Updated Checked by the Tax-Services.ca editorial team How we check
A Canada tax review starts when your notice of assessment lands. Read it against the return you sent, and if something’s off, you can ask the CRA to change any return for a tax year that ended in the last 10 calendar years. A request made in 2026 has to relate to a year after 2015.
What should you check on your notice of assessment?
Start with the money in and the money already paid. Your T4 shows employment income in box 14 and income tax withheld in box 22, and both should match what the notice lists. Then look at the deductions. An RRSP contribution, child care costs or union dues left off the return is the usual gap.
Credits come next. The notice won’t spell out everything you could have claimed, only what you did claim. So keep the return beside it and tick through your own paperwork: receipts, slips, the RRSP contribution receipt. Missing one slip is common, and the CRA often has a copy that you don’t.
Last, look at the result. Does the refund or balance owing look close to what you expected? If it’s far off, one of the boxes above is usually the reason.
How much can one missed deduction change the result?
Take an Ontario employee with $75,000 of employment income and $14,000 of tax withheld. Our tax refund calculator puts the tax for the year at $12,704.65, so the refund is $1,295.35.
Now say the same person put $5,000 into an RRSP and forgot to claim it. With the deduction, tax drops to $11,072.15 and the refund grows to $2,927.85. That’s $1,632.50 back for one line on the return, or about 33 cents on each dollar contributed.
| Case (Ontario, $75,000 salary, $14,000 withheld) | Tax for the year | Refund |
|---|---|---|
| No deductions claimed | $12,704.65 | $1,295.35 |
| $5,000 RRSP deduction claimed | $11,072.15 | $2,927.85 |
The RRSP calculator shows how much room you have before you lean on that line, and the income tax calculator lets you test other deductions the same way.
How do you ask for a change to a filed return?
Wait for the notice of assessment first. The CRA says you can’t request a change before you have it. After that there are three routes.
| Route | How | Processing time the CRA gives |
|---|---|---|
| Change my return | Online, in your CRA account | 2 weeks |
| ReFILE | In certified tax software, tax years 2022 to 2025 | 2 weeks |
| Form T1-ADJ | By mail, with supporting documents | 19 weeks |
You can add income, claim a deduction or credit, or add a slip you missed. You can’t use these routes to update your address or marital status. Save the confirmation number and the summary of changes when you finish online.
The answer comes back as a new notice of reassessment, a partial approval with an explanation, or a letter saying nothing was changed and why.
You must wait until you receive your notice of assessment (NOA) before you can request a change to your return.
Changing a tax return, Canada Revenue Agency, canada.ca
The same page also asks you to wait for the CRA’s answer before sending another change request, so bundle everything you’ve spotted into one request instead of trickling them in.
What if you disagree with the result?
Then it’s an objection, which is a separate process. For an individual, the deadline is the later of two dates: one year after the filing due date for that return, or 90 days from the date on the notice. Don’t sit on it. The 90 days pass quickly, and a missed date makes everything harder.
What does a typical fix look like in real life?
Sam in Calgary gets his notice and notices the refund is a few hundred dollars lighter than the ballpark he had in mind. He digs out his folder and finds a tuition slip that never made it onto the return. He logs in to his CRA account, picks “Change my return”, adds the slip, saves the confirmation number and waits. Two weeks later a new notice of reassessment shows up with a bigger refund. The whole thing took half an hour, and most of that was finding the slip.
Not every case ends that neatly. If the CRA changes only part of what you asked for, the letter explains which part and why. Read that explanation before you decide whether it’s worth pushing back. For help understanding the process around letters and notices, see our guide to dealing with the CRA.
Mistakes and limits of a self-review
People often change one number and forget the ripple. A new deduction can also move your credits and some benefits that depend on net income, so a refund estimate is only a first look. The calculators here work with federal and provincial income tax and the basic credits. They don’t model benefits or refundable credits, so use the tax credits calculator only as a starting point (it is federal only).
Another trap is the year. ReFILE covers 2022 to 2025 only, so an older return goes through your CRA account or paper form T1-ADJ. And a change request isn’t a way to update personal details. That takes a separate step.
What about a review by the CRA itself? A letter asking for receipts is a different thing from your own check, and we haven’t covered how those are handled, since we couldn’t confirm the detail on an official page. If a letter arrives, read the deadline on it first.
What this page is based on
The change routes, processing times and the 10 year limit come from the Canada Revenue Agency‘s page on changing a return. The objection deadline comes from its objection guide. The tax figures in the example use 2026 federal and Ontario rates from our own data files, checked in September 2026. No level of government runs or endorses this site.
Frequently asked questions
How far back can I change a tax return?
Generally for a tax year ending in any of the 10 previous calendar years. A request made in 2026 has to relate to a year after 2015.
Do I need my notice of assessment before asking for a change?
Yes. The CRA says to wait until you receive it before you request a change.
Which is faster, online or mail?
The CRA gives 2 weeks for Change my return and ReFILE, and 19 weeks for a mailed form T1-ADJ.
Can I change my address with a change request?
No. These processes are for income, deductions, credits and missing slips, not personal details.
How long do I have to object to an assessment?
For an individual it's the later of one year after the filing due date and 90 days from the date on the notice.
- How Canadian income tax works: brackets and credits
How federal and provincial Canadian income tax stack up, what marginal and average rates mean, and how credits and deductions change your bill