Law firm cross-border tax: US clients, treaty cover, and the Reg 105 mirror

US clients, Canadian desk: paperwork, not US tax

When a New York company retains your Toronto firm for a cross-border deal, the fees are income from services performed in Canada — not US-source, and under the treaty's business-profits article not taxable by the US without a permanent establishment there. What their AP department needs is a Form W-8BEN-E from your professional corporation or LLP certifying foreign status and the treaty claim. Their systems default to requesting a W-9; sending one back as a Canadian firm is the wrong answer every time. The W-8BEN-E stays valid until the end of the third calendar year after signature, so calendar the renewal.

The subtler trap is HST. Legal services supplied to non-residents are generally zero-rated — but there is a carve-out that pulls services connected to litigation in Canada back into tax. Advising that same US client on a share purchase agreement is 0%; defending them in an Ontario courtroom is 13% HST despite their address. Firms that zero-rate every US-billed file build an HST liability with every invoice, and we see it on almost every new law firm file we take over.

Working physically in the US: sourcing follows your feet

Fly to Chicago for depositions, argue an arbitration in New York, close a deal in person in Boston — fees attributable to those days are US-source services income. The treaty still protects you if the firm has no US permanent establishment, but a PE can arise from a services test: 183 or more days of US presence in any twelve-month period on the same or connected project, not just from an office lease. For a firm with recurring US-side work, the prudent posture is a protective Form 1120-F with a Form 8833 treaty disclosure: it costs little, preserves the right to deduct expenses if the IRS ever asserts a taxable presence, and avoids the disclosure penalty. We track partner day counts the same way we track trust reconciliations — routinely, not heroically.

The Reg 105 mirror: when your firm is the withholding agent

Every rule your US clients' payment systems apply to you, CRA applies in reverse when your firm brings talent north. Pay a US expert witness, jury consultant, or foreign counsel for services performed in Canada and the firm must withhold 15% under Regulation 105, remit it, and issue a T4A-NR slip — even if the expert clearly owes no Canadian tax under the treaty. The fix for a treaty-exempt expert is an R105 waiver obtained before payment, not an apology after. Engagement letters matter here too: US experts often demand gross-up clauses, which quietly move the withholding cost onto the file.

Dual-bar practitioners and USD money in trust

A partner admitted in both Ontario and New York is taxed by where each dollar is earned, not where they are called to the bar. Ongoing US of-counsel income means a US filing with a matching Canadian report and foreign tax credit; US bar dues and CLE remain deductible against the practice income they support. Spend enough of the year stateside and personal residency itself needs managing — the treaty tie-breaker keeps you Canadian, but only if the facts are documented.

USD retainers add a books problem rather than a tax problem. Client money in your USD trust account is not firm income, but the fee portion must be valued in CAD when it is earned and transferred, and the FX difference lands in the firm's P&L — LSO trust reporting and CRA both expect the same numbers. The firm's own US bank and investment accounts (never trust funds) count toward the $100,000 T1135 threshold. Our law firm bookkeeping service keeps the trust ledger, FX, and tax reporting reconciled monthly so year-end is a formality.

Source: IRS — About Form 8833, Treaty-Based Return Position Disclosure.