Portland city councilors could vote Wednesday on a plan to charge every city property owner a new stormwater fee based on the size of their rooftops and driveways.

The new fee would take effect in January 2016 and help pay for $170 million in infrastructure improvements required by the federal government to prevent untreated sewage and polluted runoff from flowing into local waterways during storms.

The new fee would be partly offset by a decrease in sewer fees, although the average homeowner would see an estimated $4.50 increase in combined monthly fees for sewage and stormwater. Some businesses with large buildings or parking lots could pay thousands of dollars a month. Property owners would be able to reduce their fees by controlling stormwater runoff, such as with retention ponds to capture contamination washed off parking lots.

The City Council is scheduled to hold a public hearing on the proposal at its 7 p.m. meeting. It could vote on the fee or postpone action to a later date.

The proposal has been working its way through city committees for several months and received relatively little public comment, although some large property owners have challenged the fairness of some elements.

The city has set up a website – www.cleangrowth cleanwater.com/calculator – that will allow property owners to see how many “monthly billing units” staff has estimated for each property based on impervious area – rooftops and pavement. Each billing unit is equivalent to $6, so a property with 3 billing units would be assessed $18 a month in fees. When entering an address, leave off the words “street” or “avenue.”

Such a fee is assessed in other cities as a way to spread the costs of managing stormwater that rolls off rooftops, down driveways and across parking lots before entering the sewer system or waterways. The improvements have been paid from sewer fees, which don’t take into account a property’s effect on stormwater.