Actually, the bank safe deposit box is accessible to the successor trustee and is a very good place to store your original documents. Small fireproof home safe's (like Sentry brand) are also good for the purpose. If you only store your original doc's in such a safe, then there's no reason not to give the combo-code to your successor trustee now, so s/he can access it later.
I agree with the previous response. You can buy a fireproof safe at many retail stores, such as Home Depot or Lowe's. As far as the combination goes ... you could write a set of instructions detailing any information that your successor would need to know and store that somewhere in your home. Then you could just inform the successor of the location of those instructions. Or you could store those instructions in an encrypted document on your computer or on a removable memory device, providing your successor the necessary information as to its existence and location in the event something happens to you.
The bottom line is that it is really up to you where you store these documents, and how you inform your named successors and executors.
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If you use the safe deposit box-just authorize your PR or successor trustee to open the box with an authorized signatory card on file at the bank.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.