Testamentary Capacity: Definition and How It Works in California

By Eric Ridley, attorney, Ridley Law. Updated September 2026.

Testamentary capacity is the mental competence California law requires to make a valid will or trust: understanding what the document does, what property is at stake, and who would naturally receive it.

How it works in California

Prob. Code, § 6100.5 sets the standard. A person lacks testamentary capacity if they cannot understand the nature of the testamentary act, the nature and extent of the property involved, or their relationship to the people who would naturally inherit from them, such as a spouse, children, or parents. Attorneys who want a plan that survives a later challenge often build in capacity documentation at the time of signing, not after a dispute starts.

Capacity is measured at the moment the document is signed, not before or after. A person can have good days and bad days, including in the early stages of dementia, and still sign a valid document on a day they meet the standard.

Capacity to make a will is a lower bar than the capacity needed to manage one’s own finances or enter a complex contract. Someone found unable to manage their own money can still, on the right day, have enough capacity to sign a will. For a trust or trust amendment, courts use the same standard when the document is about as simple as a will, and a higher standard when it is more complex.

Why it matters

For example, a family that waits until a parent’s dementia has progressed far before updating an estate plan risks a court later finding the parent lacked capacity when they signed, which can undo the changes and revert to an earlier document or to intestacy.

Common mistakes

Families sometimes assume a dementia diagnosis alone rules out capacity, when the real question is what the person understood on signing day. Others wait for a better time that never comes, instead of documenting capacity while it still exists.

Related terms

• Guardian ad Litem: appointed to protect a minor or incapacitated person’s interests when capacity is at issue in a case.
• Holographic Will: a handwritten will that still has to meet the same capacity standard to be valid.
• Trust Contest: often argues lack of capacity as one of its grounds.
• Undue Influence: a separate ground for a challenge, sometimes raised alongside lack of capacity.
• Will: the document capacity is measured against at the moment of signing.
• Testator: the person who makes a will.
• Trust Amendment: a signed document that changes specific terms of a trust.

Part of the California estate planning glossary. For the full treatment, see Building a Contest-Proof Estate Plan: The Capacity Documentation Playbook for California Families.