California Code, Health and Safety Code - HSC § 42301.18

(a) Any agricultural source that existed prior to January 1, 2004, that becomes subject to a permit requirement pursuant to a district rule or regulation that was adopted prior to that date shall be permitted as an existing source and not as a new source.

(b) Any agricultural source that is an existing source pursuant to subdivision (a) shall be permitted by the district based upon its maximum potential to emit air contaminants, to the extent that level can be determined, as of January 1, 2004.

(c) A district may not require an agricultural source to obtain emissions offsets for criteria pollutants for that source if emissions reductions from that source would not meet the criteria for real, permanent, quantifiable, and enforceable emission reductions.