A letter from CRA is not automatically an audit. The large majority are processing reviews — routine, narrow, and resolved by sending in the document that was asked for.
Work out what you are actually holding
There is a real difference between a pre-assessment or processing review (verifying one claim on one return), a request for information, and an audit (an examination of your books, usually with a named auditor and a scope letter). The response effort is not remotely the same, and treating a review like an audit wastes money while treating an audit like a review creates risk.
Note the deadline before anything else
CRA letters carry response deadlines, typically 30 days. Missing one usually means the claim is simply disallowed and you are reassessed — then you are arguing to get something back rather than to keep it, which is a materially worse position.
If you need more time, ask for it. Extensions are commonly granted when requested before the deadline rather than after.
Answer narrowly
Send what was asked for. Not the rest of the file, not a fuller explanation than the question requires, not context that was not requested.
This is not about concealment — it is that a review is scoped to a specific claim, and volunteering unrelated material invites questions that were never on the table. Answer the question that was asked, completely and accurately.
Keep the paper trail
Send documents in a way that produces proof of delivery and date. Keep a copy of everything sent. If you speak to an agent by phone, note the date, time and agent identification number.
When to bring someone in
If the letter concerns more than one year, uses the word audit, proposes an adjustment you disagree with, or concerns an area where you are unsure the original position was right — get a representative authorised before you respond. An authorised representative can deal with CRA directly, and the first response is much easier to get right than to correct later.
If you have already been reassessed, there is still a route: a Notice of Objection. For an individual the deadline is the later of 90 days from the date on the notice and one year after the filing due date for that return — which is often more time than people assume, though never enough to be casual about. For a corporation it is the 90 days alone. Miss it and you can apply for an extension, but that is a discretionary application rather than a right.
General information only, current at the date of publication. Tax rules and CRA positions change. This is not advice for your circumstances.