What should I do if I made a mistake on my Canadian tax return?

If you have not received your Notice of Assessment yet, file a T1 Adjustment through CRA My Account's 'Change My Return' tool — or re-NETFILE an amended return using your tax software. If you have already received your NOA, you can still amend via the same tools. You have up to 10 years to request adjustments. Do not file a second return for the same year.

Will the CRA penalize me for making an honest mistake on my taxes?

Generally no — honest errors and omissions (such as forgetting a deduction or entering a wrong number) are corrected through the normal reassessment process without penalties. Penalties are mainly applied for late filing, gross negligence, or deliberate misrepresentation. If you proactively fix an error before CRA contacts you, the outcome is almost always better than waiting.

How long does the CRA have to reassess my tax return?

The standard reassessment period is 3 years from the date of your original Notice of Assessment. For suspected misrepresentation or fraud, there is no time limit. This means if you find a mistake from 2022 or later, the CRA can also find it — so fixing it proactively is always the right move.

Can I file a second tax return to correct a mistake?

No — you cannot file a second return for the same tax year. The correct process is to amend your existing return using a T1 Adjustment request through CRA My Account or by mailing a T1-ADJ form. Filing a second return will be rejected or cause delays.

What if I underreported income — should I just hope the CRA does not notice?

No. The CRA receives copies of T4s, T5s, and most other slips directly from issuers and routinely cross-checks returns. Unreported income is one of the most common audit triggers. Proactively correcting it through a T1 Adjustment or the Voluntary Disclosures Program is far less costly than waiting for CRA to find it — which adds penalties and interest on top.

What is the difference between a T1 Adjustment and the Voluntary Disclosures Program?

A T1 Adjustment (Change My Return) is for fixing honest errors — a wrong number, a missed slip, a forgotten deduction. The Voluntary Disclosures Program (VDP) is for more serious situations: multiple years of unreported income, foreign assets never disclosed, or returns never filed. The VDP can reduce or eliminate civil penalties in exchange for coming forward proactively before CRA contacts you.

I Made a Mistake on My Tax Return

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 amendments, and honest mistakes corrected proactively almost never result in penalties. Here is exactly what to do.

First: what kind of mistake did you make?

The right action depends on the type of error.

Mistake type	What to do
Missed deduction or credit (medical, 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 (T4, T5, T3, etc.)	T1 Adjustment — likely owe more tax
Wrong province claimed	T1 Adjustment
Wrong SIN or personal information	Call the CRA directly: 1-800-959-8281
Multiple years of unreported income	Consider the Voluntary Disclosures Program
Return never filed	File now — or use the VDP for multiple missed years

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. Check your software for this option.

How to amend a filed return: three methods

Option 1: CRA My Account — Change My Return (fastest)

This is the recommended method 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 — you will not be charged late-payment penalties if you pay promptly after receiving the revised NOA.

Option 2: Re-NETFILE via tax software (2–3 weeks)

Open your saved return in your tax software, correct the error, and re-submit electronically. Confirm your software supports prior-year amendments. Most certified software does for the current and previous two tax years.

Option 3: Mail a T1-ADJ form (6–8 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 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. Keep all records for 6 years.
Reassess differently — in rare cases, the CRA may adjust the return in a way other than what you requested. You can appeal a CRA decision if you disagree.

Learn how to read the revised assessment at how to read your Notice of 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, set up a CRA payment plan or read what to do when you cannot pay your taxes.

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). For the 2026 filing season, that means you can amend returns as far back as 2016.

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 — making early correction especially valuable for those years.

What not to do

Do not file a second return for the same year — it will either be rejected or create significant processing delays
Do not just ignore 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
Do not call 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

Common specific mistakes and where to get help

If your mistake falls into one of these common categories, there is a dedicated guide:

For a full list of deductions and credits to review before submitting an amendment, use the tax deductions checklist.