Cal. Health & Safety Code § 40405

(a) As used in this chapter, “best available control technology” means an emission limitation that will achieve the lowest achievable emission rate for the source to which it is applied. Subject to subdivision (b), “lowest achievable emission rate,” as used in this section, means the more stringent of the following:
1. (1) The most stringent emission limitation that is contained in the state implementation plan for the particular class or category of source, unless the owner or operator of the source demonstrates that the limitation is not achievable.
2. (2) The most stringent emission limitation that is achieved in practice by that class or category or source.
(b) “Lowest achievable emission rate” shall not be construed to authorize the permitting of a proposed new source or a modified source that will emit any pollutant in excess of the amount allowable under the applicable new source standards of performance.