Whenever the governing body shall have acquired lands for public parking stations it shall cause to be made by some competent person an estimate, under oath, of the cost of improving the land for parking stations, which estimate shall be filed with the city clerk. The percentage of the cost of improving such parking stations to be assessed against the benefit district shall be the same as determined for the cost of acquiring the lands therefor. The assessment against the benefit district shall be apportioned among the various lots, tracts, pieces and parcels of land within the benefit district in accordance with the special benefits accruing thereto, this apportionment of benefit assessments to be made by three disinterested property owners appointed by the governing body of such city within thirty days after the filing of the estimates of the cost of said improvement with the city clerk.

As soon as the amount chargeable against each piece of property is ascertained, the governing body of such city shall by ordinance levy such amount against this said real estate in the benefit district, which ordinance shall be published once in the official city paper. No suit to question the validity of the proceedings of the city shall be commenced after thirty days from the awarding of a contract for such improvements and until the expiration of said thirty days the contractor shall not commence work under his or her contract. If no suit shall be filed within such thirty days then all proceedings theretofore had shall be held to be regular, sufficient and valid.