A general rule of thumb¹ is that residential rain gardens average about $3 to $4 per square foot, depending on soil conditions and the density and types of plants used.  Commercial, industrial and institutional site costs can range between $10 to $40 per square foot, based on the need for control structures, curbing, storm drains and underdrains. In any bioretention cell design, the cost of plants varies substantially and can account for a significant portion of the facility's expenditures. While these cost estimates are slightly greater than those of typical landscaping treatment (due to the increased number of plantings, additional soil excavation, backfill material, use of underdrains etc.), those landscaping expenses that would be required regardless of the bioretention installation should be subtracted when determining the net cost.

Perhaps of most importance, however, the cost savings compared to the use of traditional structural stormwater conveyance systems makes bioretention cells quite attractive financially. For example, the use of bioretention can decrease the cost required for constructing storm water conveyance systems at a site. A medical office building in Maryland was able to reduce the amount of storm drain pipe that was needed from 800 to 230 feet - a cost savings of $24,000.² And a new residential development spent a total of approximately $100,000 using bioretention cells on each lot instead of nearly $400,000 for the traditional stormwater ponds that were originally planned.³ In addition, in residential areas, stormwater management controls become a part of each property owner's landscape, reducing the public burden to maintain large centralized facilities. Detailed cost estimates are given below, as adapted from Prince George's County Bioretention Manual.⁴

Data or studies that compare construction, maintenance, and life cycle costs for stormwater management systems are limited. The wide range of site conditions and design requirements also makes it difficult to determine the life cycle cost benefits. It is recommended that each potential application be evaluated on a site-by-site basis. However, a range of cost estimates for the basic installation of permeable paver materials is given in the table below for comparison purposes.⁷ The wide range of costs for the paver systems should be noted.
 

Paver System	Cost Per Square Foot (Installed)
Asphalt	$0.50 to $1.00
Porous Concrete	$2.00 to $6.50
Grass / gravel pavers	$1.50 to $5.75
Interlocking Concrete Paving Blocks	$5.00 to $10.00*

*dependent on depth of base and site accessibility, per conversation with Maryland Unilock® representative (2002)

Users should also keep in mind that a more accurate price comparison would involve the costs of the full stormwater management paving system.  For example, a grass / gravel paver and porous concrete representative stated that when impervious paving costs for drains, reinforced concrete pipes, catch basins, outfalls and stormwater connects are included, an asphalt or conventional concrete stormwater management paving system costs between $9.50 and $11.50 per square foot, compared to a permeable paving stormwater management system at $4.50 to $6.50 a square foot. The savings are considered to be even greater when pervious paving systems are calculated for their stormwater storage; if designed properly, they can eliminate retention pond requirements.⁸

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References

¹ Krueger, G., 2000: Pervious paving offers one solution to city's flooding problem. Savannah Morning News, web posted February 12, 2000. Search the News archives for the Local section at  http://www.savannahnow.com/.

² Canadian Water and Wastewater Association (CWWA), 2001: Porous pavement cleans up water run-off: 'Green' roads would improve the environment. Bulletin, 15 (5) June. Contact CWWA for archived copied. http://www.cwwa.ca/home_e.asp 

³ North Carolina State University Cooperative Extension Continuing Education Course on Permeable Pavements developed by Bill Hunt in the Department of Biological & Agricultural Engineering.  Visit the following links for more information:
http://greene.ces.ncsu.edu/content/Continuing+Education+for+Real+Estate+Professionals and http://www.bae.ncsu.edu/stormwater/PublicationFiles/BMPs4LID.pdf

⁴ Ferguson, B.K., 1996: Preventing the problems of urban runoff. Washington Water RESOURCE, the quarterly report of the Center for Urban Water Resources Management, 7(4) Fall.  Accessible at http://depts.washington.edu/cuwrm/ under Subscriptions.

⁵ White, P., 1996: A whole lot of turf - permeable paving permits mall expansion in Connecticut. Turf Magazine, February.  Accessible at Invisible Structures, Inc. Grasspave² web site http://www.invisiblestructures.com/GP2/whole_lotof_turf.htm.

⁶ For more information on the project contact Kinston, NC City Engineer. http://www.ci.kinston.nc.us/publicservices/Engineering.htm 

⁷ Numbers compiled from:
     Peterson, C., 2001: Pervious Paving Alternatives. http://www.petrusutr.com/paving_paper.htm.

     EPA, 2000: Low Impact Development (LID) - A Literature Review. EPA-841-B-00-005, Office of Water, Washington, D.C.

     Booth, D.B., J. Leavitt and K. Peterson, 1997: The University of Washington Permeable Pavement Demonstration Project – Background and First-Year Field Results. Accessible at http://depts.washington.edu/cuwrm/ under Research.

⁸ Chere Peterson of PETRUS UTR, Inc., 2002, personal communication