Filing your taxes accurately the first time is the goal, but life doesn't always cooperate. A forgotten RRSP contribution slip, a T4 that arrived late, or a Disability Tax Credit approved years after the fact — these situations happen regularly, and the Canada Revenue Agency has a formal process to address them. A T1 Adjustment lets you correct or update a tax return you've already filed and had assessed, and the window to do so extends up to 10 years into the past.
A T1 Adjustment is the formal process of amending a personal income tax return that the CRA has already assessed. It is not the same as filing a new return or sending a letter to the CRA explaining an error. When you request a T1 Adjustment, you are asking the CRA to reassess a specific tax year by changing one or more line items on your original T1, based on information that was missing, incorrect, or unavailable when you first filed.
The official form used for mailed adjustments is the T1-ADJ, T1 Adjustment Request. If you file online, you use the Change my return feature inside CRA My Account, which accomplishes the same outcome without the paper form.
Once the CRA processes your adjustment, they issue a revised Notice of Reassessment. If the change results in a refund, they pay it — with interest. If it results in a balance owing, interest begins accruing from the original tax deadline of the year in question.
There is a wide range of situations that legitimately warrant a T1 Adjustment. Some of the most frequent include:
A T1 Adjustment is only available for tax years that have already been assessed. If you filed your return and have not yet received your Notice of Assessment for that year, do not submit a T1-ADJ. The CRA asks that you wait for the original assessment to be issued and then contact them directly — either by phone or through My Account — to provide the corrected information before the return is fully processed.
Similarly, if you are disputing the CRA's assessment on legal or interpretive grounds (rather than correcting a factual error), a Notice of Objection under the formal appeal process is the appropriate route, not a T1 Adjustment.
The fastest and most straightforward method is using Change my return inside your CRA My Account. This is the recommended approach for most taxpayers because adjustments are confirmed immediately and typically processed within eight weeks.
To use this method:
You do not need to attach supporting documents when filing online, but you should keep them on hand. The CRA may contact you to verify the changes, particularly for larger adjustments.
If you prefer paper or do not have CRA My Account access, you can submit Form T1-ADJ by mail. The processing time for mailed adjustments is longer — typically eight to ten weeks — so factor this into your timeline if you are expecting a refund.
To complete the T1-ADJ:
Do not send original documents — send copies and retain the originals for your own records.
The CRA generally allows taxpayers to request adjustments going back 10 calendar years from the end of the tax year in question. In practical terms, this means:
This window is particularly significant for Disability Tax Credit claims, where a retroactive approval can result in substantial refunds across multiple years. If you or a family member received a DTC certificate in 2025 covering years back to 2015, you have until December 31, 2025 to capture that full 10-year range.
If your T1 Adjustment results in a refund, the CRA pays refund interest on the amount. Interest is calculated from the later of May 1 of the year following the tax year being adjusted, or 30 days after the date the CRA receives your T1-ADJ request. The current prescribed interest rate for refunds is set quarterly by the CRA and is applied on a compound daily basis.
If your adjustment results in a balance owing — for instance, if you discovered you under-reported income — arrears interest applies from the original tax filing deadline of the year in question. This can mean years of accumulated interest on an unreported amount, which is one reason to address known errors promptly rather than leaving them unresolved.
Filing a T1 Adjustment does not automatically trigger an audit. The CRA processes thousands of adjustment requests every year as a routine matter. That said, adjustments requesting large refunds — particularly for multiple years at once or involving certain credit types — may prompt the CRA to issue a request for supporting documentation before issuing the reassessment. This is not an audit; it is a verification step.
Providing clear, complete documentation with your original request — whether online or by mail — reduces the likelihood of a follow-up letter and speeds up processing.
At Swift Accounting Calgary, we regularly assist clients with multi-year T1 Adjustments, including retroactive DTC claims, missed RRSP deductions, and corrected employment income. Getting the documentation organized correctly from the start makes the process significantly smoother.
Simple one-line corrections — such as adding a single T5 slip — are generally straightforward to handle through CRA My Account. But if you are looking at adjustments spanning multiple years, a retroactive DTC, a business income correction, or a situation where you are unsure whether an adjustment or a formal objection is the right tool, professional guidance is worth the investment. Errors in an adjustment request can create new complications or delay your refund.
If you are unsure about your situation or want a professional to review your adjustment before it is submitted, contact Swift Accounting for a consultation. We can assess which years are eligible, calculate the expected refund, and manage the submission process from start to finish.
You can generally request a T1 Adjustment for any tax year within 10 calendar years of the end of that tax year. For example, a 2015 return can be adjusted until December 31, 2025. After that window closes, the CRA is not obligated to reassess the year, though there are limited exceptions in cases of fraud or misrepresentation.
Adjustments submitted online through CRA My Account's Change my return tool are typically processed within eight weeks. Adjustments submitted by mail using the paper T1-ADJ form take longer — generally eight to ten weeks, and sometimes more during peak periods. You can check the status of a submitted adjustment through CRA My Account under View mail or Check CRA processing times.
Yes, if your return has already been assessed and you want to add a deduction or credit you missed — such as an RRSP contribution, a charitable donation, or a medical expense — you need to file a T1 Adjustment. You cannot simply send a letter or call the CRA to change a line on an assessed return; the formal adjustment process is required to generate a revised Notice of Reassessment.
Yes. If you are adjusting a return to correct income you under-reported, the reassessment will show a balance owing, and CRA arrears interest will apply from the original tax deadline of that year. However, in most cases where taxpayers voluntarily identify and report the error before the CRA discovers it, the Voluntary Disclosures Program (VDP) may offer interest and penalty relief — a different process from a standard T1 Adjustment.
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