What Evidence Do You Need to Challenge a Will in Ontario?

Believing a will doesn't reflect what a loved one truly wanted is not, on its own, enough to overturn it. Ontario courts need evidence to challenge a will — specific, relevant proof connected to one of a small number of recognized legal grounds. Understanding what kind of evidence actually matters can save you from pursuing a challenge that feels justified but isn't legally supported, or from underestimating one that genuinely is.

The type of evidence you need depends entirely on which ground you're arguing. A single medical diagnosis, for instance, proves far less than most people assume.

The Recognized Grounds for a Will Challenge

Ontario courts will only set aside a validly signed will on specific grounds:

• Lack of testamentary capacity
• Lack of knowledge and approval of the will's contents
• Undue influence
• Fraud
• Failure to meet the formal signing and witnessing requirements

Each ground calls for a different kind of proof.

Evidence for a Capacity Challenge

Testamentary capacity is a legal standard, assessed at the specific moment the will was made, not a general medical diagnosis. A dementia diagnosis, for example, does not by itself prove someone lacked capacity to make a valid will at that particular time. Useful evidence tends to include:

• Medical records from around the time the will was signed, not just a general diagnosis at some other point
• The drafting lawyer's file notes, including their own observations of the testator's understanding
• Evidence about whether the testator understood the nature and extent of what they owned and who might reasonably expect to benefit
• Witness observations of the testator's behaviour and communication around the relevant date

Evidence for a Knowledge and Approval Challenge

This ground focuses on whether the testator actually understood and approved the specific contents of the will, separate from whether they had general capacity. Relevant evidence includes the circumstances of how the will was prepared and reviewed, whether the testator read it or had it read to them, and anything unusual about how instructions were given — for example, entirely through a third party who stood to benefit.

Evidence for an Undue Influence Challenge

Undue influence is often the hardest ground to prove, because it requires showing that someone's free will was actually overborne, not just that they were persuaded or influenced in an ordinary way. Evidence that tends to matter includes:

• The testator's isolation from other family members or friends
• Dependency on the person who benefited from the alleged influence
• A significant, unexplained change in the will's terms that benefits the person accused of influence
• That person's direct involvement in giving instructions for the will

Evidence for Fraud or Improper Execution

Fraud claims require evidence of deliberate misrepresentation or concealment connected to the will. Improper execution claims focus on the mechanics of signing — whether the testator's signature is genuine, whether two witnesses were actually present at the same time, and whether the formal requirements were followed. Witness affidavits and, where necessary, forensic document analysis are common in this category.

Why the Drafting Lawyer's File Often Matters Most

Across nearly every ground, the file kept by the lawyer who drafted the will is often the single most persuasive piece of evidence in either direction. Contemporaneous notes about the testator's instructions, demeanour, and understanding, made at the time rather than reconstructed later, tend to carry significant weight with a court, which is one reason a properly documented will-signing process matters.

Frequently asked questions