N.Y.C. Admin. Code § 28-305.1.1: § 28-305.1.1 Structures located on the lot line of adjacent properties and partially on both properties.

The owners of adjacent properties shall be responsible jointly for the proper maintenance and repair of retaining walls, partition fences and other site structures, or portions thereof, that are located along the common lot line and on both their properties; and each such owner shall be responsible for one-half of the costs of maintaining and repairing such fences, retaining walls and other site structures, or such portions thereof. Where an owner elects to remove temporarily a retaining wall or partition fence that is required to support a grade differential between the two properties, or for any other reason is required by this code, such owner shall protect the adjacent property, shall not impair its safe use, and shall replace the retaining wall or partition fence at his or her own cost. Refer to chapter 33 of the New York city building code for additional requirements during construction and demolition operations.