N.Y.C. Admin. Code § 28-305.1.2: § 28-305.1.2 Structures located entirely on one property.

Where such retaining walls, partition fences or other site structures, or portions thereof, are located entirely on one property, the owner of such property shall be wholly responsible for the proper maintenance and repair of the retaining wall, partition fence or other site structure. If, however, the proper maintenance and/or repair of such retaining wall, partition fence or other site structures requires access to the adjoining property, the owner of such adjoining property shall allow such access. Refer to chapter 33 of the New York city building code for additional requirements during construction and demolition operations.