Cal. Code Regs. tit. 18 § 122
Improvements consist of buildings, structures, fixtures, and fences erected on or affixed to land; planted fruit and nut trees and vines that are taxable, other than date palms between four and eight years of age; and planted ornamental trees and vines. Where a substantial amount of materials other than land, such as concrete, is added to an excavation, both the excavation and the added materials are improvements, except that whenever the addition of other materials is solely for the drainage of land to render it arable or for the drainage or reinforcement of land to render it amenable to being built upon, the land, together with the added materials, remains land. In the case of property owned by a county, municipal corporation or a public district, fill that is added to taxable land is an improvement.
Cal. Code Regs. Tit. 18, § 122
Note: Authority cited: Section 15606, Government Code. Reference: Sections 105, 110, 401 and 401.5, Revenue and Taxation Code.