In a document I am correcting the manager of the building is referred to as the Facilities Manager. Should it be Facility Manager if they are only the manager of one building? And when would, if ever, the Facility Manager be referred to as the Facilities Manager? My Boss said that it should always be Facilities Manager.
If you are referring to facility manager and facilities manager generically (as common nouns, lowercase spelling), then either form might be acceptable. A Google Ngram shows facility manager being more common in American English and facilities manager more common in British English, but they are not far apart.
if you are talking about formal titles, it is in the interest of accuracy to keep the original form and not exchange it for another one. In other words, if an organization refers to one of its staff as Facility Manager, you should use Facility Manager. If someone introduces herself as the Facilities Manager, you should use Facilities Manager.
Since this is for work, however, the overriding rule will be to use whatever your organization or publication may prefer as a matter of house style. If your boss says to always use facilities manager, you are well-advised to follow your boss's instructions.
Second, facility can be defined with a broad range of scopes. It can refer to an entire complex of buildings, like NASA's Michoud Assembly Facility. It can refer to a single toilet. Even a small building may contain all manner of equipment and spaces, so it would not be inappropriate for someone to be the facilities manager of that building.
Facilities Manager or Facility Manager. In terms of actual work (if the titles are used correctly) the scope of work will vary in scope.
Facility Manager, manages the complete building or site. Cleaning, Gardening, Maintenance, Car Parks etc.
Facilities Manager will usually be in charge of building maintenance (Lighting, heating, Air conditioning etc), but the scope may be more inclusive.
facility noun (BUILDING) a place, especially including buildings, where a particular activity happens: Cambridge English Dictionary
facilities [ plural ]: the buildings, equipment, and services provided for a particular purpose: