If you spot an error after you file, do not file a second return. Ask the CRA to change the one you already sent.
How do you change a return?
- Wait until you get your notice of assessment, so the CRA has finished the first one.
- Sign in to your CRA account and use the change my return option, or send the T1 adjustment request by mail.
- Say what changed, and attach any slips or receipts for the new amounts.
What if the mistake means you owe more?
Fix it as soon as you can. The CRA may charge interest on the extra tax, and a correction you make yourself can be treated more gently than one the CRA finds. Ask about the voluntary disclosures program if it is a big omission.
What if the mistake means you are owed money?
You can generally ask for a change for up to 10 calendar years back, so a missed credit or slip can still be claimed. Keep receipts for the years you ask about.
What should you watch out for?
A mismatch with a slip can trigger a letter. See our CRA letters decoder and dealing with the CRA. A tax preparer can help with bigger corrections.
Where does this information come from?
The steps follow the Canada Revenue Agency guidance on changing a tax return. Checked in October 2026. This is an independent information site: we provide no services, we do not represent the CRA or any organization, and we do not guarantee that anything here is accurate or current. We accept no liability for decisions based on it. Check the official source before you rely on it. See the full disclaimer.