Who this is for
- Canadian corporations with missed Form 1120-F filings after US business activity
- Canadian corporations with an unfiled treaty claim or protective return
Not covered here
- Determining whether a US trade or business or permanent establishment exists
- US state returns and Canadian T2 catch-up
- Computing a US tax refund or Canadian foreign tax credit
We never filed Form 1120-F. Are our deductions already lost?
Not necessarily. A Canadian corporation that files a true, accurate Form 1120-F within the applicable deduction window can generally claim deductions tied to its effectively connected US business income. After that window, most deductions and credits are unavailable unless the IRS grants a waiver; a few statutory exceptions remain (26 CFR 1.882-4).
| Situation for a missed year | What is at risk |
|---|---|
| US business income, return within the deduction window | Ordinary filing penalties may still apply, but the return can preserve eligible deductions and credits |
| US business income, return outside the window | Most deductions and credits are unavailable unless the IRS grants a waiver |
| Treaty protects business profits because there was no US permanent establishment | File the treaty disclosure if required; a protective return can preserve deductions if the treaty position is later rejected |
What is the 18-month rule, and when does it start?
Start with each year's ordinary Form 1120-F due date. For a December 31 year-end with no US office, the usual due date is June 15 of the next year; the deduction cutoff is December 15 one year after that, subject to weekend or holiday shifts. The due date depends on a US office or place of business, not simply a US customer or treaty permanent establishment (IRS filing responsibilities; 26 CFR 1.882-4).
| US office or place of business | General Form 1120-F due date after year-end | Deduction window, generally |
|---|---|---|
| Yes | 15th day of the fourth month | Due date plus 18 months |
| No | 15th day of the sixth month | Due date plus 18 months |
Check weekend and holiday shifts and IRS notices. With a US office, a June year-end that began before January 1, 2026 uses the 15th day of the third month; later June years use the fourth month (IRS filing manual). After the first required year, if the preceding year's return was not filed, the deduction window ends at the earlier of 18 months after the due date or the date the IRS mails a specified missing-return notice. A filed preceding year, or the first required year, uses the general 18-month rule (26 CFR 1.882-4(a)(3)). A filing extension does not move this regulation's starting date.
Does having no US permanent establishment change the catch-up?
Yes. If the corporation qualifies for the Canada–US treaty and has no US permanent establishment, Article VII generally protects its business profits from US federal income tax. But a US trade or business can still require Form 1120-F, with Form 8833 for a reportable treaty claim (treaty Articles V and VII; IRS instructions).
A protective Form 1120-F matters if the IRS later finds a permanent establishment or rejects treaty eligibility: timely filing preserves deductions against taxable US business income. Known taxable income requires a full return, not a protective one.
What is the penalty for a missed treaty disclosure?
A C corporation can face a US$10,000 penalty for each required treaty-based position it failed to disclose, even if the treaty leaves no US business-profits tax. Section 6712 measures the penalty per failure, not simply per late return (US Code, section 6712).
Form 8833 instructions require a separate disclosure annually for each reportable treaty position. If effectively connected business profits are claimed exempt because they are not attributable to a US permanent establishment, disclose that position on Form 8833 for each affected year (26 CFR 301.6114-1(b)(5)). Check other treaty positions for exceptions.
Can the IRS waive the deduction deadline or the disclosure penalty?
Yes, but the two waivers use different rules and neither is automatic. For late deductions, the corporation must satisfy the IRS that it acted reasonably and in good faith, and must cooperate in determining its tax liability. Knowing a return was required and choosing not to file fails that test (26 CFR 1.882-4(a)(3)(ii)).
The regulation asks whether the corporation came forward before the IRS discovered the gap, knew about protective filing, had filed before, exercised reasonable diligence, or faced events beyond its control. Explain the facts year by year and provide records. For the treaty-disclosure penalty, section 6712 separately lets the IRS waive some or all of it for reasonable cause and good faith (US Code, section 6712).
How many years back must we file?
Review every year from the first US activity that may have required Form 1120-F; there is no safe three-year catch-up cutoff for an unfiled required return. Under section 6501, the IRS can assess tax at any time when no return was filed (US Code, section 6501(c)(3)).
Build a year-by-year timeline before selecting forms. Some years may need a full return, some a treaty-based protective return, and some none.
What usually starts a catch-up, and what should we gather?
A new accountant, buyer, or IRS notice can reveal the gap. Record when the corporation learned of it and whether the IRS had contacted it; both affect a possible waiver under 26 CFR 1.882-4. Before closing, a buyer may demand returns or a holdback.
Gather the same core records for each year:
- Canadian and US returns, US employer identification number, extensions, IRS letters, and filing proof.
- US contracts, invoices, sales by activity, payment records, Forms 1042-S, and tax withheld.
- Staff and contractor travel dates, job-site records, US office or warehouse details, and who concluded contracts.
- Year-end financial statements, expense support, related-party transactions, and the basis for treaty residence and eligibility.
Decide each year's US trade-or-business status, permanent establishment, filing due date, deduction deadline, and forms. File missing returns with required Forms 8833. If the US business had reportable related-party transactions, attach Form 5472 for each related party; failure to file can cost US$25,000 per year (IRS instructions; missed Form 5472 filings). Where US tax was withheld, file promptly: a late original return generally can refund only tax treated as paid within three years before filing, plus any filing extension; chapter 3 or 4 withholding counts as paid on the original return due date (26 USC 6511; 26 USC 6513). Seek deduction and treaty-penalty relief separately; answer ordinary late-return penalty notices when received (IRS instructions). Keep filing proof. US tax paid may affect the Canadian T2; see claiming US tax on a corporate return.
Example
Illustrative only. All amounts are US dollars.
No US permanent establishment, three missed years
A Canadian corporation had a US trade or business but qualifies for treaty protection in three missed years. Assume it had no US permanent establishment and each year needed Form 8833. Business profits may be exempt from US federal income tax, yet one undisclosed position each year could bring US$10,000 per year, or US$30,000 total. It files the missing returns and disclosures and seeks penalty relief if assessed. Relief is not assured.
A US job site created a permanent establishment
Assume a Canadian corporation's job site created a US permanent establishment. US$300,000 of revenue less US$250,000 of attributable deductible expenses leaves US$50,000 of profit. If it misses the deduction window and gets no waiver, and none of those costs are cost of goods sold, the taxable base can rise to US$300,000. At the 21% federal rate, tax rises from US$10,500 to US$63,000 before any branch tax (26 USC 11). Cost of goods sold still reduces gross income (Form 1120-F, Section II); actual treaty attribution can differ.
A buyer finds the missing returns
Before a share sale, a buyer asks for US filing history. The corporation rebuilds the affected years and submits required returns and treaty disclosures before closing, keeping proof for due diligence. The buyer may still request a holdback; filing does not settle the tax or penalty outcome.
Different for you?
- Unsure whether any Form 1120-F was required or whether there was a permanent establishment: see Canadian corporation US tax returns.
- Staff trips may have changed the treaty result: see employees working in the US.
- You may owe state returns too: see business income tax in other states.
- Your Canadian T2 returns are also late: see catching up on unfiled corporate returns.
- A buyer is reviewing the company: see selling your business.
- Several years, a deduction waiver, or an IRS notice are involved: bring the timeline and records to cross-border tax.
Figures on this page
| Figure | Value | Source |
|---|---|---|
| Usual Form 1120-F filing window to preserve deductions and credits Measured from the return due date; exceptions and earlier IRS notice rules can apply | 18 months | IRS: Foreign corporation Form 1120-F filing responsibilities Checked |
| Penalty for not disclosing a treaty-based return position (C corporation) Per failure, under section 6712, for a C corporation. | US$10,000 | IRS: Form 8833 (Rev. December 2022) Checked |
| Form 5472 penalty for failure to file or keep records For tax years beginning after December 31, 2017. Per reporting corporation, per tax year. A substantially incomplete Form 5472 counts as a failure to file. IRC 6038A(d)(1); Treas. Reg. 1.6038A-4(a). | US$25,000 | IRS: Instructions for Form 5472 Checked |
| Refund lookback for a late original income-tax return Refund generally limited to tax paid within three years before filing the late original return, plus any extension of time to file | three years | US Code: section 6511(b)(2)(A) Checked |
Primary sources
About this guide
Edited and reviewed by Di Lu, CPA on . It explains general rules for the tax year shown. It is not advice for your situation.
Changes
- : First published.