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If you’ve found a tax filing error after you sent your return, don’t refile and don’t wait. You fix it by asking the CRA to change the return, either online through “Change my return” in your CRA account or on paper with form T1-ADJ. Online requests are usually processed within about two weeks.
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. Mail takes much longer, and complicated requests (several years, carrybacks, a deceased person’s return) take longer again. When the change is done, you get a notice of reassessment that shows what the CRA changed.
You can ask for changes going back up to ten calendar years. Older than that, the CRA page says it can’t issue a refund.
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. We haven’t modelled CPP or income-tested benefits, which 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 mistake you know about and leave alone is the thing that grows.
What if the mistake means you owe tax?
Fix it anyway, and do it before the CRA finds it. The CRA’s page on changing a return doesn’t say how interest is worked out, and we didn’t confirm the rules for penalties, so we won’t put numbers on them here. You can check the CRA’s guidance on your own notice of reassessment.
For a mistake that’s older, the Voluntary Disclosures Program can apply. The CRA page says it gives relief case by case when the error is at least one year or one period past the filing due date. The program changed on 1 October 2025, and we didn’t confirm the current conditions, so read the program page before you apply. Even with relief, expect to pay the tax itself.
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.
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 the numbers come from
The process, processing times and the ten-year limit come from the CRA page on changing a tax return, read in September 2026. Voluntary disclosure details come from the CRA program overview. 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.
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 certified tax software, or mail form T1-ADJ with your documents.
How long does a correction take?
The CRA page says online requests are processed within about two weeks. Mailed requests take longer, and complex cases take longer still.
How far back can I change a return?
Up to ten calendar years. The CRA page says it cannot issue a refund for an adjustment request beyond that.
Should I file a second return?
No. Request a change to the return you filed. The CRA then sends a notice of reassessment showing what changed.
What if my mistake is old and I owe tax?
The Voluntary Disclosures Program can give relief case by case for errors at least a year past the due date. Check the program page for current conditions.
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Information only. This page is general information, not tax, legal or accounting advice. Tax-Services.ca is an independent publisher: we don't prepare or file tax returns or offer tax services, and we have no connection with the CRA, Revenu Québec, any government or any company named on this page. Check the figures that matter with an official source or a qualified professional.