We’ve been design ALOT of rain gardens in Maryland. Right now, the rationale is squeezed into the current Maryland Design Manual by “qualification” for a rooftop disconnect. This will hopefully change in the future to facilitate more opportunities to apply these at-the-source measures. A nice DNR doc on rain gardens can be found here.
However, like most made-for-homeowner docs on this subject, they COMPLETELY MISSED the fact that – unless you have very sandy soils, you really should incorporate an underdrain system and man-made soil mixes. This probably illustrates the difference between what a homeowner can do in their yard and what a professional specifies on a plan for someone else to implement.