Cal. PCC § 21551

(a) All contracts for the construction of any unit of work estimated to cost over ten thousand dollars ($10,000) shall be let to the lowest responsible bidder in accordance with Article 3.5 (commencing with Section 20120) of Chapter 1.5. If cost of the project or service will not exceed ten thousand dollars ($10,000), the district may have the work done by force account. The district may purchase in the open market, without advertising for bid, materials and supplies for use in any work either under contract or by force account.

(b) The board of directors may, by ordinance, resolution, or board order, authorize the county engineer or other county officer to order changes or additions in work being performed under construction contract. When so authorized, any change or addition in the work shall be ordered in writing by the county engineer, or other designated officer, and the extra cost for any change or addition to the work so ordered shall not exceed five thousand dollars ($5,000) when the total amount of the original contract does not exceed fifty thousand dollars ($50,000), nor 10 percent of the amount of any original contract that exceeds fifty thousand dollars ($50,000), but does not exceed two hundred fifty thousand dollars ($250,000).

(c) For contracts whose original cost exceeds two hundred fifty thousand dollars ($250,000), the extra cost for any change or addition to the work so ordered shall not exceed twenty-five thousand dollars ($25,000), plus 5 percent of the amount of the original contract costs in excess of two hundred fifty thousand dollars ($250,000). In no event shall any such change or alteration exceed one hundred fifty thousand dollars ($150,000).