Buffer requirement. There shall be established along any boundary of the tract which coincides with a single-family residential zone boundary, and along any boundary of the project which coincides with an existing single-family detached residential lot, a minimum buffer width of 65 feet. The buffer area shall contain no building or other impervious coverage. Further, site grading and other earthwork shall be prohibited within the buffer area, except to enhance the integrity of the buffer, such as earth berming.