Is "premises" always plural?

Neither premise nor premises actually means a company. Premises refers to

a piece of land together with its buildings, esp considered as a place of business

According to Dictionary.com, both premises (plural noun) and premise (singular noun) can have this meaning. Both can also refer to logical propositions.

The answer I pointed you toward would seem to indicate that on-premise ought to be the proper adjective, although you could probably make a good argument for on-premises if you assert that "premises" in this meaning is a singular collective noun.

A Google Books search for "on-premise inspection" finds a dozen unique matches for that particular phrase, going back to at least 1955. All appear to be from the United States, and many occur in federal government regulations. Unfortunately the dating on such published regulations (which may be revised multiple times) is extremely misleading, and in some cases the matches are shown in snippet view, so there is no way to confirm the actual date of origin of many of the matches.

Here are a few examples from the Google Books search. From The Code of Federal Regulations of the United States of America, volume 7 (revised in 1955):

§989.158 Inspection of raisins on dehydrator's premises—(1) Application and agreement for dehydrator on-premise inspection, (i) Any dehydrator may submit to the committee for approval, and the committee may approve, in accordance with the provisions of this paragraph an application and agreement, on a form furnished by the committee, for dehydrator on-premise inspection of natural condition raisins produced by the dehydrator by subjecting raisins variety grapes to artificial heat.

From The Federal Reporter, Second Series, volume 398 (1969[?]):

72(2). — Acts constituting, and sufficiency in general.

Ct.Cl. 1968. "Plan for the Inspection Job" which continued off-premise inspection as originally instituted, rather than on-premise inspection as called for by supply contracts, did not constitute an agreement modifying the contract specifications to establish a single contractually specified inspection procedure.—Red Circle Corp. v. U. S., 398 F.2d 836.

From U.S. General Accounting Office, "Alcohol and Tobacco Excise Taxes: Laws and Audits Need Modernizing" (1977):

Our review has shown that the on-premise inspection system in the distilled spirits and industrial alcohol industry results in an unnecessary expenditure of resources to insure proper payment of excise taxes. The review has also shown that an inordinate amount of staff-time is expended in beer, wine, and tobacco inspections while only negligible monetary results in terms of tax deficiencies are found.

From Bill Rodgers, "Pesticide Safety - Standard Operating Procedures," in Proceedings of Seminar/Workshop on Tick Eradication Measures (September 3–6, 1985):

2. Spraying Livestock with The Spray-Dip Machine

a. When spraying livestock with the spray-dip machine, the guidelines in VS Memorandum 556.5 should be followed.

b. An on premise inspection should be made by the supervisor, or inspector in charge of the spraying operation, to see that suitable facilities are available to do a safe and effective job.

From a translation of "Anti-Monopoly Law of the People's Republic of China" (August 30, 2007) in Anti-Monopoly Law and Practice in China (2011):

Article 39

When investigating suspected monopoly conduct, the AMEA [Anti-Monopoly Enforcement Authority] can take the following measures:

(1) Conduct on premise inspection of the place of business of the undertakings under investigation or other relevant places,

(2) Question the undertaking under investigation, interested parties, and other relevant organizations and individuals, requiring them to provide relevant information;

These examples suggest that "on-premise/on premise" has been in use in U.S. government publications for at least 60 years. In the past 30 years or so—and especially in the past decade— references to real-estate property and fixtures as "a premise" or "the premise" rather than as "premises" or "the premises" have become much more common, in and outside government texts.

As I note in answer to the related question Is "premises" referring to a single property considered a plural noun? Google Books searches turn up multiple matches for various phrases such as "enter the premise," "search the premise," and "allowed on the premise"—where premise refers to land, a house or other structure, and/or other property—suggesting that increasingly in actual U.S. practice people are flouting the traditional rule that plural premises is the correct form to use in such cases.

As prescriptivists we may regret this development, but as descriptivists we need to take seriously the evidence that the old idea that premise[s]-as-property is always plural no longer reflects reality.

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Update (August 19, 2020): instances of 'on-premise' from the 1930s

A check of the Elephind newspaper database tuns up a spate of matches for "on-premise consumption [of alcohol]"—from Virginia and New York newspapers in 1934 and 1935. This suggests that both Virginia and New York may have enacted post-Prohibition statutes that used the form "on-premise" in place of "on-premises"—which in turn suggests that both states may have drawn on a model code that did the same thing.

The earliest example appears in a brief untitled notice in the [Norton, Virginia] Crawford's Daily (June 6, 1934):

Please take notice that the undersigned, on June 16, 1934, will apply to the Virginia Alcoholic Control Board for retail license to sell beer for on-premise consumption at his place of business at Coeburn, Va., Route 1. —L. C. ROBINETTE

From a similar notice in the Smithfield [Virginia] Times (June 28,1934):

This is to notify the public that on Monday, July 9, I will apply to the Virginia Alcoholic Beverage Board for a retail license to sell beer for on-premise consumption at the store of J. H. Carter, Battery Park, Va. —J. H. CARTER

From "Sunday Outdoor Affairs to Get Beer Delivery," in the [Rockland, New York] Journal News (July 11, 1934):

New rulings on liquor packages also have been received by the [Rockland County Alcoholic Beverage Control] board. Holders of licenses for on-premise consumption of liquor may not have in their places any bottled goods in containers which hold less than one-fifth of a gallon.

From "New Liquor Rules Listed," a followup story in the same newspaper (July 14, 1934):

A ruling to prevent on-premises consumption places from competing with liquor stores provides that the former cannot keep or sell upon the premises any liquor in containers less than what are commonly known as fifths. Distillers or wholesalers can't sell less than fifths to such places.

No off-premises licensee will be permitted to keep or sell upon the premises any liquor in less than the apothecary 12 ounce pint container except that bonded whisky may be sold upon such licensed premises in containers of one half pint of eight ounces. Distillers or wholesalers cannot sell less than 12 ounce pints to such premises unless it is bonded whisky. These rulings pertaining to off-premise and on-premise consumption go into effect tomorrow.

A new ruling permits on-premise consumption places to display in the window or inside the establishment signs advertising particular brands of beverages but these must not exceed 15 by 18 inches in size. Signs cannot be displayed outside the premises and liquor stores cannot display such signs either inside or outside.

On-premise consumption establishments had not previously been permitted to display signs advertising any particular brand of beverage.

In this example, the writer consistently refers to "the premises" (plural) but mixes "off-premises," "off-premise," "on-premises," and "on-premise" as adjectives. There is no consistent pattern in the usage of these compound adjectives.

Additional instances of "on-premise consumption [or sale]" appear in the [Norton, Virginia] Crawford's Weekly of August 17, 1934, and November 9, 1934; in the Rockland [New York] Journal News of March 8, 1935, and July 24, 1935; and in the Bronxville [New York] Review of November 1, 1935. The upshot here is that "on-premise" has appeared in place of "on-premises" in connection with consumption or sale of alcoholic beverages since at least the middle 1930s in at least two parts of the United States.