High season for spare bedrooms.

Winter, Mary

Lodging has been a golden goose for the so-called sharing economy, the system in which people rent items rather than buy them, often directly from the owners instead of companies. In cities around the world, travelers are bypassing commercial hotels and booking private digs--from spare bedrooms in Atlanta to villas in Tuscany--through websites such as Airbnb, HomeAway and VRBO. The biggest, Airbnb, has more than 600,000 listings in 34,000 cities in 192 countries, according to its website.

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Renters love the rates--often half those of hotels, or less--and property owners ("hosts") love the extra income. Local economies gain from the boost in tourism, advocates argue.

For the most part, short-term rentals have been unregulated and untaxed, but that's changing. States see substantial tax revenue to be tapped, and neighborhood groups increasingly want more regulation of the rentals, which they complain bring noise and traffic and lower property values. Other critics say short-term rentals displace lower-income families, and that owners of short-term rentals often violate zoning, fire and occupancy codes but are rarely punished for it.

In 2010, New York made it illegal to sublet apartments for less than a month, but State Attorney General Eric Schneiderman says the law is routinely flouted by owners of multiple properties.

Critics of the blanket ban on short-term rentals, however, say it hurts average homeowners--people who could use much-needed cash from letting a room--so in New York, lawmakers this year introduced bills to allow rentals in those circumstances.

Short-term rentals are especially popular in San Francisco, where affordable housing is scarce. The city sued two apartment owners it claims evicted tenants in favor of more lucrative short-term rental deals and has threatened stiff fines against others.

Airbnb officials say they've taken steps to start collecting a 14.7 percent tax in New York state and a 14 percent tax in San Francisco.

Less populated areas are also wrestling with regulations. In Teton County, Wyo., commissioners recently told vacation rental managers their properties must meet commercial building codes with room sprinklers and other safety devices.

Tourism-dependent Florida passed a law in 2011 prohibiting local governments from regulating or banning vacation rentals. But after widespread complaints about rentals turning neighborhoods into "motel alleys," legislators dialed back the law so that localities have some power to regulate noise and other issues.

The debate is likely to move to more statehouses. According to a study by PhoCusWright, the share of online sales in vacation rentals doubled from 12 percent in 2007 to 24 percent in 2012, and is expected to increase to 30 percent this year.