How do I correct a mistake on a previously filed Ontario tax return?

If you made an error or omission on a previously filed T1 return, you can request an adjustment rather than filing a new return. The most common method is to use the CRA's Change My Return service through My Account online, which allows you to correct most line items electronically. Alternatively, you can mail a completed T1 Adjustment Request (form T1-ADJ) to the relevant CRA tax centre.

The CRA generally allows adjustments for up to 10 years after the original assessment. For a typical individual, this means you can correct returns going back a decade if you discover missed deductions, unreported income, or calculation errors. However, the CRA may audit or ask for supporting documents, particularly for credits or deductions you are adding retroactively.

If your mistake resulted in you underpaying tax, correcting it will lead to a bill for the difference plus interest. If you overpaid, the CRA will refund the difference. Voluntarily disclosing previously unreported income before the CRA contacts you is generally treated more favourably than if the CRA finds it first — the Voluntary Disclosures Program (VDP) can waive penalties in certain situations. A tax professional should guide you through any significant amendment, especially if unreported income is involved.