This is allowed in Colorado.
Colorado's version of the Uniform Probate Code authorizes such a document for tangible personal property which is usually called a "Personal Property Memorandum." This is authorized by Colorado Revised Statutes § 15-11-513. The section of Colorado's probate code states:
Whether or not the provisions relating to holographic wills apply, a
will may refer to a written statement or list to dispose of items of
tangible personal property not otherwise specifically disposed of by
the will, other than money. To be admissible under this section as
evidence of the intended disposition, the writing shall be either in
the handwriting of the testator or be signed by the testator and shall
describe the items and the devisees with reasonable certainty. The
writing may be referred to as one to be in existence at the time of
the testator's death; it may be prepared before or after the execution
of the will; it may be altered by the testator after its preparation;
and it may be a writing that has no significance apart from its effect
on the dispositions made by the will.
New York State Law
NYS has a requirement that the Will is signed by 2 Witnesses and (I
believe -- keep me honest) notarized. So I'm hoping this is a way for
me to add items to the Appendix without needing to re-sign, re-witness
and re-notarize every time I think of something else. Is this strategy
valid & enforceable?
This is generally not permitted in New York State
The legal formalities for the execution of wills and trusts in New York States are among the most strict in the United States (Florida and Maine have been more strict from time to time).
This cannot be done in a way that is valid and enforceable in New York State.
One can determine this from a review of the relevant New York State Statute which is called the Estate, Powers, and Trusts Law (EPTL), that does not contain a parallel provision to the Colorado statutory section.
Therefore, in New York States, all dispositive provisions of a Will related to tangible personal property must be executed with the same formalities as any other Will or Codicil (i.e. it must be signed by the testator and two witnesses, a notarization is not required).
Oral and unwitnessed handwritten wills are rarely allowed in New York State
New York State does recognize oral wills (called nuncupative wills) and wills in the handwriting of the person making them (called holographic wills) as well, but only in very limited circumstances. The pertinent provision of the EPTL state:
(a) For the purposes of this section, and as used elsewhere in this
(1) A will is nuncupative when it is unwritten, and the making thereof
by the testator and its provisions are clearly established by at least
(2) A will is holographic when it is written entirely in the
handwriting of the testator, and is not executed and attested in
accordance with the formalities prescribed by 3-2.1.
(b) A nuncupative or holographic will is valid only if made by:
(1) A member of the armed forces of the United States while in
actual military or naval service during a war, declared or undeclared,
or other armed conflict in which members of the armed forces are
(2) A person who serves with or accompanies an armed force engaged
in actual military or naval service during such war or other armed
(3) A mariner while at sea.
(c) A will authorized by this section becomes invalid:
(1) If made by a member of the armed forces, upon the expiration of
one year following his discharge from the armed forces.
(2) If made by a person who serves with or accompanies an armed force
engaged in actual military or naval service, upon the expiration of
one year from the time he has ceased serving with or accompanying such
(3) If made by a mariner while at sea, upon the expiration of three
years from the time such will was made.
Wills valid where executed will generally be honored
If a Will accompanied by a Personal Property Memorandum were drafted and executed in Colorado while someone was domiciled there, and then was presented to a Surrogate's Court in New York State for probate, because the person who wrote it was domiciled in New York State when they died, however, the New York courts might honor the Personal Property Memorandum on the theory that the validity of the execution of a Will is usually governed by the law of the place where it is signed and not by the law of the place where it is probated.
The alternative of a revocable trust in New York State
A workaround somewhat similar to a personal property memorandum could be done with a revocable trust, but this has its own limitations.
In New York State a trust or amendment to a trust not created by a last will and testament must be either (1) signed and notarized by the creator of the trust and also by the trustee if there is one separate from the creator of the trust, or (2) signed by the creator of the trust and witnessed by two witnesses in essentially the same way that a will would be witnessed.
Unlike most U.S. states, New York State does not recognize trusts that are signed but are not notarized or witnessed, and unlike most U.S. States, New York State does not recognize orally created trusts.
Trusts validly formed and amended outside of New York will generally be honored
This is, of course, assuming in both cases that New York State law governs the formation of the trust. New York State choice of law rules, however, will generally recognize the validity of a trust formed with formalities that were valid in the place where it was executed if the person executing it was domiciled there.