It may cost from about $5,000 to "tens of thousands of dollars" to set up one or more foreign corporations, depending on fees of service providers and the complexity involved, said Jeffrey Kolodny, a member and an attorney in the private client services group at Cozen O'Connor in New York.

International buyers from about 70 different countries purchased $82 billion in residential real estate in the U.S. in the year through March 2011, compared with purchases of $66 billion by buyers from 53 countries the year before, according to the National Association of Realtors. The figures include non-resident foreigners and recent immigrants.

Such purchases may have unforeseen estate-tax consequences for those who don't live in the U.S., said Gideon Rothschild, a partner with Moses & Singer LLP in New York.

"They'll see a broker, buy an apartment here, and the broker doesn't know the first thing about tax issues," Rothschild said. While the owner is alive planners generally may be able to transfer the interests to a more tax-efficient structure such as an offshore company. If the buyer has died, "there aren't too many options at that point other than to write out a big check."

Some countries, including France, the United Kingdom and Japan, have tax treaties with the U.S. that may provide credits to avoid double taxation on U.S. assets at death, Kolodny said. Hong Kong and Singapore don't have tax treaties with the U.S., said G. Warren Whitaker, a partner with Day Pitney LLP in New York. Residents of those countries may consider holding U.S. stocks through a foreign corporation, he said.

Failing to plan ahead may come with high costs for those leaving the U.S. tax system as well, Basha said. The U.S. may impose a so-called exit tax on those giving up residency or citizenship, if they have a net worth of $2 million or more or paid an average of $151,000 or greater annually in net income taxes over previous five years, for those expatriating in 2012.

One former World Bank employee who worked with Basha was moving from the U.S. back to Europe upon retiring. The woman had already given up her U.S. green card before consulting with Basha, which triggered U.S. exit taxes of about $500,000 on the value of her total future pension payments from the World Bank. She declined to name the client citing privacy concerns.

Some foreigners who formerly might have ignored U.S. tax rules have been forced to reconsider, Whitaker said.

"In the old days people moved to the U.S. and some of them just wouldn't comply," he said. "They'd count on hiding money and not reporting it, and those people are having a lot of difficulties now as the world shrinks and information-exchange increases."

Disclosing Gifts