Assuming this is an abbreviation for 'Nutzfläche', this is the term we used when I worked at MOD Defence Estates. It refers to the total floor space of the building, including areas such as cellars/basements, laundry rooms, attics, etc.. As such it's a bit misleading to use it in conjunction with 'Wohnfläche', which is the space taken up by living areas (living rooms, kitchens, bathrooms, etc.).

You could use 'total floor space/area' as well.