Impervious area is considered to be ‘disconnected’ when, “the appropriate water quality volume has been retained” in compliance with the MS4 permit. As a reminder this means when the first 1” of rainfall (i.e., minimum “Water Quality Volume”), infiltrates into the ground or is otherwise permanently retained on site. Detention does not count. You can retain stormwater on site in many ways as long as it infiltrates the ground or is reused without a surface or storm sewer discharge. In most cases, disconnection will be achieved through common GSI practices or by simply directing runoff to a vegetated area such as a lawn rather than into the stormwater system.

An impervious area conversion is met when, “an existing impervious surface is converted to a pervious vegetated surface and the pre-development infiltration rate and storage capacity of the underlying soils is restored”. This credit is more geared towards construction or industrial development or re-development. The existing impervious surface must be replaced with a pervious vegetated surface and the original soils must tested and adjusted to restore the pre-development infiltration rate and support vegetation.

While not as common as credits for non-residential properties, residential credits can offer great incentives to boost impervious cover disconnection and fee reductions for any residents who many not be as apt to pay a new utility fee. It is important to evaluate your stormwater utility fee and ensure that any residential credits which area provided create a push towards GSI implementation that actively reduces stormwater pollution all while no creating a loss of profit with the fee. In a study performed in 2014 evaluating the hydrologic and economic efficacy of stormwater utility credit programs in the US for residential properties, it was found that of the examined utilities, the ones which had minimum treatment requirements provided “a higher reduction in runoff than constituted in the discount” while other utilities experienced a higher discount for a fraction of the reduction in runoff (Kertesz, et al 2014).

Most often, residential credits provide a percentage for fee reduction or an amount in billing units in return for the implementation and maintenance of GSI on the property. Rain gardens, rain barrels, cisterns, permeable pavements, and green roofs are all common examples of qualified implementations:

Residential properties can earn a credit of 0.5 Stormwater Billing Unit (SBU) for every whole increment of 600 sq. ft. of impervious area treated, up to a maximum of 0.5 SBU for properties with less than 1800 sq. ft. of impervious area, and 1.0 SBU for properties with 1800 or more square feet of impervious area… Credits are available to residential properties that treat impervious area with the following structural controls: cisterns, dry wells, modified French drains, permeable pavers and rain gardens (Stormwater Credit Manual; Portland, ME).

To receive a 25% credit, at least 25% of the property’s total on-site impervious surface area must drain to the rain garden. In order to receive a 50% credit, at least 50% of the on-site impervious surface area, including rooftops, must drain to the rain garden (Stormwater Utility Fee Credit Manual for Residential; Harrisonburg, VA).

50 percent of the property’s roof area must be connected to rain barrels or other storage devices that provide at least 50 gallons of storage per downspout…. Must drain in no less than 24 hours and no more than four days after each rainfall event. Overflows from storage must be directed to appropriate outlets or areas and away from neighboring properties, sidewalks, steep slopes or retaining walls (Single Family Residential Property Credit Manual; Richmond, VA).

While these are typical residential credits, there are a variety of non-structural BMP options. For example, Alexandria, VA offers residential credits for ‘No Fertilizer’ pledging and native tree planting, while Senoia, GA offers septic tank maintenance and natural land conservation residential credits.

In order to maximize stormwater pollution reduction without a profit loss on the fee, most cities and towns place a cap on the amount of credits a residential property can receive. This is calculated based on impervious area size, peak flow of a storm, and the ability of a BMP to retain stormwater runoff. Cost of stormwater management and administration expenses must also be taken into consideration when deciding on credit caps. However caps are not required and there is no one correct cap to use. For example,  Roswell, GA’s approach offers 100% reduction for the first year of a residential credit for rain barrels, which is reduced to 25% for the following years.

The process for credits typically requires an application from property owners for the specific credit or credits they believe they qualify for. It is typical for property owners to have to apply each year for the same credit to ensure proper maintenance of the BMP and proper stormwater management. It is also typical that the credit be attached to the person, not the property. Therefore, if a person leaves their property and it goes into a new name, the new owner of the property will have to apply for all of the credits which the property qualifies for. Credit timelines vary from no expiration so long as applications are approved and the BMP is proven to be properly maintained to having a certain time cap on how long a property can have a credit for. See an example application from Salem.