It came as a big surprise to the real estate industry last year that second homes were accounting for more than one-third of annual sales.

Before 2005, the National Association of Realtors’ survey techniques failed to accurately gauge the size of this market, even though anecdotal evidence had indicated a substantial increase in second-home purchases over the previous decade.

The percentages don’t tell the whole story: Though baby boomers make up a large part of the second-home market, a growing number of foreign buyers are in it, taking advantage of a weaker dollar. And the opposite is true, too: Many U.S. buyers are finding the dollar purchases more in foreign markets, especially Latin America and Eastern Europe.

The second-home market has become so large that 140,000 of the nation’s more than 1.6 million Realtors acknowledged specializing in it in 2005, a 30 percent increase over 2004.

A clear indicator of its importance came at the Realtors’ annual meeting in San Francisco in late October, when the NAR announced a new certification program for the Resort and Second-Home Professional Specialist.

When the second home can be as near as 50 miles — or as far away as 5,000 — from the primary residence, knowing the market is critical to an agent’s success, said Maire Rosol, a Park City, Utah, Realtor who specializes in resort sales.

“We have a lot of buyers coming here from countries that have security problems, and we are finding that these buyers favor gated communities,” Rosol said. “From what they’ve told us, they find being able to ride a bicycle for long distances safely worth the price.”

The recent increase in the number and intensity of hurricanes in Florida has shifted a lot of buyers “away from the ocean and to the golf-course communities of the interior,” said Ron Acker, principal broker at Re/Max 200 Realty in Winter Park, Fla.

Some of that shift can be attributed to what Steven Friedman, national director of housing at Ernst & Young, calls “the Aspen effect,” meaning that when prices get high enough, “second-homeowners start leaving Naples, Fla., for the mountains of Tennessee.”

Meanwhile, Rosol said, 1 million Americans have retired to Mexico, Costa Rica, and even emerging markets in Eastern Europe.

Buyers from abroad often capitalize on other countries’ problems.

“Foreign buyers started considering Argentina when we devalued our currency and property values dropped 40 percent,” said Paul Reynolds, owner of Reynolds Propiedades in Buenos Aires. “Those values have only just recovered to 1997 levels, and with airfare from Miami only $700 round-trip, we are drawing in a lot of Florida investors.”

Short-term rentals are popular, because a fully furnished apartment in Buenos Aires averages $400 a week, while one night’s stay in a comparable hotel runs from $100 to $300, Reynolds said. “Occupancy rates on these are 80 to 90 percent, so you can recover your investment. But, then again, you can buy a 220,000-acre farm in the interior for $1.5 million, so your dollar is going farther.”

Mexico is becoming the biggest draw for Americans because, according to AARP magazine, “for $600 a month, retirees can live in a three-bedroom home with a gardener.”

Tom Kelly, who with Mitch Creekmore wrote “Cashing In on a Second Home in Mexico” (Crabman Publishing, $19.95), said significant changes in Mexican law offering protections for American investors created the boom.

Americans have realized that the rates of return and appreciation on Mexican properties exceed those in the United States, Kelly said.

Contributing to the change have been the passage of real estate licensing laws, third-party escrow and title services, and the emergence of U.S. lenders in the Mexican market.

Like the British do in Florida or Philadelphians at the Jersey Shore, some second-home buyers rent out their properties part time. Christine Hrib Karpinski, author of “How to Rent Vacation Properties by Owner” (Dearborn Trade, $19.95), said many investors rush into buying without knowing the first thing about the rental market in a particular area.

“When purchasing vacation homes, it’s a given that you need to know a lot about the area where you want to buy,” she said. “Included in that is knowing about strict governing rules, regulations and laws that apply exclusively to vacation rental homes.”

The boom in second homes doesn’t come without pain. In some resort areas, an abundance of second homes has led to a decline in affordable housing for permanent residents, said William S. Hettinger, president and chief executive officer of Wyndham Financial Group.

“Maintaining a balance in support housing is critical to protecting the core of the community — its residents and workers,” he said.

Nor can the second-home boom be sustained at current levels.

“We can’t do another 35 percent market share of second homes,” said Ernst & Young’s Friedman. “That is the peak.”

That’s not to mention the uncertainty of the future.   A presidential commission recommended the end of mortgage deductions on U.S. income taxes for second homes.  

The real estate industry, stunned by the proposal, objects in a big way.