APPROXIMATELY 20 percent of all American homeowners now own second houses, according to the National Association of Realtors. This does not include the number of second homes abroad; that figure has yet to be calculated. But Delores Conway, director of the Casden Real Estate Economics Forecast at the University of Southern California’s Lusk Center for Real Estate, says it’s significant and increasing. “There is a very broad trend that shows that Americans, especially baby boomers, are pursuing unique living experiences,” she says, “and vacation homes outside the country are part of that trend.” Still, prospective buyers need to be aware of local laws, customs and real estate regulations. Here is a guide to some of the most dynamic foreign second-home markets.