Why Your Invoice Isn't a 'Proper Invoice' Under the Ontario Construction Act: 7 Common Mistakes

The Ontario Construction Act’s 28-day payment clock only starts when the invoice qualifies as a “proper invoice” under section 6.1. Most contractors never stop to check the rules, and most off-the-shelf invoice templates fail at least one requirement.

Here are the seven most common ways invoices fall short, pulled from actual contractor invoices reviewed by Ontario construction lawyers and ODACC adjudicators. Each one resets the 28-day clock to zero.

Mistake 1: Missing the period of work

Section 6.1 requires the invoice to specify the period during which services or materials were supplied. A date range like “April 7 to April 18, 2026” satisfies it. The invoice date alone does not, an invoice dated April 20 with no indication of when the work was actually performed is missing a required field.

This is the single most common deficiency in Ontario contractor invoices. US-built invoice tools default to a single invoice date because US billing law does not require a service period. The owner can issue a notice of improper invoice within 7 days, and the clock resets.

Mistake 2: No contract reference

Section 6.1 requires identification of the contract or authority under which the work was done. This can be a contract number, a signed quote reference, the project address with a contract date, or a purchase order number. Just naming the customer is not enough.

The reason: the Act needs the invoice to be tied to a specific contract so the owner can verify scope and pricing. Multi-job contractors who reuse the same client across multiple contracts especially need this field, without it, the owner cannot tell which contract is being invoiced.

Mistake 3: Vague description of services or materials

“Renovation work” or “labour and materials” is not specific enough. Section 6.1 requires a description that lets the owner verify what was supplied. Specific tasks (“install kitchen tile, 120 sqft”) and material counts (“12 pot lights, Lutron dimmer, 250ft 14/2 NMD”) make the invoice defensible against a deficiency challenge.

Line items also matter for ITC defence: an HST audit may require you to show what each invoiced item was. Detailed line items are protection in both directions.

Mistake 4: Missing payment recipient details

Section 6.1 specifically requires the name, title, mailing address, and telephone number of the person to whom payment is to be sent. For sole proprietors that is usually the contractor themselves, but the title (“Owner”, “Principal”) must be explicit. For incorporated contractors, name the specific person plus title (e.g., “Jane Doe, President”).

US accounting tools usually print the business name and address only. Without an explicit named individual and title, the invoice does not meet s.6.1(1)(6).

Mistake 5: No HST/GST number on the invoice

Although not strictly required by s.6.1, the contractor’s GST/HST Business Number is required for the owner to claim Input Tax Credits. An invoice without a valid BN cannot support an ITC claim, and many owners will treat that as a deficiency and request a corrected invoice, effectively pushing the 28-day clock back to zero.

Always include your full HST Business Number (the format is 9-digit BN + RT0001 program account, e.g., 123456789RT0001). Markup validates this format on invoice generation.

Mistake 6: Issuing the invoice before substantial performance

Most prompt-payment work assumes invoices are issued at the end of a billing period (typically monthly) for work already completed in that period. Issuing an invoice for work scheduled but not yet performed creates a problem: the owner can challenge the invoice as covering services not yet supplied, which is a valid deficiency challenge.

For multi-month projects, issue progress invoices on a clear schedule (last business day of each month is the safest) and invoice only for work completed in that period. Document the percentage complete in the line items if possible.

Mistake 7: No proof of delivery

The 28-day clock starts on the date the owner receives the proper invoice, not the date the contractor issues it. If there is no proof of delivery (email timestamp, courier tracking, signed acceptance), an owner can dispute the receipt date and push the clock back.

Email is the easiest evidence. Always send the invoice as an email attachment with a clear subject line, and keep the sent email and any read receipt. For high-value invoices, consider courier delivery with signature confirmation.

Bonus pitfall: relying on the customer to tell you about a deficiency

Under the 2026 amendments to the Ontario Construction Act, the owner now has only 7 days from receipt to issue a notice of improper invoice. If they do not, the invoice is deemed proper even if it has technical defects. This is a contractor-favouring change, but it only helps if you can prove the date of receipt. See mistake 7.

That means an invoice that you suspect is technically deficient becomes legally proper after day 7. Don’t voluntarily resend a deficient invoice within the 7-day window, you may be resetting a clock that would have started running. Get legal advice before re-issuing.

This article is general information for Ontario trades contractors, not legal advice. The Construction Act’s proper-invoice requirements have specific procedural implications. Before relying on an invoice in a dispute or adjudication, consult a construction lawyer licensed in Ontario.