Article 5. Annexation of California Health And Safety Code >> Division 3. >> Chapter 8. >> Article 5.

At any time after the incorporation of a district, land contiguous to it may be annexed, if the board of trustees finds that the annexation will benefit both the land to be annexed and the district.
Annexations may proceed under Article 2 (commencing with Section 2822), as though territory to be annexed were a new district, except that the pest or pests to be controlled shall be the same as those which the annexing district is authorized to control.