When a sole shareholder in Kelowna, British Columbia receives a demand letter in January 2025 asserting personal liability of $318,174 under guarantees signed years earlier, the immediate question is whether those guarantees remain enforceable after everything that happened in between. The guarantor in this scenario signed personal guarantees supporting 9 loans totaling $570,000 advanced by a federally chartered development bank to a technology software company between 2018 and 2024. During those 6 years, the lender repeatedly deferred payment obligations, restructured repayment terms, and continued advancing fresh capital even as the borrower's revenue collapsed from $1.09 million in 2018 to $225,000 by 2023 and cumulative deficits reached $469,000. The guarantor now faces enforcement proceedings and must evaluate whether the lender's course of conduct during the lending relationship—modifications undertaken without obtaining fresh guarantor consent—operates to discharge the guarantee in whole or in part. This lesson examines the legal principles governing material modification as a guarantor defense, the distinction between modifications that prejudice a guarantor and those that do not, and the contractual provisions that typically govern whether consent was required in the first instance.
The foundation of guarantor defense analysis rests on the suretyship principle that a guarantee is a secondary obligation parasitic upon the underlying primary obligation. A guarantor agrees to answer for the debt of another, and the scope of that undertaking is fixed at the moment of execution by reference to the terms of the loan then in existence or contemplated. Canadian common law has long recognized that if the creditor and principal debtor subsequently agree to vary the underlying contract in a manner material to the guarantor's risk, the guarantor may be discharged unless the guarantor consented to the variation or the guarantee itself authorized modifications of that character. The rationale is that the guarantor bargained to stand behind a particular obligation with particular terms; if those terms change materially, the guarantor is being held to an undertaking never made. This principle protects guarantors from finding themselves exposed to risks they never assumed—longer repayment periods that increase default probability, higher principal amounts, altered interest structures, or forbearance arrangements that allow deterioration to deepen before the guarantor is called upon. The doctrine operates as a shield for the guarantor and imposes discipline on the lender: modify the deal, lose your guarantee, unless you preserved your position contractually or obtained consent.