Stormwater Fee Structure Analysis

Rather than calculating and collecting stormwater utility fees based on water usage, the City is proposing a new structure where these fees would be based on how much impervious area is on each individual property within the City.

Impervious area is any area within a property that prevents or significantly impedes the infiltration of stormwater into the soil (e.g., buildings, parking lots, walkways, sheds, patios, decks, swimming pools, etc.). Impervious area better reflects the service demands that a property places on the stormwater system, since excess runoff that cannot be absorbed into the ground must be conveyed through the City’s infrastructure. For a background of the stormwater utility and further explanation of this change, please visit this page for more information.

Impervious area is now the most common measure used to calculate stormwater fees in the United States. Generally, these types of fees are calculated for each property using a base billing rate called an Equivalent Resident Unit (ERU). For Wheaton, the ERU would represent 3,300 square feet of impervious area, which is the average amount of non-porous surface on single-family residential properties within the City. The City would review each property, assign it an ERU based on its impervious area characteristics, and then charge the property a monthly utility fee based on its ERU.

Although the ERU billing setup is most common, there are several variations of this fee structure. For example, each property could be billed for its exact amount of impervious area (Actual ERU) or residential properties could be assigned ERUs based on a range (Tiered ERU). Staff explored several of these structures and analyzed how each of them would affect the proportion of stormwater fees assessed to different types of property in Wheaton (Residential, Commercial, Institutional, & Industrial). Below is a summary of these fee structures and their estimated impact on utility fees for different types of properties within the City.

Proposed Options for Stormwater Utility Fee

Option 1 - Actual ERU:

Under this option, every property is billed for the exact amount of impervious area on the property proportional to the ERU. For example, a property with 4,950 sq ft of impervious area would be calculated like so:

4,950 sq. ft. / 3,300 sq. ft. = 1.5 ERU

Pros: The amount of impervious area for each property is easy to identify. Additionally, this option would be consistently applied across every property type in the City, so the administration of the fee would be relatively simple.

Cons: This option does not consider the pervious area of each property. Properties with higher amounts of pervious area will have more opportunities for stormwater to percolate into the soil than properties with lower amounts. Considering the amount of pervious area is important, so the fee accurately reflects each property’s burden on the stormwater infrastructure.

Option 2 - Tiered Residential ERU:

Under this fee structure, the City would group single-family properties and assign them a standard ERU based on their impervious area. The table below shows the tiered system if the City implemented the Village of Downers Grove’s fee structure. Non-single-family residential properties would be charged based on their actual amount of impervious area. Vacant parcels would be charged 0.3 ERUs.

Property Classification # of ERU on Property
Tier 1 Single Family (1 – 2,500 sq ft of impervious area) 0.75 ERU
Tier 2 Single Family (2,501 – 4,000 sq ft of impervious area) 1 ERU
Tier 3 Single Family (4,001 – 7,000 sq ft of impervious area) 1.5 ERU
Vacant Properties 0.3 ERU
All Other Properties Actual ERU (#)

Using the example from Option 1, a property with 4,950 sq. ft. of impervious area would be placed in Tier 3.

Pros: From the City’s perspective, this option provides the simplest administration and assessment of fees. A property’s impervious area is unlikely to change significantly, so a property’s tier is expected to remain consistent.

Cons: This option is less proportional than the other two since there will be properties who will pay slightly more or slightly less than their actual impervious area. Like Option 1, it does not consider the amount of each property’s pervious area.

Option 3 - Actual ERU & Intensity of Development (ID):

This fee structure incorporates an Intensity of Development (ID) that assigns an ERU to a property in addition to the property’s Actual ERU. A property’s Intensity of Development is calculated as the ratio of impervious area on a property compared to the total area of the property. Essentially, the more developed a property is, the more ERUs are assigned to it. The tiers for the ID are shown below.

Intensity of Development % Impervious Area Adjusted ERU
Tier 0 < 1% 0.1
Tier 1 1% – 20% 0.2
Tier 2 21% – 40% 0.4
Tier 3 41% – 60% 0.6
Tier 4 61% – 80% 0.8
Tier 5 81% – 100% 1.0

Using the same example, a house with 4,950 sq ft of impervious area with 10,000 sq ft of total area would be calculated like so:

4,950 sq ft / 10,000 sq ft = 49.5% = Tier 3 = 0.6 ERU

This ERU would be added to the property’s Actual ERU, as shown in the following calculation:

1.5 ERU + 0.6 ERU = 2.1 ERU

Pros: This fee structure, unlike the other two options, considers a property’s pervious area. Therefore, this option best reflects a property’s burden on the City’s stormwater infrastructure, resulting in the most proportional distribution of the fee burden.

Cons: This option is the most complicated to administer since the fees would be more sensitive to development within a property.

Projected Impact of Fee Options on Stormwater Customer Base

For each option, staff performed a preliminary analysis of how each fee structure would affect the proportion of the stormwater fee paid by different property types. The classifications of property types are as follows:

  • Residential - all single and multi-family properties
  • Commercial - all properties used to conduct commercial business
  • Institutional - all properties owned by public entities, non-profit organizations, and utilities
  • Industrial - all properties used for industrial purposes

It is assumed that the fee per ERU would be $4.24 in 2024, the minimum the fee needs to be to cover all operating stormwater costs. This fee is dependent on Council approval and is subject to change.

The table below demonstrates how the incidence of the fee is expected to shift among property types compared with the current method.

Proportion of Stormwater Utility Fee Allocation by Property Type

Count of 
Current Fee 
(Water Use)
Option 1 
Actual ERU
Option 2 
Tiered ERU
Option 3
Residential 16,725 83% 65% 67% 72%
Commercial 764 9% 19% 18% 16%
Institutional 450 8% 16% 15% 12%
Industrial 23 0% 1% 1% 0%

All three options decrease the overall proportion of stormwater fees borne by residential properties while fees borne by other property types increase. There are two reasons for this shift. Under the current fee structure, residential properties pay the highest portion of fees since these properties use the most water. Moreover, the higher amount of development and impervious area on the other property types shifts the burden of fees to those properties since these generate more stormwater runoff.

The City of Wheaton is proposing Option 3 for its next stormwater fee. The earliest the City would adopt this new fee structure would be at the end of 2023. Adopting this fee structure would result in a more proportional distribution of these fees and a more dependable source of revenue for the City.

If you would like additional information on the City’s analysis and proposed fee structure you can review several of the previous city council planning sessions that covered this topic:

For more information about the Stormwater Utility Fee, please visit this page or contact