A year from now, the amount of mortgage interest they pay will be moot for vast swaths of the public. The same goes for deductions for state and local taxes.

While the lowering of the cap on mortgage interest will affect the wealthy, it will do so only marginally. The cap now limits the deduction to the first $1 million spent on a house plus $100,000 borrowed through home equity loans. The new law limits it to the first $750,000. It also limits deductions for state and local taxes to $10,000.

The lower cap will cost nearly $10,000 in mortgage interest deductions on the first year of a $1 million mortgage fixed at 4% for 30 years. Mortgages taken out before December 2017 remain under the $1 million cap.

But that's a pittance for the 3% of filers making $500,000 or more in 2015. IRS statistics show their average total deductions were $200,122. If you made less than $100,000 — but still enough to itemize — your average deduction was $17,911.

The cap means there's not much difference in mortgage interest deductions: $6,779 for those making under $100,000 and $19,704 for those making $500,000-plus.

But the statistics mean mortgage interest expense is the key element in determining how a homeowner is taxed. As households file now on 2017 income, they will compare their itemized deductions with a standard deduction of $6,350 per person. And this year, regardless of whether they itemize, they will also take a personal exemption of $4,050 for each person in their household. This year about 44% of homeowners will benefit from itemizing, according to the Urban Institute.

A year from now the Urban Institute estimates only about 14% of homeowners will benefit from itemizing. They will be comparing the amount of their deductions against a standard deduction of $12,000 per filer. (That sounds like a near doubling of deductions, but less so for taxpayers who realize that the new law has eliminated personal exemptions entirely).

Taxpayers who will drop out of the itemizing class are more likely to live in smaller markets, where wages and housing prices are relatively low — places like Winston-Salem, N.C., home of Allegacy Federal Credit Union ($1.4 billion in assets, 139,475 members).

The reduction in caps will have more of an impact on the coasts and in large metro areas with higher wages and housing prices — places like, say, Long Island, N.Y., the home of Nassau Educators Federal Credit Union ($2.8 billion in assets, 178,373 members). NEFCU ended 2017 with $700 million in home equity lines of credit and $450 million in first-lien residential mortgages.

“We are going to see a bunch of people bump up against it,” Chuck Price, NEFCU's vice president of lending, said. “Long Island has a relatively high real estate tax, and we tack on a high state income tax. There were certainly a ton of people at the local tax collecting offices at the end of the year taking on early 2018 payments to try to get the deductions on the 2017 taxes.”

In Winston-Salem, few households will be affected by the lower caps on mortgage size and state and local taxes.

“On property taxes, anything less than the Biltmore House will be deductible,” Elkins said, referring to the 178,926-square-foot mansion built in the late 1800s in western North Carolina by George Vanderbilt, one of the earlier, but not last, rich Yankees to build a summer home in the southern mountains.

Most Winston-Salem borrowers will be affected by the near doubling of the standard deduction, which is likely to offer a higher tax reduction than itemizing if you earn less than $100,000.

At $100,000, “you're exceeding the normal household pretty handily in our part of the country,” Elkins said. “On Long Island, between blue-collar jobs being union and executive pay up there, it's subsistence living. It's pretty good living down here.”

The difference between the members of the two credit unions can be seen in Home Mortgage Disclosure Act data on average applicant incomes on purchase loans in 2016.

Among Allegacy borrowers, the average income was $80,570 to take out an average purchase loan of $171,376. The first year's interest on a $171,376 30-year mortgage fixed at 4% is about $7,800.

At NEFCU, the average borrower made $141,426 and the average purchase loan was $349,651. The first year's interest would be about $13,900.

Economists at the Mortgage Bankers Association in Washington expect the new tax law will have little overall impact on mortgage originations, but the greatest negative impact on home equity loans.

Federal tax laws previously allowed deductions on the interest for up to $100,000 of home equity indebtedness. The new law eliminated the provision for home equity indebtedness, but retained deductions for acquisition indebtedness.

MBA Chief Economist Mike Fratantoni expects this will lessen demand for home equity loans, and it might crimp cash-out refinancings, depending on how the IRS drafts its regulations.

“In our thinking, there's going to be more scrutiny on how the funds are used,” Fratantoni said. “If you have something called a home equity loan, and you use the funds for a renovation or addition or other substantial improvement on your house, we're thinking that's likely still going to be deductible. But there is still some uncertainty about that. Previously if you used a home equity loan to buy a car or pay for a vacation that was probably deductible. That almost certainly will go away.”

Banks have decreased their HELOCs sharply since the Great Recession, when plummeting property values wiped out much of the equity in home equity loans. “They're really only providing lines to very high credit borrowers,” he said.

Overall, home equity loans account for about 9% of credit union loans, down from 14% in 2011. First mortgages account for about 40% of credit union loan portfolios, a share that has been stable over the past several years, said George Hofheimer, chief knowledge officer for Filene Research Institute, a non-profit credit union think tank in Madison, Wis.

Nationally, credit unions granted $32.8 billion in second-lien mortgages in the 12 months that ended Sept. 30, 2017, up 16% from a year earlier. First-lien originations grew 9% to $144.2 billion, while non-real estate originations grew 9% to $304.1 billion.

NEFCU originated $300 million in home equity lines in 2017, about the same as the year before, and $160 million in first-lien residential mortgages, up 10%. This year it expects home equity originations to increase about $20 million to $30 million, while first-lien mortgages to be flat.

NEFCU advises its members to consult with their accountants or tax lawyers to understand how the laws affect their particular circumstances, Price said.

“Our primary message is going to be that at the end of the day, if you have work you want to do on your property or you want to consolidate debt, there's still a good possibility that home equity credit line rates in the interest-only period are really going to be cheaper than any of your other alternatives.”