Better Site Design Fact Sheet: Green Parking



Green parking refers to several techniques applied together to reduce the contribution of parking lots to the total impervious cover in a lot. From a stormwater perspective, application of green parking techniques in the right combination can dramatically reduce impervious cover and consequently, the amount of stormwater runoff. Green parking lot techniques include setting maximums for the number of parking lots created, minimizing the dimensions of parking lot spaces, utilizing alternative pavers in overflow parking areas, using bioretention areas to treat stormwater, encouraging shared parking and providing economic incentives for structured parking.


All of the techniques can be applied in new developments and some can be applied in redevelopment projects, depending on the extent and parameters of the project. In urban areas, application of some of techniques like encouraging shared parking and providing economic incentives for structured parking can be very practical and necessary. Commercial areas can have excessively high parking ratios and application of green parking techniques in various combinations can dramatically reduce impervious cover of a site.


Many parking lots designs result in far more spaces than actually required. This problem is exacerbated by a common practice of setting parking ratios to accommodate the highest hourly parking during the peak season. By determining actual average parking demand instead, a maximum number of parking spaces can be set as well. Table 1 provides examples of conventional parking requirements and compares them to average parking demand.

Table 1. Conventional Minimum Parking Ratios
(Source: ITE, 1987; Smith, 1984; and Wells, 1994)

Land Use Parking Requirement Actual Average Parking Demand
Parking Ratio Typical Range
Single family homes 2 spaces per dwelling unit 1.5 - 2.5 1.11 spaces per dwelling unit
Shopping center 5 spaces per 1000 ft2 GFA 4.0 - 6.5 3.97 per 1000 ft2 GFA
Convenience store 3.3 spaces per 1000 ft2 GFA 2.0 - 10.0 --
Industrial 1 space per 1000 ft2 GFA 0.5 - 2.0 1.48 per 1000 ft2 GFA
Medical/ dental office 5.7 spaces per 1000 ft2 GFA 4.5 - 10.0 4.11 per 1000 ft2 GFA
GFA = Gross floor area of a building without storage or utility spaces.

Another green parking lot technique is to minimize the dimensions of the parking spaces. This can be accomplished by reducing both the length and width of the parking stall. Parking stall dimensions can be further reduced if compact spaces are provided. While the trend toward larger sport utility vehicles (SUVs) is often cited as a barrier to implementing stall minimization technique, stall width requirements in most local parking codes are much larger than the widest SUVs (CWP, 1998).

Utilizing alternative pavers is also an effective green parking technique. They can replace conventional asphalt or concrete in both new developments and redevelopment projects. Alternative pavers can range from medium to relatively high effectiveness in meeting stormwater quality goals. The different types of alternative pavers include gravel, cobbles, wood mulch, brick, grass pavers, turf blocks, natural stone, pervious concrete, and porous asphalt. In general, alternate pavers require proper installation and more maintenance than conventional asphalt or concrete. For more specific information on alternate pavers, refer to the Alternative Pavers Fact Sheet.

Bioretention areas can effectively treat stormwater in a parking lot. Stormwater is directed into a shallow, landscape area and temporarily detained. The runoff then filters down through the bed of the facility and is infiltrated into the subsurface or collected into an underdrain pipe for discharge into a stream or another stormwater facility. Bioretention facilities can be attractively integrated into landscaped areas and can be maintained by commercial landscaping firms. For detailed design specifications of bioretention areas, refer to the Bioretention Fact Sheet.

Shared parking in mixed use areas and structured parking are also green parking techniques that can further reduce the conversion of land to impervious cover. A shared parking arrangement could include usage of the same parking lot by an office space that experiences peak parking demand during the weekday with a church that experience parking demands during the weekends and evenings. Costs may dictate the usage of structure parking, but building upwards or downwards can help minimize surface parking.


Applied together, the green parking techniques can effectively reduce the amount of impervious cover, help to protect local streams, result in stormwater management cost savings, and visually enhance a site. Proper design of bioretention areas can help meet stormwater management and landscaping requirements while keeping maintenance costs at a minimum.


Some limitations to applying green parking techniques include applicability, cost, and maintenance. For example, shared parking is only practical in mixed use areas and structured parking may be limited by the cost of land versus construction. Alternative pavers are currently only recommended for overflow parking because of the considerable cost of maintenance and bioretention areas can be costly to construct.

The pressure to provide parking spaces can come from fear of complaints as well as requirements of bank loans may. This may pressure developers to construct more parking than necessary and be a possible barrier to providing the greenest parking lot possible.


Utilizing green parking lots can dramatically reduce the amount of impervious cover created. The level of the effectiveness depends on how much impervious cover is reduced as well as the combination of techniques utilized to provide the greenest parking lot . While the pollutant removal rates of bioretention areas have not been directly measured, it's capabilities is considered comparable to a dry swale which removes 91% of total suspended solids, 67% of total phosphorous, 92% of total nitrogen, and 80-90% of metals (Claytor and Schueler, 1996).

Cost Considerations

Setting maximums for parking spaces, minimizing stall dimensions, and encouraging shared parking can result in considerable construction cost savings. At the same time all of the green parking techniques can also reduce stormwater management costs. Bioretention areas costs about $6.40 per cubic foot of quality treatment.


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Brown, Whitney E. and Schueler, Thomas R. The Economics of Stormwater BMPs in the Mid-Atlantic Region: Final Report. Center for Watershed Protection, Ellicott City, MD. 1996.

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Wells, Cedar. Impervious Surface Reduction Technical Study. Draft Report. City of Olympia Public Works Department. Washington Department of Ecology, Olympia, WA. 1994.