Finding an error on your tax return after you have filed can feel alarming, but in most cases it is a straightforward fix. The CRA has a formal process for changing a filed return, and honest mistakes corrected proactively rarely result in penalties. This page covers that process: the three ways to submit a change, how long each takes, how far back you can go and when the Voluntary Disclosures Program applies instead. For the rest of the filing process, see the guide to filing a tax return in Canada.
First: what kind of mistake did you make?
The right action depends on the type of error. Each common mistake also has its own guide covering the line, slip or form involved.
| Mistake type | What to do |
|---|---|
| Missed deduction or credit: medical expenses, charitable donations, moving expenses, etc. | T1 Adjustment: you are owed a refund |
| Wrong income amount (typo, transposed numbers) | T1 Adjustment: may owe more or get a refund |
| Missing T-slip: a T4 left off the return, a T5 with interest income you didn’t report, a T3, etc. | T1 Adjustment: likely owe more tax |
| Province of residence entered incorrectly | T1 Adjustment |
| SIN given incorrectly or other personal information wrong | Call the CRA directly (numbers on the CRA contact information page) |
| Multiple years of unreported income | The Voluntary Disclosures Program may apply (see below) |
| Return never filed | File now, or use the VDP for multiple missed years |
The most common tax mistakes Canadians make are collected in one place if you are not sure which category yours falls into.
If you have not received your Notice of Assessment yet
Your return is still being processed. You have two options:
Option 1: wait and then amend. It is usually easier to let the CRA process your original return first, then submit a T1 Adjustment once you receive your Notice of Assessment. This avoids any processing conflicts.
Option 2: re-NETFILE now. If you filed electronically and have not yet received your NOA, most tax software will let you open your completed return, make the correction, and re-NETFILE it before the CRA processes the original. The CRA will use the most recently received version. Not every package offers this option.
How to amend a filed return: three methods
Option 1: CRA My Account, Change my return (fastest)
This is the quickest of the three methods for most corrections.
- Log in to CRA My Account (see how to set up CRA My Account if needed)
- Select Change my return under the Tax Returns section
- Choose the tax year you want to amend
- Find the specific line you need to correct and enter the new amount
- Add a brief explanation (e.g., “Adding T5 slip missed on original filing”)
- Submit
Processing time: approximately two weeks. If the change results in a refund, it is issued automatically. If you owe more, the CRA will send a revised assessment with a new balance due date; late-payment penalties don’t apply if you pay promptly after receiving the revised NOA.
Option 2: ReFILE through tax software (2 to 3 weeks)
Open your saved return in your tax software, correct the error, and re-submit electronically. Support for prior-year amendments varies by package. Most certified software supports it for the current and previous two tax years.
Option 3: mail a T1-ADJ form (6 to 8 weeks)
Download the T1 Adjustment Request (T1-ADJ) form from canada.ca. Fill in your SIN, the tax year, the line numbers being corrected, the original and new amounts, and a brief explanation. Attach any supporting documents (e.g., a T-slip you forgot to include). Mail it to your CRA tax centre and keep a copy of everything.
What happens after you submit a T1 Adjustment?
The CRA reviews the amendment and will:
- Accept it and send a revised Notice of Assessment showing the corrected tax and any refund or new balance
- Request more information: the CRA may write to ask for receipts or supporting documents for the claim you are adding. Records are kept for 6 years.
- Reassess differently: in rare cases, the CRA may adjust the return in a way other than what you requested. You can dispute a CRA decision if you disagree.
The guide to reading your Notice of Assessment explains each section of the revised assessment.
Do you owe more tax because of the error?
If your correction means you owe additional tax, you generally will not face late-payment penalties as long as you pay promptly after receiving the revised NOA. The CRA charges prescribed interest on outstanding balances from the original due date (April 30), but proactively filing the amendment before the CRA finds the error on their own typically avoids any civil penalties on top of that interest.
If you cannot pay the balance owed, read what to do when you cannot pay your taxes, including how a CRA payment arrangement works.
When the T1 Adjustment is not enough: Voluntary Disclosures Program
If the situation goes beyond a single honest error (for example, you have unreported income across multiple years, foreign assets you have never declared, or returns that were never filed), a T1 Adjustment is not the right tool. In those cases, the CRA Voluntary Disclosures Program (VDP) may be more appropriate.
The VDP lets you come forward proactively before the CRA contacts you. In exchange, the CRA can:
- Cancel civil penalties entirely
- Provide partial interest relief on the oldest years
You still owe the underlying tax and some interest: the VDP does not forgive the tax itself. But it is far less costly than being caught. See the full VDP guide to understand whether your situation qualifies.
How far back can you go?
You can request a T1 Adjustment for any tax year that falls within the 10-year window (the current year plus the 9 prior tax years). A request made in 2026 can go back as far as the 2016 tax year.
The CRA’s own reassessment window is 3 years from your original NOA for most taxpayers (longer if they suspect misrepresentation). This means errors from the past three years are the most likely to be caught independently, which makes early correction especially valuable for those years.
What to avoid
- Filing a second return for the same year: it will either be rejected or create significant processing delays
- Ignoring it: the CRA cross-matches T4s, T5s, and T3s from issuers and will likely catch discrepancies, adding penalties and interest on top of what you owe
- Calling the CRA to ask them to fix it for you: they do not proactively correct errors in your favour; that requires a formal T1 Adjustment
Related situations
If the CRA changed your return rather than you, why the CRA reassessed your taxes explains the usual reasons, and what to do when you get a CRA audit letter covers a review or audit. If the problem is a missed RRSP contribution rather than an error on the return, see what to do if you missed the RRSP deadline. Before submitting an amendment, the deductions and credits guide lists what else you may be able to claim.