I am referring to the style of home created when two single trailers are bolted together and lived in as a single unit.
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2You can find answers to such questions by simply looking in a dictionary. If you still can't find an answer, add a note stating what you looked up etc. – coleopterist Aug 02 '12 at 11:17
3 Answers
Both double-wide and doublewide are acceptable. However, it is, IMHO, better to stick to the hyphenated option.
"Double wide" is incorrect. The rules/guidelines of hyphenation are explained in this excellent answer.
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It can't be a double wide trailer because modifiers that come before nouns must be single words (see John Lawler on the "eleven-year-old boy rule").
Google Ngram Viewer suggests that double-wide trailer is the most common spelling, but doublewide trailer is also in use.

Compound terms often start out hyphenated, but as people become familiar and start to treat the term as a unit, the hyphen disappears. For example, ball-point pen, initially the more common spelling, was overtaken by ballpoint pen around 1975:

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British English doesn't generally use double with an adjective in this way, it's normally a noun (or a past participle like double-fronted).
Double-width would be correct. Double is always† hyphenated when compounded like this, and *doublewidth is wrong.
† Except in neologisms like doublethink.
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@GarethRees That's *exactly* why I prefaced my answer with "British English"; from your link: "A double wide is a very common phrase in American English though, apparently, not in UK English." – Andrew Leach Aug 02 '12 at 11:54