There are many myths and misconceptions surrounding whether a handwritten will is valid. The short answer is- it depends.
A handwritten will is also referred to as a “holograph will”. Section 6 of the Succession Law Reform Act establishes that a testator may make a valid holograph will. There are, however, criteria that the holograph will must comply with in order for the court to declare it as valid. The will must be wholly in the testator’s handwriting and include the signature of the testator. Holograph wills do not have the same requirement that they be witnessed.
The Supreme Court of Canada decision (Bennett et al. v. Gray / Bennett et al. v. Toronto General Trusts Corporation, 1958 CanLII 49 (SCC),  SCR 392), added an additional requirement, which is that the holograph will must contain a “deliberate or fixed and final expression of intention as to the disposal of property upon death.”
These cases can get messy when the testator has used a pre-populated template that contains some writing other than that of the testator, or when the testator is attempting to use a suicide note as their last Will.
A recent amendment under section 21.1 (1) of the Succession Law Reform Act grants our courts a broad discretion to determine that a holograph will is valid, even if it does not fully comply with formalities.
We always suggest that you seek legal advice when making preparations for your Last Will and Testament.