The stormwater rate system bills properties based on usage of the storm water system, as represented by impervious area. Impervious surfaces do not absorb water. Examples of impervious areas include roofs, pavement, sidewalks, patios, and gravel or crushed stone surfaces.
How is stormwater usage measured by impervious area?
A computer analysis of infrared aerial photographs is able to distinguish hard, impervious surfaces in contrast to areas that can absorb stormwater, such as lawns and gardens. The computer program assigns the residential property into one of four billing tiers to more equitably distribute costs proportional to use instead of using a flat fee. Homes with larger impervious areas pay more. You can review your property's stormwater assessment online and, if desired, submit an appeal (see below).
What are the current stormwater rates?
Single and two-family residential properties are placed in one of the following four rate tiers, depending on the square footage of impervious area.
Measured impervious area
Midpoint of Tier listed on the Water Utilities Bill
Up to 2,187 square feet
> 2,187 to 4,175 square feet
> 4,175 to 7,110 square feet
> 7,110 square feet
* Plus a $4.07 customer service charge per quarter.
Commercial and other properties (e.g., multi-family, office, institutional, and industrial land uses) are billed directly on the impervious areas at a rate of $767.06 per acre per quarter, plus a $4.07 customer charge per quarter.
How do I view my stormwater calculation online?
- Link to your online stormwater calculation (impervious area and stormwater fee information) through
- Type your address into the search bar in the upper right.
The property image will show the impervious area analysis for the property you have requested. This analysis has primarily been done with a computer, so some errors may have occurred. The following pervious materials might be interpreted by the computer as impervious:
- Wood chips
- Areas in shadows
- Decks with spaces between the boards with underlying pervious material
Please note that driveways are impervious regardless of the material, unless it is constructed with permeable pavers, permeable asphalt, or permeable concrete (these permeable system installations are rare).
How do I submit an appeal?
If you have reason to believe that the impervious area has been incorrectly identified, please take the following steps:
Save or print the online stormwater assessment
Identify the areas that have been incorrectly identified as impervious (contrasting pen or highlighter works best)
Email it to email@example.com
or mail the document to:
City of Ann Arbor – Stormwater
Ann Arbor, MI 48107-8647
We will review your revisions, and return the results of the analysis to you. Please remember that if you are a one or two-family residential property, you will need to reduce your impervious area by an amount sufficient to enter a lower tier. For example, if you are currently at 4,775 square feet, you will need to lower your impervious area by 600 square feet (to 4,175) in order to enter a lower tier.
You can take advantage of credits to lower your storm water bill.
Stormwater Credit Events
Several workshops on rain gardens and/or rain barrels are often conducted by local organizations and businesses. Please contact these organizations for more specific topics and upcoming dates. Rain garden presentations are available to local neighborhood and service groups from the Washtenaw County Water Resources Commissioner's Office.
Frequently Asked Questions
Stormwater Utility Policy Documents
A2 Stormwater Utility Rate Study 5-3-2018
A2 Stormwater Utility Update Report FINAL Sept 08.pdf
A2 Stormwater Utility Regulations FINAL 8-06-07.pdf
A2 Stormwater Utility Policies Procedures Manual FINAL 9-15-08.pdf
Stormwater Management on Residential Construction Projects
Did you know that a stormwater management plan is required for residential construction projects with 200+ sq. ft. of impervious surfaces? Visit the Residential Stormwater Code Requirements page for more information about the code changes and to download an impervious area worksheet that will help you determine if your project will require stormwater management as part of the grading permit application process.
If you are interested in receiving stormwater credits for your treatment measures visit the Residential Stormwater Credits page to verify that your treatment measure will meet those minimum requirements before the design is complete and plans are submitted.