The legislative body may, without further notice or hearing, omit or decrease any portion of or make such alterations or changes in the work proposed to be done which will not increase the estimated cost by more than one-tenth of the total estimated cost as stated in the report and no changes or alterations which will increase said estimated cost by more than said one-tenth shall be made. The person who prepares the plans and specifications shall estimate separately the cost of each addition to or omission from or decrease in the things proposed to be done as set forth in the report, and file such estimate with the clerk of the legislative body. Such filed estimate shall be conclusive evidence of the cost of such changes.