You must file Form CT‑1040X in the following circumstances:
1. The IRS or federal courts change or correct your federal income tax return and the change or correction results in your Connecticut income tax being overpaid or underpaid.
-File Form CT‑1040X no later than 90 days after final determination. If you file Form CT‑1040X no later than 90 days after the date of the final determination, any Connecticut income tax overpayment resulting from the final determination will be refunded or credited to you even if the Connecticut statute of limitations has otherwise expired.
2. You filed a timely amended federal income tax return and the amendment results in your Connecticut income tax being overpaid or underpaid.
-File Form CT‑1040X no later than 90 days after final determination. If you file Form CT‑1040X no later than 90 days after the date of the final determination, any Connecticut income tax overpayment resulting from filing the timely amended federal income tax return will be refunded or credited to you even if the Connecticut statute of limitations has otherwise expired.
3. You claimed a credit for income tax paid to a qualifying jurisdiction on your original income tax return and the tax officials or courts of the qualifying jurisdiction made a change or correction to your income tax return and the change or correction results in your Connecticut income tax being overpaid or underpaid (by increasing or decreasing the amount of your allowable credit).
-File Form CT‑1040X no later than 90 days after final determination. If you file Form CT‑1040X no later than 90 days after the date of the final determination and you claimed credit for income tax paid to a qualifying jurisdiction on your original income tax return, any Connecticut income tax overpayment resulting from the final determination will be refunded or credited to you even if the Connecticut statute of limitations has otherwise expired.
4. You claimed a credit for income tax paid to a qualifying jurisdiction on your original income tax return and you filed a timely amended income tax return with that qualifying jurisdiction and the amendment results in your Connecticut income tax being overpaid or underpaid (by increasing or decreasing the amount of your allowable credit).
-File Form CT‑1040X no later than 90 days after final determination. If you file Form CT‑1040X no later than 90 days after the date of the final determination on a timely-amended return with a qualifying jurisdiction and you claimed credit for income tax paid to a qualifying jurisdiction on your original income tax return, any Connecticut income tax overpayment resulting from the final determination will be refunded or credited to you even if the Connecticut statute of limitations has otherwise expired.
5. If none of the above circumstances apply, but you made a mistake or omission on your Connecticut income tax return and the mistake or omission results in your Connecticut income tax being overpaid or underpaid.
-File Form CT‑1040X no later than three years after the due date of your return, or if you filed a timely request for an extension of time to file, three years after the date of filing the return or three years after the extended due date, whichever is earlier.