More and more, travelers are skipping hotels in favor of private homes when they hit the road. Web sites like Airbnb make it easy for both travelers and hosts.

But Airbnb's business model doesn't always jibe with city zoning laws.

By the book

Andrew Ross is flipping through a thick book in a windowless room. He's a lawyer for the city of Philadelphia who works in the Housing & Code Enforcement Unit.

Ross says renting out your home via Airbnb isn't illegal per se; it just falls under tight regulation.

"Short term rentals like this, which would also include hotels or B&Bs, are permitted under the zoning code in certain areas only," said Ross.

Those areas include most commercial districts, but, literally, only a handful of residential blocks. We've mapped them above.

Still, even if you did live in those zones, Ross says, you'd have to get a license from the city to have a paying guest stay the night.

According to Ross, that's the legal way to rent your space via Airbnb in Philadelphia.

Which means the hundreds of Philadelphia rooms, houses and apartments listed on Airbnb right now are technically breaking the law.

"And of course the website itself wouldn't be a violator," Ross said. "It would be the property owners who are listing their premises and renting them to people."

Trying it out

David Stern-Gottfried lives in a rowhome in the city's Mount Airy neighborhood with his wife and 2-year-old son. Every year around Christmas they take a trip to California.

"Usually someone's away on vacation and they let us use their home," said Stern-Gottfried. "This year, no one would let us use their home because they rented out all their spaces on Airbnb."

So Stern-Gottfried decided to give it a try.

He says his experience with the web site was great, and in short order a traveler had signed up, willing to stay a whole week.

"We probably wouldn't have been able to do the trip if we didn't subsidize it somehow," Stern-Gottfried said. "So this was a really important piece in our ability to get out of town."

But, technically, becoming an impromptu hotel is a no-no, according to city code, which Stern-Gottfried did not know during the holidays.

"Now that I know that what I'm doing is illegal, would I have a problem with that? Is that where you're going with this?" Stern-Gottfried asks.

Sure.

"I don't really have a problem with it."

Gray area

Airbnb renters in Philadelphia are not alone when it comes to unsteady legal footing. In New York City, someone who made $300 renting out their apartment is facing $30,000 in fines.

Matthew Yglesias, the business and economics correspondent for Slate, has written about Airbnb's zoning problem.

"It became clear to me that they were taking, as a matter of business strategy, a sort of 'shoot first, ask questions later' approach to going into different markets," said Yglesias.

Airbnb says that approach has changed.

With an army of customers now behind it, the company says it's trying to get lawmakers to see its side of the story.

"We are actively engaging with governments everywhere to help clear the path for our hosts to engage in this amazing and economically beneficial activity that's good for cities," writes David Hantman, Airbnb's head of global public policy.

A new economy

Slate's Yglesias says the clash between rigid zoning codes and new digital business models reveals something more.

It's a bump in the road toward a "sharing economy," where housing, cars, even parking spaces can be quick sources of income.

"Business is supposed to be ubiquitous in the sharing economy," said Yglesias. "In a sense, everyone is going to have a little bit of a business. And that doesn't work with the concept of a rigid separation."

Yglesias says smart cities will probably look to accommodate services like Airbnb, with zoning policies that prevent abuse without stifling innovation.

City officials in Philadelphia say rentals via Airbnb haven't led to any citations here yet. They say the process is typically driven by complaints — not officials trolling web listings.

It always pays to follow the law, the city says. But chances are Airbnb will continue to fill beds here without issue.