Stormwater Retrofit Manual

Stormwater Retrofit Manual 

This manual provides guidance on how to approach the management of stormwater through installation of SCMs in existing or reconstructed development situations where regulatory requirements do not dictate prescribed stormwater control measures.  For the purposes of this manual, a retrofit is defined as the addition of stormwater controls on a currently developed site. Retrofits may include controls incorporated into planned construction work or controls implemented as a stand-alone stormwater improvement project. Retrofit stormwater control measures (SCMs) are designed to minimize the impacts of existing development at sites either lacking or having insufficient stormwater controls in place. The intent of the manual is to provide research-based guidance on a fundamental approach for planning, siting, and design of retrofit stormwater control measures as well as an approach for crediting pollutant reductions associated with these measures.

Situations where retrofits are encouraged, and this manual will assist, could include:

  • implementing stormwater controls as part of maintenance and planned construction
  • demonstrating stormwater treatment to the maximum extent practical
  • implementing stormwater controls to meet total maximum daily load (TMDL) targets
  • identifying and prioritizing stormwater management interventions to protect or improve a local water resource
  • quantifying the improvement of select structural controls (e.g., pollutant load reductions)
  • understanding cost/benefit for various control measures and options


This manual does NOT intend to replace existing federal, state or local requirements for stormwater management. Instead it should be used as supplemental guidance and a resource to support smart choices in site design as it relates to the mitigation of stormwater impacts from existing developed areas. This manual supports the choice to do some improvement on a site over bypassing an opportunity because it does not fit within a current regulatory framework.

The Stormwater Manual will be ready to download by June/July 2022. Please check back soon. 

This Stormwater Retrofit Guidance Manual has been developed by SNEP Network Partner, The University of New Hampshire Stormwater Center and project consultant VHB with technical support of other SNEP Network partners, state agencies, and EPA Region 1.