Cal. Civ. Proc. Code § 1240.320

(a) Any public entity authorized to exercise the power of eminent domain to acquire property for a particular use may exercise the power of eminent domain to acquire for that use substitute property if all of the following are established:
1. (1) The owner of the necessary property has agreed in writing to the exchange.
2. (2) The necessary property is devoted to or held for some public use and the substitute property will be devoted to or held for the same public use by the owner of the necessary property.
3. (3) The owner of the necessary property is authorized to exercise the power of eminent domain to acquire the substitute property for such use.
(b) Where property is sought to be acquired pursuant to this section, the resolution of necessity and the complaint filed pursuant to such resolution shall specifically refer to this section and shall include a statement that the property is necessary for the purpose specified in this section. The determination in the resolution that the taking of the substitute property is necessary has the effect prescribed in Section 1245.250.