BOSTON (State House News Service) – With the voter-approved surtax on Massachusetts’ highest earners set to start in the new year, there are still big questions around the constitutional amendment that changes the state tax code, according to the Bay State accountants.

The so-called millionaire’s tax was cleared by voters in November, amending the state constitution for the first time in 22 years and shifting Massachusetts away from its flat income tax adopted over a century ago.

It adds a 4 percent surtax on top of the state’s 5 percent flat tax for the portion of annual household income that exceeds $1 million. The money that comes into the state from the surtax is intended for public education and transportation investments, although the Legislature maintains discretion of its annual allocation.

The surtax will be collected when taxpayers remit taxes owed for the tax year 2023, according to the Department of Revenue. This means the first quarterly estimated payments affected by the new rate will be due on April 15, covering January through March of 2023, for most taxpayers.

In the short term there’s a lot of “unknowns” and “uncertainty” about what classifies as “taxable income,” said the vice president of advocacy at the Massachusetts Society of CPAs, Zachary Donah.

The accountants’ organization said they have been working with the DOR on questions such as if the surtax will apply to trusts and estates and how to calculate the tax for individuals who are married and filing separately.

Other questions include uncertainty around whether businesses have to withhold the extra 4 percent on behalf of W2 employees who make over $1 million per year, Donah said, and how the SALT tax deduction will interact with the surtax.

“Tax is complicated, so every unique situation creates a whole list of questions,” Donah said. “The DOR has a lot to answer, and I know they’re trying to get it done as quickly as they can. They’ve been really helpful trying to get us answers.”

Donah also said that he believes the revenue the state will collect from the surtax will be more volatile than predicted. Some estimates predict $2 billion will come in annually from the additional levy, while others have guessed around $1.2 billion.

The Tufts University Center for State Policy Analysis estimates there were 21,000 state taxpayers in 2019 — the last year for which they had complete data — with incomes of more than $1 million, amounting to 0.6 percent of all households. Yet, these households earned 22 percent of all taxable income in Massachusetts.

“It’s very hard to predict how much will come in with so many questions up in the air,” Donah said.

Surtax opponents argued the tax hike would cause wealthy Bay Staters to flee Massachusetts for lower-tax states, especially as more and more people can now work from home.

Accountants advising individuals — especially those with houses in other states — are telling clients they could change their listed home address easily, Donah said.

When surveyed by the CPA society, 75 percent of member accountants reported that they are more likely to advise their clients to change their domicile out of Massachusetts because of the surtax.

Opponents of the surtax, Question 1 on November’s ballot, also argued throughout the election cycle that the additional state tax revenue is not certain to actually be spent in the intended areas of public education and transportation as it is subject to legislative appropriation.

The DOR confirmed that generally, under the Constitution and Chapter 29 of state finance law, all revenue payable to the state is deposited into the General Fund, unless there is a statute that says otherwise.

To guarantee the funds from the new tax will go toward education, Secretary of State William Galvin called upon the Legislature to establish an education trust fund for the money when he certified the constitutional amendment earlier this month.

Already, education advocacy groups are eyeing the new large funding pot ahead of the upcoming session.

The Board of Higher Education hopes to double the amount of state-funded financial aid for public higher education students, and among the Massachusetts Teachers Association’s priorities, next year is increasing cost-of-living adjustment in payment to retired educators.

Galvin also proposed that the incoming Legislature amend current law to define “annual taxable income” as excluding a one-time profit on the sale of a homeowner’s primary residence if the homeowner is elderly and income-limited.

The move would address another concern of surtax opponents, who ran advertisements during the campaign saying the surtax would penalize retired homeowners whose financial futures weigh on the value of their house.

“With the results of the election now certified, this Constitutional amendment will go into effect immediately on the first day of the new year,” Galvin said earlier this month. “Immediate action will be needed to assist elderly homeowners who may have been intending to sell their current primary residence, in an effort to downsize and fund their retirement.”

When asked recently if the secretary had filed any such legislation, an aide said he was in conversations with legislators “perfecting a proposal” to be filed in the new session.

“While the language is still being worked out, the proposal would involve amending the definitions in chapter 62 of General Laws,” the Galvin aide said.

When asked if he would support Galvin’s proposed legislation to further regulate the surtax, House Speaker Ron Mariano said in a statement, “The House will review any legislation filed next session through the formal legislative process.”

A spokesperson for Senate President Karen Spilka also said lawmakers would review specific legislation when it is filed, but added that Spilka has said in the past that she is dedicated to ensuring the money goes toward education and transportation as the ballot question intended.

“The Senate President has been committed since the Fair Share amendment was proposed years ago to ensuring that the money raised is dedicated to education and transportation, two sectors that need our sustained investment to ensure the economic vitality and quality of life we enjoy here in the Commonwealth,” the spokesperson said in a statement. “She looks forward to working with her colleagues and stakeholders to make sure that all revenues received from the Fair Share amendment are transparently and responsibility used to spur new investments.”

In response to a question on if she would support a bill that put forward the changes Galvin proposed, a spokesperson for Gov.-elect Maura Healey said Healey is “committed to ensuring the revenue … goes to fund critical investments in public education, affordable public colleges, and universities, and for the repair and maintenance of roads, bridges, and public transportation.”

“She also continues to have discussions with the Senate President and Speaker and looks forward to working together with the Legislature to move forward on efforts to make Massachusetts more affordable and support residents and businesses during these times,” the spokesperson added.

Healey this month also discussed the surtax — which she publicly supported during her campaign — on GBH Radio’s “Boston Public Radio.”

“I have been very clear that I respect the will of the voters and the voters voted to allocate that money to make sure that money was to be directed — ultimately, it’s up to the Legislature — but to have that money directed towards important investments in transportation, education, and infrastructure,” she said.

Co-host Jim Braude asked Healey if her promises to cut taxes would cancel out new funds from the income tax as “roughly a wash” in the state budget.

“I don’t view it as a wash, I think these are different things. On the one hand, we’re talking about tax reform, and we’re talking about something I support, which is to lower taxes for seniors, for renters, for low-income, middle-income folks, to do something that I promised to do around tax credits, $600 per child per family in the state, also raising the threshold of the estate tax,” she said.

The Massachusetts Fiscal Alliance, which has long opposed the surtax, has urged Healey to reveal more details on the tax cuts she plans to support.

“Question 1 is going into effect in less than two weeks, and Gov.-elect Healey is missing an opportunity to address her administration’s first big test. Question 1 will easily drag the state back to the days of ‘Taxachusetts’ and the new governor is missing a great opportunity to take on a challenge head-on. She can do that by making good on her ‘tax cut’ pledge and offering some specifics for what kind of broad tax cuts she has in mind… The governor-elect could easily come out with specifics for what kind of broad tax cuts she wants to be passed into law, but she is so evasive on the topic that she isn’t even comfortable describing them as ‘broad,’ ” said foundation spokesperson Paul Craney.

Massachusetts ranked 34th in the Tax Foundation’s annual business tax climate index released in October, but analysts at the think tank said the state’s ranking could slide down to 46th with the implementation of the new tax.

“This spring is the first time we’ll see from the tax filing how much revenue is generated from the surtax,” Donah said. “But between folks who might move and questions on how it will actually look in practice, I don’t think we’ll really see the full picture of the revenue impact until April 2024.”