What is the office establishment of a Commissioner called?

I have seen people using the word "Commissionerate" for the office establishment of a Commissioner.

I have made search on the Free Dictionary and on Oxford Dictionaries. But, I did not find the word there.

Is the word correct? Is so, will it be spelled as "Commissionerate" (i.e. with "e" at the end) or "Commissionerat" (i.e. without "e" at the end)?

I think it is a Indian English term:

From The Hindu: What commissionerate means:

• After much deliberation, the State government accepted in principle that Kochi along with capital city Thiruvananthapuram would have magisterial commissionerates. However instead of bringing clarity to the issue, the decision seems to have stepped up the confusion.

• At present, Kochi with a Deputy Inspector General (DIG) of police as chief comes under the Inspector General of Police, Kochi Range, along with Ernakulam Rural, Alappuzha, Kottayam and Idukki. The Kerala Police Act says that when the commissionerate is constituted, it should have an officer not less than the rank of a DIG.

From Encyclopaedia of North-East India (2007):

• Assam was then a mere commissionerate (Division) of Bengal and Bengali officialdom convinced the authorities that Assamese was but a dialect of Bengali; hence the change which hurt the Assamese pride. Persistent agitation bore not fruit.

'Commissionerate' doesn't appear in the OED Online. The sense it would convey is attested for 'commissionership' instead.

1. The district under the jurisdiction of a commissioner.... Now chiefly hist.
   Recorded earliest in chief commissionership.
   Freq. with reference to the British Empire, esp. British India.

2. The position, office, or term of office of a commissioner (in various senses).

["commissionership, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/37148?rskey=EZ6AnF&result=55&isAdvanced=false (accessed December 19, 2015).]

Examples of this use of 'commissionership' are given dating from 1625 to 2007 (Encycl. N.-E. India).

As for the correctness of 'commissionerate' or 'commissionerat', I'm not sure what 'correct' means.

1. Because there are examples of the word in contemporary use, the word is in that limited way correct.
2. The word will be understood with either spelling.
3. Google and DuckDuckGo searches produce approximately the same number of results for 'commissionership' and 'commissionerate'.
4. Other dictionaries than the OED list 'commissionership', but not 'commissionerate'.
5. The '-ate' suffix is used chiefly to form

a. Substantives denoting office or function, or the persons performing it ....

6. This meaning of '-ate' does not apply so readily to what I take to be your intended meaning as does the meaning of '-ship':

Added to ns. designating an official or person of rank to denote the office, position, dignity, or rank of the person designated ....

7. The '-at' suffix is the original form of the '-ate' suffix with the meaning shown under (5) above. It is less likely than the '-ate' suffix to be considered correct.

All the foregoing aside, I suspect that by "office establishment" in your question, you intended "office building" or the "office suite" (where "suite" is a collection of physical offices within a building or set of buildings). In that case, the usual American English and likely the usual British English word you want is probably

Commission

where "Commission" is an abbreviation of the full title of the commission. For example, this

Commissionerate of Higher Education, Education Department - Government of Gujarat

(From the website of the Commissionerate of Higher Education, Education Department - Government of Gujarat.)

would be this

Commission of Higher Education, Education Department - Government of Gujarat

and this

Office of the Commissioner of Higher Education is responsible for executing policy initiatives, regulating & funding higher education system of the State, in a network of universities, affiliated colleges and research institutions. ...

would be this

Office of the Commission etc.

idiomatically in American English. If "Commission" (or the full title of the commission) is not sufficient to unambiguously refer to the buildings or suite of offices housing the Commission of Higher Education,

office (or offices) of the Commission of Higher Education

would be used idiomatically.

None of the foregoing is intended to suggest that the American or British dialects of English are any more correct than other dialects of English.