Section 28-305.2. Retaining walls required

Hereafter, when an owner elects to
  set  his  or  her  grade  either  higher  or  lower than the grade of an
  adjoining property  at  the  property  line,  such  owner  shall  erect,
  maintain and repair a retaining wall of sufficient height, structure and
  foundation to support such grade differential, and with proper drainage,
  in  accordance  with  this  code, such that the adjacent property is not
  impacted, and shall do so at the sole expense of such owner and entirely
  on the property of such owner without access to the adjoining property.