Under state stormwater guidance, found in the current edition of the Washington State Department of Ecology Stormwater Management Manual for Western Washington (SWMMWW), there are instances when bioretention of stormwater and rain gardens are either NOT required, or not allowed. These include when professional geotechnical evaluations recommend against infiltration, or in areas of erosion or landslide hazard. Other requirements related to stormwater may still apply; consult the SWMMWW and your local government planning department.
Reference: SWMMWW, Volume 5, BMP T7.30: Bioretention Cells, Swales, and Planter Boxes. http://www.ecy.wa.gov/programs/wq/stormwater/manual.html