Contracts, bids, plans and specifications; time for levy

All contracts for improvements costing more than one thousand dollars shall be in writing, and there shall be no liability to any contractor or his or her assignees for the construction of any improvement costing more than one thousand dollars unless the contract therefor is in writing. At least twenty days before the board shall let any contract for any improvement, they shall cause accurate detailed plans and complete specifications therefor, together with a detailed estimate, under oath, of the cost of such improvement, to be made by some competent person and filed in the office of the clerk of such board, for inspection by all persons interested therein, and thereafter, and before letting of such contract, the board shall advertise for bids to do such work in accordance with such plans and specifications for at least ten days in the official city paper. All bids shall be made in writing and signed by the bidder, and presented by the bidder, his or her agent or attorney, to the board, at a public meeting thereof, and all bids shall be considered and accepted or rejected immediately after their submission. The board may reject any bids and shall not accept a bid in excess of the estimated cost of the work, and a contract let at a price in excess of the estimated cost of the work shall be void. No special assessments shall be levied for the cost of any work until the contract therefor has been let; but such assessments may be levied before the construction of an improvement, when the cost of the same is definitely ascertained.