"Portability is a safety valve for married couples that didn't have wills that utilized the exemptions," said Marco Svagna, a certified public accountant at New York-based Berdon LLP, who advises clients with net worth of $100,000 to more than $1 billion.

Having a will that includes a bypass trust and other tax- planning strategies could cost up to $10,000, Olivieri said.

"Portability is meant to address the perceived problem that although there is an estate-tax exemption, unless you do some level of sophisticated planning, a married couple could lose the benefit of one exemption," Olivieri said.

Passing all assets to spouses upon death and using the portability provision rather than forming bypass trusts is best suited for taxpayers who have estates worth about $7 million or less, with the main assets being real estate and retirement plans, said Linda Hirschson, a trusts and estates lawyer at Greenberg Traurig LLP in New York.

That's because they might not want put those assets in a trust, since they need to live in the house and may want to easily access the money in the retirement account, she said. Taxpayers with estates worth more than $10 million should continue to use bypass trusts, she said.

Lower Portable Amounts

Wealthy taxpayers should beware of undoing bypass trusts and relying on portability because it may not be extended by Congress, said Carlyn McCaffrey, a partner in the law firm McDermott Will & Emery LLP where she is co-head of the New York private client group.

Even if it is extended, the carryover amount may change if the tax thresholds are altered, Olivieri said. That's because the legislation says the portable amount is the lesser of the unused exemption amount of the first spouse to die or the exemption amount at the time of the second spouse's death.

That means if the threshold switches to $1 million in 2013 and the surviving spouse in the above example dies that year, her estate would only be able to use $1 million under portability instead of $2 million from her deceased spouse.

Trusts can also protect assets from creditors and election rights of spouses, said McCaffrey. And they ensure that assets, as well as their appreciation, are outside of the estate, she said.

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