(1) Excavations for foundations shall be taken down to firm natural ground, except as otherwise provided in section 9.

(2) Unless a foundation is placed on solid rock, the bottom of the foundation shall be not less that 450 millimeters below the adjoining finished ground-level unless otherwise authorized by the local authority.

(3) If a foundation is placed on solid rock, the bearing area shall be cleaned and, if necessary, stepped or dowelled adequately to prevent lateral movement.