Filed an extension? What to do before October 15, 2026

The dates and rules, side by side

Both returns for tax year 2025 had the same original due date and the same extended due date. Where they differ is in what made the extension valid and in how the penalties are calculated.

First, check that your Massachusetts extension is actually valid

This is the step people miss. The federal extension is a form you file. The Massachusetts extension is a test you pass: the Department of Revenue grants the six-month extension automatically, but only if at least 80% of your total 2025 Massachusetts tax was paid on or before April 15, 2026. Withholding from your paychecks, estimated payments, any extension payment and credits all count toward the 80%.

If you fell short, there is no valid Massachusetts extension. Your Form 1 was due on April 15, 2026, and DOR's late-filing penalty applies to a return not filed by its due date or within an extension DOR granted. In that situation, waiting until October 15 does not help you. File as soon as the return is complete.

Two related rules from DOR: an extension payment of $5,000 or more had to be made electronically, and a return that shows no tax due gets an automatic extension to October 15, 2026 even if nothing was paid.

Illustrative example. Your 2025 Massachusetts tax works out to $10,000. Your employer withheld $7,000 and you paid $500 with your extension in April. That is $7,500, or 75%, so the extension was not valid and the Form 1 is already late. Had you paid $1,000 in April, you would have reached 80% and the extension would hold.

What the federal extension did and did not do

If you filed Form 4868 by April 15, 2026, the IRS gave you until October 15, 2026 to file. The IRS puts it plainly: an extension of time to file is not an extension of time to pay. Any 2025 tax you still owe has been collecting the 0.5% monthly late-payment penalty and interest since April 15, and it will keep doing so until it is paid.

If you did not file Form 4868 in April, there is no federal extension to rely on and the return was due April 15. The late-filing penalty is figured on tax that is due and unpaid, so if you are owed a refund there is usually no late-filing penalty, but you should still file promptly to claim the refund.

What to do in the next two weeks

1. 1.Find your April numbers: what you paid with the federal extension, what you paid to Massachusetts, and your 2025 W-2s and 1099s showing withholding.
2. 2.Estimate your 2025 Massachusetts tax and check the 80% test above. If you missed it, treat the Form 1 as overdue and file it first.
3. 3.Gather anything that was missing in April. For extended returns this is usually a late K-1, a corrected 1099, a brokerage cost-basis statement or records for a home or business sale.
4. 4.Finish both returns together. Most Form 1 figures start from the federal return, so preparing them separately invites mismatches.
5. 5.File by October 15, 2026, even if you cannot pay the whole balance. Filing stops the larger late-filing penalty; payment can follow.
6. 6.Pay what you can with the return: IRS Direct Pay or your IRS Online Account federally, and MassTaxConnect for Massachusetts.
7. 7.Keep copies of both returns, your e-file acceptance confirmations and your payment confirmations.

If you cannot pay the full balance

File anyway. Federally, the late-filing penalty (5% a month) is ten times the late-payment penalty (0.5% a month), so an unfiled return with a balance is the most expensive position to be in. When both penalties run in the same month, the IRS reduces the late-filing penalty by the late-payment amount, but the combined cost is still far higher than filing and owing.

Once the return is filed, the IRS offers payment plans. For individuals who filed on or before the due date and have an approved installment agreement, the late-payment penalty drops to 0.25% a month while the plan is in place. Massachusetts DOR has its own payment agreements. Our pages on the federal installment agreement and on Massachusetts tax debt explain how each works.

• Federal payment plans: see our installment agreement page
• Massachusetts balances: see our Massachusetts tax debt page
• Penalties already charged: see our penalty abatement page for reasonable cause and first-time relief

If October 15 passes

Nothing about the return changes except the cost. Federally, the 5% monthly late-filing penalty on any unpaid tax begins after October 15, and once the return is more than 60 days past the extended due date a minimum penalty applies: for returns required to be filed in 2026, the lesser of $525 or 100% of the unpaid tax. In Massachusetts, the late-filing penalty is 1% of the tax required to be shown for each month or part month, up to 25%, on top of the 1% monthly late-payment penalty.

Both the IRS and DOR can remove penalties where there was reasonable cause and not willful neglect, and the IRS also considers first-time abatement. Interest is different: DOR states it does not have authority to abate interest on unpaid tax except where the tax itself is reduced or the interest was miscalculated.

Illustrative example, before interest. You owe $4,000 on your 2025 federal return and file it on November 20, 2026. The late-payment penalty has run since April 15 at 0.5% a month. For the two part-months after October 15, the late-filing penalty adds 5% a month, reduced by the 0.5% late-payment amount, so about 4.5% of $4,000 per month, or roughly $360. Filing on October 15 instead would have avoided that $360.

Where we come in

Extension season is when we see the returns that were hard in April: a missing K-1, a property sale, a move into or out of Massachusetts, or a first year with the new federal deductions for tips or overtime. We prepare the federal and Massachusetts returns together, go through them with you before anything is filed, and tell you plainly what is owed and when. Before we start, we give you a scope and a price in writing.

If the 80% test shows your Massachusetts return is already late, tell us when you get in touch so we can prioritize it.