Updated Checked by the Tax-Services.ca editorial team How we check
If you have found a tax filing error after sending your return, do not refile. Ask the CRA to change it, either online through “Change my return” in your CRA account or on paper with form T1-ADJ. The CRA says online requests take about 2 weeks and mailed ones about 20 weeks. Online changes reach back to 2016.
How do you fix a tax filing error after you’ve filed?
There are three routes. The fastest is “Change my return” in your CRA account. The second is the ReFILE service inside certified tax software, which suits you if a preparer did the return. The third is form T1-ADJ with supporting documents, mailed to your tax centre.
Online, you only send documents if the CRA asks for them afterwards. By mail, attach everything up front and send it separately from your current-year return. The CRA’s page on changing your return (dated October 30, 2026) gives about 2 weeks online and about 20 weeks by mail, counted from when the CRA receives the form. Requests with several returns, a bankruptcy, a deceased taxpayer or a carryback can take up to 36 weeks. When the change is done, you get a notice of reassessment that shows what the CRA changed.
How far back can you go? “Change my return” covers the 2016 to 2025 tax years, and ReFILE cannot change returns before 2021. For older years you mail form T1-ADJ, but the CRA says it cannot issue a refund for a request made more than 10 calendar years after the end of the tax year.
What do you need before you ask for a change?
A short pile of paper makes the whole thing quicker. Sam in Calgary found a missing T4A in October, so he pulled together four things first.
- The slip or receipt that proves the missing amount.
- Your last notice of assessment, so you can see what the CRA already has.
- A one-line note on what changes and why.
- The tax year, since each year is a separate request.
That’s all it took. Most people overthink this part.
How much does a mistake change the bill?
The size of the error and your tax bracket set the answer. We ran two cases for an Ontario resident with $60,000 of taxable income, using the same maths as our income tax calculator.
| The mistake | Effect on income tax |
|---|---|
| Left out $1,200 of side income | about $356 more tax |
| Forgot a $2,000 RRSP deduction | about $559 less tax, so a bigger refund |
| Left out $1,200 at $120,000 of income | about $521 more tax |
Those amounts are income tax only. CPP and income-tested benefits aren’t modelled, and they can move too. The tax refund calculator gives a quick feel for how a correction moves your refund, and the marginal tax rate calculator shows what the next dollar costs you.
So is a $1,200 slip worth worrying about? Yes, but only in the sense that you should fix it. The money is small, a few hundred bucks. A mistake you know about and leave alone is the thing that grows.
What if the mistake works in your favour?
Then it’s the same process, and it’s a nice surprise. Forget a $2,000 RRSP deduction at $60,000 of income and the table says you’re out about $559, or roughly the price of a decent set of winter tires. The same request fixes it. You don’t have to do anything special, but the sooner you ask, the sooner the money lands.
What if the mistake means you owe tax?
Fix it anyway, and do it before the CRA finds it. The CRA says interest on overdue tax runs at a rate it sets each quarter. For October 1 to December 31, 2026 the prescribed rate on overdue tax is 7%, compounded daily. At that rate, $1,500 of unpaid tax adds about $26 of interest over 90 days and about $109 over a year. Rates change each quarter, so that is only an illustration, and we did not confirm the penalty rules that may also apply. Your notice of reassessment shows what the CRA charged.
For an older or bigger mistake, the Voluntary Disclosures Program can apply. Changes took effect on October 1, 2025, and the CRA’s page on the changes describes two types of application. If you come forward before the CRA contacts you, it lists 100% relief of penalties and 75% relief of interest. If the CRA prompted you, for example with a letter about a possible issue, it lists up to 100% penalty relief and 25% interest relief. People under audit or investigation, and those who were egregiously non-compliant, are still restricted. Even with relief, expect to pay the tax itself. Read the program page before you apply.
A simple rule of thumb: a recent slip goes through “Change my return”. An old or large one is worth a look at the disclosure program first. If the error was on a business return, see our page on fixing incorrect business deductions, and for what you can claim in the first place, see business deductions you can claim.
Common mistakes when fixing a return
People refile a whole new return, which makes a mess. Others send a T1-ADJ with no documents and wait weeks for a request to send them. Some change one line and forget the knock-on effect: a corrected income figure also changes credits that depend on income. Check the tax credits calculator if you think a credit could move.
If a T4 or T5 turns up after you’ve filed, that’s a change request too. Keep copies of everything you send.
Watch your mail and your CRA account for the notice. It’s your record that the change went through.
Where did we check this?
In November 2026 we read the CRA page on changing a return (dated October 30, 2026), the CRA Voluntary Disclosures Program changes page and its June 2026 tax tip on the program, and the CRA fourth quarter 2026 interest rates notice. The tax examples use 2026 federal and Ontario rates in our calculator, for one Ontario resident with no other credits. Your own figures will differ. The interest example is our own arithmetic. This site is independent of the CRA.
Frequently asked questions
How do I fix a mistake on a return I already filed?
Use Change my return in your CRA account, ReFILE in the certified tax software you used, or mail form T1-ADJ with your documents. Do not file a second return. The CRA then sends a notice of reassessment that shows what it changed. Online you only send documents if the CRA asks.
How long does a correction take?
The CRA page says about 2 weeks for Change my return and ReFILE, and about 20 weeks for a mailed T1-ADJ counted from when it receives the form. Complicated requests, such as several returns, a bankruptcy, a deceased taxpayer or a carryback, can take up to 36 weeks.
How far back can I change a return?
Change my return covers the 2016 to 2025 tax years, and ReFILE cannot change returns before 2021. Older years go by mail. The CRA says it cannot issue a refund for an adjustment request made more than 10 calendar years after the end of the tax year, though you can still report an amount owing.
Will I pay interest if my mistake means I owe tax?
Yes, interest on overdue tax runs from the original due date. For October 1 to December 31, 2026 the CRA prescribed rate is 7%, compounded daily, so $1,500 owing for 90 days adds about $26. The rate changes quarterly. Fixing the error early keeps the interest smaller.
What does the Voluntary Disclosures Program offer?
Since October 1, 2025 the CRA lists 100% penalty relief and 75% interest relief if you come forward before it contacts you, and up to 100% penalty relief with 25% interest relief if you were prompted. You still pay the tax. People under audit or investigation are restricted, so read the program page.
- How to file your tax return in Canada: steps and dates
What to gather, when your return is due and how to file in Canada, from the first slip to the notice of assessment that closes the year