Each year during the period of time for which the improvement is to continue, and not less than 30 days prior to the making of the general tax levy for city purposes, the board, commission, officer or person designated pursuant to the provisions of Section 18040 shall furnish to the city council an estimate of the costs of the improvement for the next year. For the first year, the estimate of the cost of the improvement set forth in the resolution ordering the improvement shall be used.