On Friday, President Trump signed the biggest federal tax overhaul in three decades, and readers, understandably, have lots of question about it. Today I’ll answer one about how it will affect payroll tax withholding and another on prepaying your property tax through an escrow account.

Q: Will the new tax law impact how much I have withheld from my paycheck?

A: Yes, but it’s not clear when or how. The Internal Revenue Service generally sends out new withholding tables once a year to account for changes resulting from inflation indexing and new tax laws. Payroll processors use these tables to adjust the tax withheld from employee paychecks, and the change is usually so nominal it goes largely unnoticed.

The changes that will take effect in 2018, however, are so sweeping that payroll processor ADP was predicting that the IRS would require all employees to fill out a new Form W-4.

On Friday, however, the IRS said in an email that it is working on guidelines for implementing the new tax law. “We anticipate issuing the initial withholding guidance in January, and employers and payroll service providers will be encouraged to implement the changes in February. The IRS emphasizes this information will be designed to work with the existing Forms W-4 that employees have already filed, and no further action by taxpayers is needed at this time.” It added that the new 2018 withholding guidelines “will allow taxpayers to begin seeing the changes in their paychecks as early as February.”

Form W-4 tells payroll processors how much tax to withhold from employee paychecks based on their filing status and how many “allowances” they claim. Allowances are based loosely on the number of people in the employee’s family. However, people can claim more or fewer allowances than they have family members to adjust their withholding based on their itemized deductions, outside income and other factors. It’s a blunt instrument that has little to do with current tax law and even less to do with future tax law.

For example, in 2017, taxpayers can deduct from their income $4,050 for each adult and dependent listed on the tax return. This “personal exemption” goes away next year. However, the child tax credit will double, to $2,000 per dependent under 17, and there is a new “family” tax credit worth $500 per person for dependents 17 and older. The standard deduction will also nearly double to $24,000 for married couples and $12,000 for singles, but many itemized deductions will be limited or eliminated. Tax rates are also going down for most people.

Pete Isberg, vice president of government relations for ADP, thought the IRS would devise a new W-4 form aligned to the new tax code and require all employees to file it, but was happy to hear the IRS will tweak the old one because it will save work for employers and employees.

Whether it produces accurate results remains to be seen. After the change, “you will want to look at withholding to make sure you are still satisfied with the amount being withheld,” said Kathy Pickering, executive director of the Tax Institute at H&R Block. “Some people really like to have a tax refund as almost a forced savings plan. Others prefer … to get no refund at all.” With tax rates going down, “presumably you will have less tax withheld from your paycheck,” she added.

While that might be fine for most people, it could result in an unpleasant surprise for people whose taxes go up in 2018.

Q: My property taxes are paid by my mortgage lender via my escrow account. If it makes sense for me to pay the second half of my 2017-18 property taxes due in April 2018 before year end, would my lender do that by Dec. 31?

A: If your lender can’t do it, you should be able to pay these taxes yourself before the year ends, although you might have to wait up to a year to have any excess payments refunded through your escrow account, said Greg McBride, chief financial analyst with the Bankrate website.

Under the law signed Friday, you can pay property taxes that have been billed but are not due until 2018 by Dec. 31 and take them as an itemized deduction on your 2017 tax return. Starting in 2018, taxpayers who itemize can deduct a maximum of $10,000 in all state and local taxes (including property taxes) combined.

In California, 2017-18 property taxes have been billed, but the second installment is not due until April. As discussed in my previous column, if you itemize deductions and are not subject to the alternative minimum tax this year, consider making all or part of the April payment by Dec. 31 so you can deduct it in 2017, when state and local taxes are fully deductible.

If you pay your property taxes — along with your mortgage and insurance premiums — through an impound or escrow account, ask your lender if it can pay the second installment before the end of the year. If it can but there is not enough money in the account, the lender might let you go into arrears and make up the deficit later. If the lender can’t make this payment before year end, you should be able to pay it directly to your county tax collector.

“You could absolutely pay it, the question is how to reconcile it” with your lender, said Amanda Fried, a spokeswoman for the San Francisco Treasurer’s office. In San Francisco, homeowners can go to www.sftreasurer.org to see whether the second installment has been paid. If not, the homeowner can pay it online, by mail or wire transfer or in person.

“We would certainly take their payment,” Fried said.

When your lender makes its next annual analysis of your escrow account, it will see that you have prepaid the tax bill and at that point refund any excess that may have been paid into your account, McBride said. “You could be out the cash flow for a while,” he added. “Don’t use January’s grocery money to do this.”

I asked five large lenders how they would handle this but only Bank of America provided a response. It said in an email, “If the tax bill is available, in either physical or electronic format, Bank of America will make its best effort to process the disbursement to the taxing authority before end of year. To take advantage of this option the borrower must contact Bank of America customer service by December 26, 2017, and make a verbal request for early payout.”

Alternatively, “the borrower may contact the taxing authority and (make) a direct payment.” Borrowers who choose this option should contact BofA customer service, say the payment has been made and instruct BofA not to issue the disbursement.

You cannot prepay 2018-19 property taxes this year because those bills have not gone out.

Kathleen Pender is a San Francisco Chronicle columnist. Email: kpender@sfchronicle.com Twitter: @kathpender