70 ILCS 2805/51
The officer specified in Section 50 shall estimate what portion of the total cost of such improvement will be of benefit to the public and what proportion thereof will be of benefit to the property to be benefited, and he shall apportion the total cost between the district and that property so that each will bear its relative equitable proportion. Having found these amounts, such officer shall apportion and assess the amount so found to be of benefit to the property upon the several lots, blocks, tracts and parcels of land in the proportion in which they will be severally benefited by the improvement. No lot, block, tract or parcel of land shall be assessed a greater amount than it will be actually benefited. When the proposed improvement is for the construction of a sewer, it is the duty of such officer to investigate and report the district which will be benefited by the proposed sewer, describing the district by boundaries.
Where the improvement is to be constructed with aid from any agency of the federal government, or other governmental agency, the proportion of the total cost of the improvement to be raised by the district in addition to such aid shall be the amount allocated between public benefits and benefits of the property affected as above provided.
70 ILCS 2805/51