Your Notice of Assessment is CRA's first answer after you file. A Notice of Reassessment is CRA changing that answer later — sometimes weeks later, sometimes years later. The two documents look almost identical, which is exactly why they trip people up. This is what to do when one shows up.

The one-sentence difference

An assessment is CRA's first review of a tax return. A reassessment is CRA changing the numbers on a return they already assessed. The wording on the document is the only easy way to tell them apart: look at the top of page 1 — it will say "Notice of Assessment" or "Notice of Reassessment." Everything else (layout, the RRSP/TFSA boxes, carryforwards) is identical.

The original assessment usually arrives within 8 business days of NETFILE. A reassessment can arrive any time within the next three years (or longer — see below).

Why CRA reassesses

Most reassessments come from one of five triggers:

1. T-slip mismatch. Your T4 says one number, your return says another. CRA's slip-matching software runs all year, not just at filing time.
2. Post-filing review. CRA asks for backup on a deduction (medical, moving, charitable), and either accepts it (no reassessment) or denies it (reassessment).
3. You filed a T1 adjustment. Asking CRA to fix something is itself a reassessment — even when you initiated it.
4. Spouse mismatch. You and your partner both claimed the same dependant, eligible-spouse credit, or pooled medical expenses incorrectly.
5. Loss carryback. You filed a current-year loss against a prior year's income — that prior year gets reassessed.

About 30% of reassessments come back in the taxpayer's favour. CRA processed a late T5008, applied a credit you missed, or fixed a math error. The document looks scary but the bottom line might be a refund.

The 90-day window

From the date on a Notice of Reassessment, you have 90 calendar days to file a formal Notice of Objection (form T400A). This is the deadline for the formal appeals path. You can still file an informal T1 adjustment after 90 days — but only an objection preserves your right to escalate to the Tax Court of Canada.

If you're past 90 days but within a year, you can request an extension under subsection 166.1(7) of the Income Tax Act. CRA usually grants these if you can show "due diligence" (you were sick, abroad, or never received the notice). Past one year, you're stuck with whatever CRA decided.

How far back CRA can reassess

• Three years from your original NOA date — the standard "normal reassessment period" for individuals
• Six years if there's "misrepresentation attributable to neglect, carelessness, or wilful default"
• No limit if CRA can prove fraud

For most Canadians, the practical window is three years. After that, CRA needs to prove something to extend it — and the burden of proof is on them.

What to actually do when you get one

1. Read the "Explanation of changes" section first. Don't react to the bottom-line number — find the line items that changed.
2. Decide if CRA is right. Pull your filed return, your T-slips, and your supporting receipts. Match each changed line.
3. If CRA is right: pay the new balance by the date on the notice. Interest accrues at 8% (2026 rate) from the original April 30 due date — not from the reassessment date.
4. If CRA is wrong on a small item: file a T1 adjustment online via CRA My Account → Change my return. Usually resolved in 8 weeks.
5. If CRA is wrong on a big item, or refuses an adjustment: file Notice of Objection (T400A) within 90 days. See our NOA dispute guide.
6. If interest is the main issue, consider a Taxpayer Relief request (form RC4288) — CRA can waive interest for hardship, error, or extraordinary circumstances. Use the late filing penalty calculator to model what's at stake first.

What it doesn't mean

A reassessment is not:

• An audit. Audits involve a CRA auditor reviewing your books in detail. Reassessments are usually automated.
• A criminal investigation. Tax evasion files go through CRA's Criminal Investigations Program — completely separate process.
• A guarantee of more reassessments. Most taxpayers reassessed once never see another one for that tax year.

The full breakdown of how these three differ is in review letter vs audit vs reassessment.

Bottom line

A reassessment is CRA pressing the "I changed my mind" button. The document looks identical to your original NOA, but the 90-day appeal clock and back-dated interest calculation make the stakes meaningfully different. Read the explanation of changes, match it against your records, and either pay or object — but don't ignore it. The penalty for ignoring a reassessment is interest at 8% per year, compounded daily, with no statute of limitations on collection.

Sources: CRA — About your Notice of Assessment, CRA — File a Notice of Objection, Income Tax Act, sections 152 and 165.