Can naming a joint account holder on my property help my estate avoid Ontario probate tax?

It can, but only where the joint ownership is a genuine joint tenancy with right of survivorship, not simply two names on an account. When property is truly held that way, it typically passes automatically to the surviving joint owner on death by operation of law, outside your estate entirely, which means it isn't part of the value your estate certificate is based on, so it doesn't add to the Estate Administration Tax calculation.

The complication is that adding an adult child's name to an account for convenience, say, to help manage your finances, doesn't automatically create that kind of ownership. Ontario courts have recognized that, in many parent-child situations, the presumption is the account is held in trust for your estate rather than truly gifted to the child jointly, unless there's clear evidence you intended to make an immediate gift of a joint interest. Get that wrong, and the account can end up back in the estate anyway, or create a dispute among your other beneficiaries about what you actually intended.

Because the probate savings only materialize if the joint ownership is properly documented and genuinely intended, this should be set up with legal advice about your specific intentions, not added informally.