How to Fill Out and File Form CT-1040 EXT: Filing, Submission, and Penalties

Form CT-1040 EXT is Connecticut’s application for a six-month extension of time to file your individual income tax return, moving your filing deadline from April 15 to October 15. For tax year 2025, the form and any payment you owe are due by April 15, 2026.¹ The extension only buys time to file. It does not extend the deadline to pay, and any unpaid balance starts accruing interest from April 15.²

When You Can Skip the Form Entirely

The Department of Revenue Services (DRS) grants an automatic six-month Connecticut extension, with no form required, if all three of these are true:²

You expect to owe no additional Connecticut income tax after withholding, estimated payments, and any pass-through entity tax credit.
You already filed federal Form 4868 with the IRS.
You do not need to send a payment to DRS.

If any one of those is off — if you owe even a dollar more, or you did not request a federal extension — you need to file CT-1040 EXT.

What You Need Before You Start

Pull these together first:

Your Social Security Number and, if filing jointly, your spouse’s.²
Your best estimate of your Connecticut income tax liability for the year, built from W-2s, 1099s, and your prior-year return.
Any Connecticut use tax you owe on untaxed out-of-state purchases. Enter zero if none.
The total of your withholding and estimated tax payments made so far.
Any Connecticut income tax credits you expect to claim, including a pass-through entity (PET) tax credit from a Schedule CT K-1.

Accuracy matters here. The closer your estimate is to the real number, the less you risk interest and the 10% late-payment penalty on anything left unpaid after April 15.

Filling Out the Form Line by Line

CT-1040 EXT is a single page with seven numbered lines.²

Line 1: Estimated Connecticut income tax liability. This corresponds to Line 14 on Form CT-1040, or Line 16 on CT-1040NR/PY for part-year residents and nonresidents. Enter zero if you expect to owe nothing, but enter something.
Line 2: Estimated Connecticut use tax (Line 15 on CT-1040, Line 17 on CT-1040NR/PY). Zero is fine if none applies.
Line 3: Add Lines 1 and 2 for your total estimated tax.
Line 4: Total payments already made through withholding and estimated payments (Line 18 on CT-1040, Line 20 on CT-1040NR/PY).
Line 5: Total expected Connecticut income tax credits (Line 19 on CT-1040, Line 21 on CT-1040NR/PY).
Line 5a: Pass-through entity tax credit (Line 20c on CT-1040, Line 22b on CT-1040NR/PY). Leave blank if you have none.
Line 6: Add Lines 4, 5, and 5a. This is your total payments and credits.
Line 7: Subtract Line 6 from Line 3. A positive number is your balance due. Send that amount with the form. If Line 6 is larger, enter zero.

Line 7 is what DRS cares about. The agency wants you as close to paid up as possible before granting the extension.

How to Submit the Form

Online Through myconneCT

The fastest route is myconneCT, the DRS online filing portal.⁴

By Mail

If you file on paper, the address depends on whether you are enclosing a payment:3Department of Revenue Services. Connecticut Income Tax Extension Form

With payment: Department of Revenue Services, PO Box 2977, Hartford, CT 06104-2977
Without payment: Department of Revenue Services, PO Box 2976, Hartford, CT 06104-2976

The wrong PO Box slows processing. If you are mailing close to April 15, use certified mail or another service that gives you a dated receipt. That postmark is your proof if the envelope is late or lost.

What the Extension Does Not Do

Your filing deadline moves. Your payment deadline does not. Two separate charges can pile up on an unpaid balance:

Interest at 1% per month, or any fraction of a month, from April 15 until you pay.⁵
A flat 10% late payment penalty on the amount still owed.⁶

Connecticut waives the 10% penalty if the amount still owed when you file is no more than 10% of your total tax for the year and you pay that balance in full with your return by the October due date.⁵ Interest still applies; only the penalty is waived. Practically speaking: pay at least 90% of your true liability by April 15 and you keep the penalty off entirely.

If You Get Hit With the Penalty Anyway

You can ask DRS to waive the 10% penalty with Form DRS-PW, Request for Waiver of Civil Penalty. The request has to be submitted within one year of the date DRS first notified you of the penalty, or within one year of filing the return that triggered it.⁷ File it through myconneCT, by mail to PO Box 5089, Hartford, CT 06102-5089, or by fax to 860-297-5727.

The Commissioner presumes reasonable cause exists if it is your first penalty, or if the IRS already waived a penalty for the same tax period.⁸ Serious illness, a death in the family, loss of records to fire or casualty, and documented reliance on erroneous DRS guidance can also qualify. Relying on a tax preparer usually does not, unless you can point to a specific calculation or typographical error despite ordinary care on your part.

One trap: DRS will deny your waiver request if you have any outstanding tax liabilities or unfiled Connecticut returns when you submit it.⁷ Clean up everything else before you ask. And interest is never waivable — only the penalty.

After the Extension Is Granted

Once DRS processes your CT-1040 EXT, you have until October 15, 2026, to file your tax year 2025 return, either Form CT-1040 for residents or Form CT-1040NR/PY for part-year residents and nonresidents.² If your actual liability is lower than what you paid, you get a refund. If it is higher, the extra will owe interest back to April 15, and the 10% penalty applies if you fell outside the safe harbor.

Missing the October deadline is worse. If you fail to file within three months of the original due date and never file at all, the Commissioner can prepare a return on your behalf and impose a penalty of 10% of the assessed tax or $50, whichever is greater, on top of the 1% monthly interest.⁶ Filing a late return yourself is always better than letting DRS build one for you.

1
Connecticut State Department of Revenue Services. Connecticut Resident Income Tax Information
2
Department of Revenue Services. CT-1040 EXT – Application for Extension of Time to File Connecticut Income Tax Return for Individuals
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Connecticut State Department of Revenue Services. myconneCT

By Mail
If you file on paper, the address depends on whether you are enclosing a payment:3Department of Revenue Services. Connecticut Income Tax Extension Form
5
Justia. Connecticut Code 12-723 – Extensions
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Justia. Connecticut Code 12-735 – Failure to Pay Tax
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Connecticut Department of Revenue Services. Form DRS-PW – Request for Waiver of Civil Penalty
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Department of Revenue Services. Policy Statement – Requests for Waiver of Civil Penalties