Introduction
An incorporated professional who has been claiming a specific deduction consistently — a home office at 25% of square footage, 80% vehicle business use, or a significant annual professional development expense — may discover that the CRA, on audit, denies the deduction for multiple prior years simultaneously. This creates a compounded reassessment that can be significantly larger than any single year's issue.
Scenario: Crestfield Legal Corp Faces a Multi-Year Vehicle Expense Denial
Crestfield Legal Corp has claimed 85% business use on a $72,000 vehicle for four years — deducting approximately $12,000 in annual vehicle expenses plus a portion of the standby charge calculation. No contemporaneous logbook was ever maintained.
In 2026, a CRA field audit covers 2022, 2023, and 2024. The auditor requests the vehicle logbook. There is none. The auditor proposes to reduce the business use percentage from 85% to 50% — a reduction of 35 percentage points — for all three years.
The adjustment per year: 35% of approximately $12,000 in vehicle costs = $4,200 per year, denied. Across three years: $12,600 of denied deductions. At the SBD rate, additional corporate tax: approximately $1,536 plus three years of arrears interest.
Additionally, the incorrect standby charge calculation — based on 85% business use rather than the adjusted 50% — creates a corresponding T4 amendment for the shareholder's personal return in each year. The shareholder benefit that was understated by the business-use adjustment is added to the personal T1 for each year, with personal tax plus arrears interest.
Why Multi-Year Denials Are Disproportionately Painful
The interest component of a multi-year reassessment is calculated from the original payment due date for each year — meaning interest on the 2022 adjustment has been accruing since February 2023. A 2022 adjustment of $1,600 has three years of interest at 9% compounded daily — approximately $480 of interest on that year alone.
The compounding effect of multi-year denials, combined with both corporate and personal adjustments, means the total cost of a systemic documentation failure can be three to four times the apparent annual exposure.
The Response Strategy
The objection to a multi-year deduction denial should:
Provide any available substitute evidence for business use — fuel receipts with dates and destinations, calendar records of client meetings, billing records showing the dates and locations of client work.
Challenge the proposed percentage where the alternate evidence supports a higher business use than the auditor's default.
Propose a negotiated business-use percentage — often more realistic than either the original 85% claim or the auditor's default 50%.
Address the standby charge amendment and the personal T1 adjustments through the same objection process.
The Forward Fix
Once the audit is resolved, the deduction must be recalibrated for future years. A contemporaneous logbook — maintained from this point forward — supports the business-use claim in any future review.
When to Speak With a CPA
Immediately upon receiving a multi-year reassessment. The 90-day objection window applies to each year's reassessment notice. Where multiple reassessments are issued simultaneously, all 90-day windows are active simultaneously.