During the time allowed in any contract for the completion of any work the commissioner of public works shall, when requested in writing by the contractor at the end of any month during which said request has been made, issue an estimate of the work completed during such month, or completed and for which no estimate has been issued, and upon completion of the work said commissioner shall issue a final estimate of such work, such estimate shall be filed with the city clerk and the city clerk shall certify the same to the city council. Said final estimate shall include in addition to a statement of the amount of money due the contractor, a statement of the amount of money expended for abstracts, advertising and engineering expenses prior to the date set for the completion of the contract. All engineering expenses incurred after said date shall be borne by the contractor as a penalty for failure to complete the work within the specified time. All contracts for local improvements shall provide that at least fifteen percent of the amount due on estimates shall be retained to secure the payment of laborers who have performed work thereon, and materialmen who have furnished material therefor, and such laborers and materialmen shall have thirty days after the work has been completed to file a claim against such fifteen percent so reserved for labor done or material furnished. Provided however, that where in any improvement or work the contract price shall exceed two hundred thousand dollars but ten percent shall be reserved on estimates in excess of said sum, or where the aggregate of previous estimates equals or exceeds said amount. No contract shall be considered as completed until the commissioner of public works shall file the commissioner’s final estimate and approval of such work. No contract for the making of any public improvement under this chapter shall be assigned, transferred or sublet, except for materials, in any manner without first having endorsed thereon the consent of the commissioner of public works and the sureties on the contractor’s bond, and no transfer or assignment thereof shall in any wise affect such bond or the liability of the sureties thereon, and any assignment or transfer of any such contract without such consent and waiver endorsed thereon, except by operation of law shall make the same null and void as to any further performance thereof by the contractor, or contractor’s assigns, without any act on the part of the city council. If any such contract shall for any reason become ended or void, or shall be abandoned, the city council may re-let such contract, or complete the same as the agent and at the expense of such contractor and the contractor’s sureties. No assignment, transfer, abandonment or surrender of any such contract, nor any change in any such contract, nor any extension of time in which to complete any such contract, shall ever operate to release sureties on any bond provided for in this chapter. Any contractor desiring an extension of time for the completion of any work under any contract shall make a written application to the commissioner of public works for such extension of time, and shall state in such application the reasons for such extension, and the commissioner of public works may grant an extension of time if, in the commissioner’s judgment, such contractor is entitled to the same. No extension of time shall be valid unless endorsed on such contract. (Prior code § 13.48.180.)