Is "inbuilt" a word? Is it alright to use it or should I use "built in"?

In the Insular English of Britain (England, Scotland, and Wales), Ireland (both the Republic and the Northern part), and surrounding bits like the Isle of Man and the Channel Islands, the preferred form is in-built, hyphenated like that. That’s the way the OED has it.

Chiefly as ˈin-ˌbuilt ppl. a. = built-in

In the North American English of the United States and Canada, the preferred form runs the other way, making it built-in. The OED says:

built-in: (a) constructed to form an integral part of a larger unit, esp. of the fittings or appurtenances of a house; (b) fig. inherent, integral, innate.

My familiarity with Antipodean English is extremely spotty at best, but I am pretty sure that I have heard some Kiwis use the Insular form and some Ozzies use the North American form. However, I don’t have enough experience there to say whether those were the norms or the exceptions.

O’Reilly, the tech publishing house, uses built-in as a matter of standard practice in its published style guide, and does so whether the word is used as an adjective or as a noun.

The reason they saw need to spell out that the word should be spelt the same way no matter its part of speech is because there is a very common tendency amongst programmers to use the unhyphenated form for the noun use. Most commonly, they wil refer to a programming language’s built-in functions and keywords as that language’s builtins. So one might say that if and while, or perhaps sin and cos, length and delete, as being some hypothetical language’s builtins.

But O’Reilly doesn’t like that, so spells out that one should hyphenate even there.