Section R. 560.418 - SlopesRule 418.
(1) A development site less than 1 acre in size or a lot shall have sufficient areas of suitable soils in areas that have slopes conducive to the construction of initial and replacement on-site sewage disposal systems.(2) If a development site less than 1 acre in size or a lot has slopes of more than 12%, then the department may require the submission of a detailed development plan by any of the following persons demonstrating competence:(a) A licensed professional engineer.(b) A professional surveyor.(c) A registered sanitarian.(d) A professional approved by the department. The professional person shall draw the detailed plan to scale with a maximum 2-foot contour interval, shall show the proposed design for the initial and replacement sewage disposal systems, and shall indicate the location intended for the home and individual water supply well. The professional person shall indicate both existing and proposed contours. The professional person shall ensure that areas defined on the plan for sewage systems and the well are at locations that are readily accessible for future maintenance or replacement, or both.
(3) If extensive site modifications are required to provide for areas that have a slope amenable to construction of the on-site sewage system, then the department may require necessary site work to be completed before final recording of a subdivision or before approval for on-site sewage disposal of a development site that is less than 1 acre in size.Mich. Admin. Code R. 560.418