Challenging a will means attacking how it was made, not what it says

Unfairness is not a ground on its own

The most common reaction to an unexpected will is that it cannot be right, because it does not reflect what the family understood or what anybody thinks is fair. That feeling is genuine and it is also, in most systems, legally irrelevant on its own. A person who could make a will is generally entitled to make a surprising one, and the freedom to disappoint your relatives is part of what testamentary freedom means.

So a challenge cannot usually be built on the contents. It has to attack the making of the document: whether it was executed properly, whether the person had the ability to make it, whether they knew and approved what it said, or whether somebody else’s pressure produced it.

The grounds that recur across systems

Formality comes first, and it is the most mechanical. Wills generally have to be executed in a prescribed way, and a document falling short may be invalid regardless of how clearly it expressed the person’s wishes. Some jurisdictions have introduced a discretion to save a document that plainly reflects intention; many have not, and in those places the formalities are unforgiving.

Capacity is the second, assessed at the time of making rather than at death. Knowledge and approval is a related but distinct requirement: that the person understood the contents of the document they signed, which becomes a live question where the will was prepared by someone who benefits under it, or signed by a person with impaired sight or unfamiliar with the language. Forgery and fraud sit at the far end, rare and difficult to prove.

Undue influence is easier to allege than to establish

The idea is straightforward: the will records somebody else’s wishes because the testator was coerced. Proving it is another matter. The person who could explain what happened has died, the conduct usually occurred in private, and influence over an elderly or dependent person can be gentle, cumulative and entirely undocumented.

Systems differ significantly in how they handle that evidential difficulty. Some require the challenger to prove coercion, treating persuasion, affection and even moral pressure as insufficient. Others recognise presumptions arising from relationships of trust and dependence, shifting the burden onto the person who benefited to show the will was made freely. That divergence can make the same facts a strong case in one country and a hopeless one next door.

A different route: provision for those who depended on the deceased

Separate from any attack on validity, many systems allow certain people to claim against an estate because they were not adequately provided for. The two traditions approach this very differently and the difference is instructive. Several civil law systems reserve fixed shares for close family, which the will cannot override, so the question is arithmetic rather than discretionary.

Many common law systems instead preserve testamentary freedom and then qualify it with a discretionary jurisdiction, allowing defined categories of applicant — spouses, children, dependants — to ask a court for reasonable provision. Which model applies where the deceased lived, or where the assets are, may matter more than anything in the will itself, and estates with a cross-border element can raise both.

Cost, and who ends up paying it

There is a persistent belief that the estate always pays the costs of these disputes. It is not reliable. Costs rules vary, and while a genuine doubt created by the testator’s own conduct may sometimes justify costs from the estate, an unsuccessful challenge can leave the challenger personally out of pocket in many jurisdictions.

That risk deserves weight before anything is started, because these disputes are expensive, slow and corrosive to families in a way that outlasts the money. Mediation is widely used and frequently succeeds, partly because the outcome at a hearing is genuinely uncertain and both sides can see it.

Move early, and take proper advice

Grounds, procedures, evidential burdens, the categories of person who may claim and the effect of a successful challenge all differ substantially between jurisdictions. Whether an earlier will revives, or the estate passes under the default rules instead, is itself a local question with consequences worth knowing before starting.

Time is the acute risk here. Claims against estates and applications for provision commonly carry short deadlines, and some run from the moment authority to administer the estate is granted rather than from the death or from when the will was seen. Once distribution has occurred, recovery becomes considerably harder even where a claim is good. Anyone contemplating a challenge should speak to a qualified lawyer in the relevant jurisdiction immediately, and should not wait for family discussions to reach a conclusion.