N.Y.C. Admin. Code § 27-916: § 27-916 General.

When alterations are made requiring the addition of two or more plumbing fixtures in an existing building, or when a building is remodeled for an extension in size or change in use in which plumbing, drainage, or gas piping work is involved, all the new work shall be made to conform to all the applicable sanitary requirements of this code and the affected portions of the system made adequate for the added load.