Take this secret to the grave, and it'll cost you

The location of your estate-planning papers shouldn't be a secret you take to the grave.

Only a third of people have told their heirs where they can find key documents like their will or powers of attorney, according to a new survey from BMO Wealth Management. Another one in 10 has provided heirs with copies of the original documents.

The survey polled 1,008 Americans in December 2016.

Keeping mum about the location of your will, powers of attorney and other important documents is dangerous, said Larry Lehmann, a partner at Lehmann Norman & Marcus LC in New Orleans.

"It's a terrible mess, and litigation results out of it," he said.

In many states, if heirs can't find your will, the law presumes you revoked it and destroyed it, said Lehmann, who is also past-president of the National Association of Estate Planners and Councils. That could mean state intestacy laws determine who gets your assets, which may not be who you'd prefer.

It could also mean if someone finds an older version of your will, that's the one that stands — even if your wishes or family dynamics have changed.

Here's how to manage these critical documents:

Keeping them safe is vital. A court may not accept copies or digital files as a replacement for lost original wills, especially if someone is contesting the estate, said Russell Fishkind, an attorney with Saul Ewing in Princeton, New Jersey. It becomes "he said-she said" as to whether you intentionally destroyed the original, or it was accidentally misplaced.

States may also require originals of healthcare proxies or powers of attorney for your agent to act on your behalf, said Lehmann.

Maintain each original document in its own sealed, marked envelope, said certified financial planner Mark Prendergast, director of tax strategies for Inspired Financial in Huntington Beach, California. That can help ensure pages don't go missing, or get jumbled with other documents.

As for where, the best place to keep your original documents is in a fire-proof box or safe in your own home, said Fishkind, who is also the author of "Probate Wars of the Rich & Famous." Some clients prefer to keep originals with their attorney, he said, although that isn't the best option for documents you'd need in an emergency, like a healthcare proxy.

Don't put these papers in a safe deposit box, said Lehmann. At best, that limits family access to when the bank is open — which may not be convenient in a crisis.

State laws and bank policies also vary in who can access the box, for your protection. If you haven't properly set up access for your heirs, they may need a court order to do so should you become incapacitated or after your death, he said. A power of attorney for financial transactions might help – but not if the paperwork your agent would need is in the box.

Financial professionals you work with — including your financial advisor, accountant and attorney — should all have copies of your estate documents, said Prendergast. It's smart to have a copy in a neutral player's hands. Those pros can also help nudge you to make updates, as they notice changes in your personal and financial details.

Put copies in the hands of trusted family members, too, especially if they are named to make health care decisions or take financial actions on your behalf should you become incapacitated, said Lehmann.

"Whoever you have appointed needs to have copies of those documents," he said, "and if you don't want them to have [a copy], you need to reconsider why you appointed them."

Even if you'd prefer to keep the location of your will close to the vest, someone other than your spouse should know where important documents are kept, said Lehmann. That helps ensure your papers are found quickly, should both of you be injured in the same accident or pass away in close proximity.

All of the financial professionals you work with should know where you keep your original documents, to help in such a circumstance, said Fishkind.

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Any trusted family members or friends you alert should also be given access to any information they might need, he said. For example, the combination to your home safe or the location of its key, or your attorney's contact details if he or she has the original documents.

After you update a will, power of attorney or other key documents, destroy the old versions so that there's no confusion over which version reflects your current wishes, Lehmann said.

"Use a cross-cut shredder," he said. "Get rid of them."

Any time you make changes, repeat the above steps to make sure all your key parties have updated information, copies or originals as necessary. Any time you make changes to your team of financial professionals, follow the same protocol, he said, to ensure your family knows who to reach out to now, and that that pro has the latest document versions.

"That way there's no weak link," Fishkind said.