Ancillary or secondary use space (Nebennutzfläche - "NNF") is, under the terms of the SIA ordinance 416, the part of the usable space (Nutzfläche - "NF") that is supplementary to the main usable space (Hauptnutzfläche - "HNF").

In residential buildings, ancillary space includes areas such as utility rooms, attic and cellar space, store rooms, bunkers, and waste disposal amenities.