Cal. PCC § 21621

Any improvement or unit of work, when the cost, according to the estimate of the engineer, will exceed ten thousand dollars ($10,000), shall be done by contract and shall be let to the lowest responsible bidder or bidders in the manner provided in this article. The board shall first determine whether the contract shall be let as a single unit or shall be divided into severable parts, according to the best interests of the district. The board shall call for bids and advertise the call by publication pursuant to Section 6061.3 of the Government Code in a newspaper of general circulation published or circulated in the district, inviting sealed proposals for the construction or performance of the improvement or work before any contract is made. The call for bids shall state whether the work is to be performed as a unit or is to be divided into parts, as stated in the call. The board may let the work by single contract or it may divide the work into parts by separate contracts, as stated in the call.