Ontario construction businesses carry meaningful tax exposure on every project, and the expensive mistakes are rarely exotic — they are recurring operational misses. The rules themselves have not moved: the harmonized rate in Ontario is still 13% (5% federal GST plus 8% provincial part), and the small-supplier registration threshold is still $30,000 of worldwide taxable sales over four consecutive calendar quarters. What does change, every few years, is where the Canada Revenue Agency publishes the rules and how easy it is to cite them. This refresh was triggered by exactly that: CRA reorganized its GST/HST pages, and guidance links that contractors saved years ago now dead-end. The substance survived the reorganization; your bookmarks may not have.
This is a practical refresher for Ontario contractors — general information, not CPA advice — organized around the mistakes that actually cost money: input tax credits claimed at the wrong time, holdback cash treated as earned cash, collected HST spent before it is remitted, and late filings that trigger formula-driven penalties. Every claim below points at the CRA page it comes from, in the current location as of October 2026.
Mistake 1: Treating holdback as earned revenue — and its HST along with it
Progress billing is where construction cash flow and HST timing collide. On most Ontario contracts, the owner holds back a statutory 10% of each progress payment, released only at substantial completion or as the Construction Act's holdback rules allow. The HST question is not about the rate — it is about what you owe and when. Where the general rule applies, GST/HST becomes payable at the earlier of when the invoice is paid or when it is due, and the CRA's guidance on charging and collecting runs through these timing rules in detail. The trap is mental accounting: the holdback slice of a progress invoice is contractually restricted cash, while its HST component follows the invoice/payment timing rules — money you may owe before you ever collect it.
Mistake 2: Claiming ITCs before you are entitled to them
You recover the GST/HST you paid on business purchases by claiming input tax credits (ITCs), and the CRA's general guide for registrants (Guide RC4022) sets the expectation: as a registrant, you generally claim your ITCs on the return covering the reporting period in which you made the purchases. ITCs you failed to claim can be carried forward — the guide's general limit is that they must be claimed by the due date of the return for the last reporting period ending within four years. The holdback-specific trap cuts the other way: documents and guidance the industry long relied on for holdback ITC timing — notably CRA publication P-028R, Holdbacks: Entitlement to Input Tax Credits — is no longer available on Canada.ca, and the underlying timing questions (when is tax on the holdback portion 'paid' for ITC purposes?) are exactly the ones worth confirming with your accountant rather than a stale link. Where a supplier invoices HST on amounts you have not yet paid, the safe operational habit is to claim ITCs in the period the tax was actually paid, and track holdback-linked ITCs separately.
Mistake 3: Spending collected HST as if it were margin
Collected HST is not your money — it is trust-adjacent cash that belongs to the CRA on your filing schedule. The contractors who get burned are the ones who let collected HST pool in the operating account until a slow month turns it into payroll. The boring fix works: a separate account that receives the HST slice of every invoice the day it lands, with remittances paid from it. If your reporting periods and cash rhythm are mismatched — common when owners pay at 60–90 days but you file monthly — the separate account is what stops the shortfall.
Mistake 4: Filing late — the penalty is a formula, not a warning
Late-filing penalties are mechanical. The CRA's published formula is A + (B × C), where A is 1% of the amount owing, B is 25% of A, and C is the number of complete months the return is overdue, to a maximum of 12. Work the example: you file three months late owing $10,000. A is $1,000; B is $250; the penalty is $1,000 + (3 × $250) = $1,750. That is before interest, which accrues on top — the CRA sets prescribed interest rates quarterly, compounds them daily, and publishes the current quarter's rate on its prescribed interest rates page. No invented numbers here on purpose: rates move, the mechanism does not.
Mistake 5: Vague invoices that break the input chain
Your commercial clients need proper invoices to claim their own ITCs, and the CRA prescribes the information requirements by sale amount — the chart lives in Guide RC4022. A description like 'site work — $14,200' invites a rejected credit downstream and signals sloppiness to any auditor. The contractor-grade fix is template-level: fixed invoice structure with HST broken out, project and phase identified, and holdback lines shown separately from amounts due.
The registration line people forget until it bites
Once your worldwide taxable sales pass $30,000 in a single calendar quarter or over four consecutive ones, you must register for the GST/HST — and charge it from there, not from whenever the paperwork happens. Subcontractors growing past small jobs hit this first, usually because several clients' deposits land in the same quarter.
Make compliance operational, not episodic
The through-line is the same as every other back-office risk: the contractors who avoid HST pain are not the ones who know more tax trivia, they are the ones whose invoicing, holdback tracking, and remittance calendar run without depending on memory. One project system that carries HST-split invoices, holdback balances, and filing dates turns this whole article into defaults instead of checklists.
This article is general information, not tax advice — rules current as of October 2026, sourced from Canada.ca pages linked above. Confirm your specific situation with a CPA, especially anything touching holdback ITC timing.