When you meet someone for the first time in San Francisco, the conversation will inevitably lead to the question: where do you live?

You trade stories about your neighborhoods, and then comes the follow-up: Do you rent or own? And then: How much rent do you pay?

That might sound impertinent, but we obsess about housing here, and with good reason. We pay too much to receive so little.

Today’s column, which came from tips from two readers (thanks!), looks at one factor that critics say has nibbled away at San Francisco’s very limited rental housing stock, depleting the supply enough to make it more difficult to find a decent place to live: turning apartments into vacation rentals for tourists.

Vacation rentals have become a travel phenomenon in recent years all over the world, especially with the recession. It’s easy to understand why. A vacation rental generally costs half as much as a hotel room, tourists love living like the locals, there’s a kitchen for meals (crucial in these frugal times), and property owners make more than twice what they would renting to a regular tenant.

But with the popularity has come a backlash.

In places as diverse as Paris, Hawaii and Key West, using homes for short-term vacation rentals has been outlawed in residential neighborhoods. Such a large number of homes were being converted to vacation rentals that local residents couldn’t find places to live.

New York City will have a ban on short-term rentals beginning in May 2011 in buildings with five or more apartments: no rentals of less than 30 days allowed.

Few people know it, but San Francisco was well ahead of just about everyone in foreseeing this issue and passed a similar ban against short-term vacation rentals 30 years ago. The law has not been enforced, and hundreds, possibly thousands, of apartments have been used for vacation rentals in recent years.

Some believe it’s time to start enforcing the law.

Good luck with that.

The Department of Building Inspection has jurisdiction, but only responds to complaints. Apparently neighbors have been hesitant to narc on vacation rentals (although, frankly, it’s unlikely they know about the law).

I cold-called several owners of vacation rentals and had some lovely conversations. No one would speak on the record when I explained the story I was working on, but these aren’t evil Snidely Whiplash landlord stereotypes. Often vacation rentals are units in multi-family properties where the owners themselves reside – and they love meeting new people and playing host. It could be a tough sell to threaten our Mrs. Madrigals with jail time for renting out to visitors.

There are also issues with the law itself. One section implies that the short-term vacation rental ban only applies to buildings with four apartments or more, but another section of the law does not make this distinction. If it takes a lawyer to interpret the law, then it’s open to debate and that makes it weak.

Even if the law is changed, made stronger, and clearer, it might still be fairly easy to avoid. Just ask folks in Hawaii. Despite the strict ban there, you can still find short-term vacation rentals. Landlords require tourists to sign a 30-day rental agreement to abide by the law, but (wink, wink) have a verbal agreement on the side for shorter stays.

But even more confounding for San Francisco is the fact that a ban on vacation rentals will reduce much-needed tax revenues.

Rentals of fewer than 30 days are charged the city’s 14 percent hotel tax. The treasurer’s office could not say how much hotel tax money the city makes from vacation rentals, but that revenue will disappear if the ban is enforced – and the city needs the cash.

No matter what is done, there are consequences. That’s always the case when an issue involves housing in San Francisco. No wonder it’s our obsession.