File an amended return only after you file your original return. Generally, a claim for refund must be filed within 4 years after the unextended due date of your original return (for example, April 15, 2025, for 2020 calendar-year returns). However, a claim for refund to recover all or part of any tax paid as a result of an office or field audit may be filed within 4 years after the date assessed. This applies only if you paid the tax and did not file a petition for redetermination (written appeal).
If you filed an amended return with the IRS or another state, you generally must also file an amended Wisconsin return within 90 days. You must file an amended Wisconsin return if the changes affect your Wisconsin income, any credit, or tax payable. You should also file an amended Wisconsin return to claim a refund or pay any additional tax due even though you are not amending your federal or other state return.
If your original return was filed on Form 1, you would file an amended return using Form 1 and checking the amended return space at the top of page 1. Exception: If you incurred a net operating loss (NOL) on your 2020 Form 1 and elect to carry the NOL back to 2018 and 2019, you must file Form X-NOL, Carryback of Wisconsin Net Operating Loss, for the appropriate year to amend your return and claim a refund.
Be sure to enclose Schedule AR with your amended return to explain all changes and the reason for the change.
Check the space on Form 1 to indicate your filing status on the amended return. If you are changing from separate returns to a joint return, both of you must sign the amended return. If there is any tax due, it must be paid in full. You cannot change from joint to separate returns after the due date for filing your original return has passed.
If you are changing your filing status, the amount to fill in on line 1 of Form 1 as federal adjusted gross income must be based on the same filing status used for Wisconsin. For example, you and your spouse both originally filed Form 1 as married filing a separate return. You are amending to change your filing status to married filing a joint return. The amount to fill in on line 1 must be taken from a federal return based on a married filing joint status. If you did not file a joint federal return, you should prepare a pro forma federal return using a married filing joint return status. The federal adjusted gross income and other amounts from this pro forma return are then used to complete the amended return. Follow the Form 1 instructions to complete your amended return.
If you are changing an amount on any line of Form 1, fill in the corrected amount on that line. If you are not changing an amount on a line, fill in the amount from your 2019 return as originally filed or as you later amended it. If your latest filed return was changed or audited by the department, use the corrected figures from the adjustment notice.
All amended returns must be printed and mailed for filing.