N.Y.C. Admin. Code § 27-286: § 27-286 Classification.

When two or more classes of construction occur within the same building, the entire building shall be subject to the most restrictive occupancy and size limitations for the classes of construction involved. However, if the occupancies within the different classes of construction are completely separated by construction that meets the fire-resistance rating requirements for fire divisions listed in table 5-2 then each occupancy so separated may, for the purposes of this code, be considered as a separate building ("building section").