HST Common Filing Mistakes in Ontario (And How Small Businesses Prevent CRA Letters)

Most CRA “letters” after an HST filing aren't random. They usually happen because something in the return doesn't match the paperwork trail—missing invoices, ITC claims that don't line up, or numbers that don't reconcile to your bookkeeping.

This post covers the most common HST filing mistakes Ontario small businesses make—and a simple way to prevent CRA follow-ups before they start.

Quick Answer

To prevent CRA letters, small businesses should reconcile HST to bookkeeping, keep ITC support (valid invoices/receipts), use the correct reporting period, and avoid claiming ITCs on non-registrant or personal expenses.

The “why” behind CRA letters (what CRA is checking)

CRA is typically looking for mismatches like:

• ITCs claimed without proper support
• HST collected not matching sales patterns
• reporting period mistakes
• returns that don't tie to records if reviewed

So the fix is simple: clean inputs + clean bookkeeping + clean support documents.

Common HST filing mistakes (Ontario) + how to prevent them

1) Using the wrong reporting period

Mistake: filing for the wrong period (or mixing dates).

Prevent it: lock your reporting period dates before you start and file only transactions inside that window.

2) Claiming ITCs without CRA-ready invoices/receipts

Mistake: you have a bank charge but no valid invoice/receipt showing HST.

Prevent it: keep invoices that show supplier, date, description, subtotal, and HST amount (or clearly tax-included).

3) Claiming ITCs from suppliers who shouldn't be charging HST

Mistake: ITCs claimed where HST wasn't properly charged.

Prevent it: confirm vendor details and keep proper invoices—especially for larger purchases.

4) Mixing personal and business expenses

Mistake: claiming full ITCs on mixed-use items (vehicle, phone, home office) with no allocation logic.

Prevent it: document your business-use % and apply it consistently.

5) Not reconciling HST to bookkeeping (biggest one)

Mistake: filing from “rough numbers” without reconciling.

Prevent it: reconcile monthly (or at least before filing) so sales, expenses, and HST control accounts make sense.

6) Reporting sales incorrectly (gross vs net confusion)

Mistake: mixing tax-included and tax-exclusive totals.

Prevent it: standardize how you record sales (tax-included vs excluded) and keep it consistent.

7) Filing late or paying late (penalties add up)

Mistake: you file, but payment is late—or you miss both.

Prevent it: set reminders and keep a simple “HST due dates” calendar.

Simple “Prevent CRA Letters” checklist (do this before you file)

• Confirm reporting period dates
• Reconcile sales + HST collected
• Reconcile expenses + ITCs claimed
• Verify you have support for larger ITCs
• Check for personal/mixed-use allocations
• Save a PDF/export of the return + your support folder

FAQ