Incorrect spelling


Incorrect spelling, explanation: build-in is incorrect because the -in part makes it an adjective, and to obtain an adjective meaning from a verb we need its past participle form – in this case, built (hence built-in). Therefore built-in is the correct spelling here.

Correct spelling


Correct spelling, explanation: as an adjective meaning, verbs need to be in part participle form, therefore built-in is the correct form. Built is the past participle form of build. For the same reason, build-in is incorrect. There is such a word as build, but we need a third form of the verb for it to make an adjective, thus built-in is the only option.

Definition of built-in:
adjective, being an integral part of a structure, building, object, etc.
A built-in swimming pool sounds a little luxurious.
-Where is the wardrobe? – Oh, it’s there, it’s a built-in wardrobe.