's flaws, but then the boarder opted to buy holiday presents rather than pay her rent.

"She felt it more important to buy her daughter (an adult) some expensive Christmas gifts and clothes, complain about how much she HAD to spend, and therefore couldn't afford to pay us her rent . . . not one dime!"

If you're considering renting out a room, you want to make sure the experience is positive for your household as well as your pocketbook. Here's some advice from legal and tax experts, as well as from those who have done it.

Know the law

You'll want to research landlord-tenant laws in your area so you understand your rights and responsibilities. You'll find a guide to the states' laws on the U.S. Department of Housing and Urban Development's website. Boarders or lodgers renting a room in a home may be treated slightly differently from a typical tenant but often have many of the same rights. Violating those can lead to lawsuits. Local zoning laws also may limit the number of nonrelated adults who can live in your home. Check with your city hall or county administrator.

Tap your social networks first

Renting to a friend, or a friend of a friend, doesn't guarantee against disaster. But several people who have experiencing renting out rooms said they were more comfortable renting to people they knew or who knew people they knew.

"All my roommates were friends of friends," Rachel wrote. "Nowadays I'd use Facebook; then it was just a matter of sending a mass email to friends."