Despite Congress’ eleventh-hour move in December to head off sharply higher estate taxes, confusion remains over the future of the levies. The tax revisions lawmakers made at the end of last year set a maximum rate of 35 percent with a $5 million exemption for 2011 through 2012. The new law also provides a choice for executors of estates of people who died in 2010, when the tax had been repealed: take the 35 percent/$5 million exemption, or pay no estate tax and forfeit the step-up in basis.

Had Congress done nothing, the estate tax law would have reverted to a $1 million exemption and a 55 percent tax rate on Jan. 1 of 2011. Now, that’s set to happen in 2013 if legislators don’t act first, said Jeremiah W. Doyle IV, senior vice president of BNY Mellon Private Wealth Management.

“We’ve got a lot of moving parts here and a lot of uncertainty,” Doyle said during a program on tax law changes, sponsored by the New York County Lawyers’ Association and Trusts & Estates magazine. “There’s a lot of confusion about what’s going on.”

Part of the problem is the IRS, Doyle said. Five months into the new year (and a month after the April 18 tax filing deadline), the agency still hasn’t released Form 8939, which allows executors to elect which regime they will use to calculate taxes for those who died in 2010; Publication 4895, which is aimed at answering questions surrounding uncertainty about the process, hasn’t been published yet either.

Another problem involves the modified carryover basis regime. For clients who died in 2010 and elected to pay no estate tax, a carryover basis may be increased by $1.3 million (but not more than the fair market value of the assets). If there are multiple assets and multiple heirs, this can present a sticky issue, Doyle said; how do you allocate how much of the carryover basis should apply to each asset, and by extension to the heirs who will by divvying up the assets? A dissatisfied heir might sue. Doyle suggested obtaining a sign-off from all the heirs on the basis allocation ahead of time.

There is some good news for advisors to high-net-worth clients. The tax law revisions unified the regimes for estate tax, the gift tax, and the generation-skipping tax: All are set at the $5 million exemption and 35 percent rate (but like the estate tax, the gift tax and GST would revert to a $1 million exemption and 55 percent rate at the start of 2013, absent action from Congress.)

The law also provides a new feature that’s helpful for couples: A deceased spouse’s unused exemption amount may be passed to the surviving spouse. This means that the surviving spouse can obtain more favorable tax treatment of gifts during his or her lifetime, or obtain a larger exemption for the spouse’s own estate when that time comes. It applies only to spouses who die after 2010 and before 2013. (The portability doesn’t apply to the GST.) Doyle said that President Obama supports making portability permanent.

“There’s a lot of information to take in, and what’s scary is it’s only relevant for the next two years,” said attorney David Leibell of Wiggin and Dana. He said the most likely scenario is that Congress will approve a $3.5 million exemption and a 45 percent rate starting in 2013, with a deal ironed out after the 2012 election. But both sides could remain at odds if there are different parties in the White House and Congress, he added.