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    City of Ann Arbor

    Excerpt from Stormwater Ordinance

    Stormwater Rates

    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. 

    Single-Family and Two-Family Residential
    Tier # Measured impervious area Representative

    Impervious Area

    ​Midpoint of Tier listed on the Water Utilities Bill



    ​Tier 1 Up to 2,187 square feet ​0.03706 acres $28.43
    ​Tier 2 > 2,187 to 4,175 square feet ​0.06486 acres​ $49.75
    ​Tier 3 > 4,175 to 7,110 square feet ​0.11117 acres $85.27
    ​Tier 4 >  7,110 square feet ​0.19456 acres $149.24


    * 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? 

    1. Link to your online stormwater calculation (impervious area and stormwater fee information) through mapAnnArbor.
    2. 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:

    1. Save or print the online stormwater assessment
    2. Identify the areas that have been incorrectly identified as impervious (contrasting pen or highlighter works best)
    3. Email it to or mail the document to:

      City of Ann Arbor – Stormwater
      Box 8647 ​
      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.

    Stormwater Credits

    You can take advantage of credits to lower your storm water bill.


    Under the Post Construction Stormwater Management control measure, the City of Ann Arbor has a program requiring new and redevelopment projects to implement on-site controls that will reduce pollutant loads in stormwater run-off. The City of Ann Arbor follows the rules of the Washtenaw County Water Resources Commissioner for post construction controls. These rules are incorporated by reference into Chapter 63 of the City Code - Stormwater Management and Erosion and Sedimentation Control. 

    City staff review site plans for compliance with regulations governing stormwater management.

    Chapter 63 (5:658) requires all site plans to include a plan for the long-term maintenance of stormwater controls:

    "A program proposal for the continued maintenance of all permanent soil erosion and sedimentation control measures that remain after project completion, including the designation of the person or party responsible for the maintenance. Maintenance responsibilities shall become a part of any sales or exchange agreement for the land on which the permanent soil erosion and sedimentation control measures are located."

    Utilizing Low Impact Development (LID)​

    Low impact development (LID) is the industry standard for green infrastructure. The LID Manual for Michigan provides the latest tools for implementing post con​struction stormwater controls.

    What is LID? In short, it is a type of development that uses a basic principle modeled after nature - to manage rainfall using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. LID provides a host of stormwater benefits, including groundwater recharge and cleaner streams.

    Keep the following in mind if you are implementing a LID project:

    • Plan for stormwater management during initial phases of site design
    • Manage stormwater close to where it falls
    • Conserve and restore natural areas
    • Minimize impervious surface
    • Manage runoff with structural controls
    • Provide maintenance and education
    • Examples of LID Practices for Stormwater

    Implementing LID practices can:

    • Reduce the volume of stormwater flows
    • Improve the quality of stormwater runoff
    • Reduce the need for future capital improvements to maintain or repair stormwater systems


    • Practices designed to facilitate the percolation of runoff through the soil to groundwater
    • Result in reduced stormwater runoff quantity and reduced mobilization of pollutants
    • Examples: infiltration basins/trenches, dry wells, permeable pavement (shown)

    Stormwater Retention/Detention

    • Control stormwater by gathering runoff in wet ponds, dry basins, or multi-chamber catch basins and slowly releasing it to receiving waters or drainage systems
    • Can be designed to both control stormwater volume and settle out particulates for pollutant removal​


    • Landscaping features that, with optimal design and good soil conditions, remove pollutants and facilitate percolation of runoff
    • Maintain natural site hydrology, promote healthier habitats, and increase aesthetic appeal
    • Examples: grassy swales, filter strips, artificial wetlands, and rain gardens


    Stormwater Ordinance 

    Michigan Stormwater Homepage

    Water Infrastructure Homepage

    Best Management Practices for Stormwater A Developer's Guide for Ann Arbor (PDF)