A rain garden is built to catch runoff from impervious sources. The garden area required depends on the source; typically sized at 10-20% of the source area.  To collect the runoff, grading is sometimes necessary to redirect the water.  The garden area is excavated, usually to a depth of four feet but variable with soil type.  It is filled with a sandy soil followed by topsoil so the garden lays about six inches below grade.  The plants must tolerate standing water as well as drought conditions.  Hardy, herbaceous, native plants generally perform best in rain gardens.