Legal Question in Wills and Trusts in California

The decedent's 1/2 share of joint tenancy property is part of his estate for the purposes of estate taxes (and in some cases 100%) (this is not a big issue for most estates nowadays, since individual exemption is $5M- but if there is a surviving spouse and her estate might exceed $5M, it is necessary to file an estate tax return in order to claim "portability" which adds the decedent's exemption to hers) but is not part of his probate estate, if any, because the surviving joint tenant acquires title by simply recording an affidavit with the recorder's office.

Property held in joint tenancy belongs to the survivor when one of the joint tenants dies. So it would not be part of the Estate of the first to die unless the joint tenancy was terminated before the person died.

My guess is you were not using the term "joint property" in the strictly legal meaning of Joint Tenancy. The previous answers are correct for property held in true Joint Tenancy. There are also some types of assets that can pass outside the estate via designated beneficiaries or pay on death provisions on bank accounts, etc. All other jointly owned property, however, is part of the decedent's estate.