N.Y.C. Admin. Code § 28-305.2: § 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.