Cal. Rev. & Tax. Code § 24919

Neither the basis nor the adjusted basis of any portion of real property shall, in the case of the lessor of such property, be increased or diminished on account of income derived by the lessor in respect of such property and excludable from gross income under Section 24309 (relating to improvements by lessee on lessor’s property). If an amount representing any part of the value of real property attributable to buildings erected or other improvements made by a lessee in respect of such property was included in gross income of the lessor for any taxable year beginning before January 1, 1942, the basis of each portion of such property shall be properly adjusted for the amount so included in gross income.