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I am pleased to discover, in Lapsing Into a Comma by Bill Walsh, corroboration for my belief that it is not necessary to compound the noun version of a compound adjective. He bases it in part on pronunciation: in the sentence "I'm having a backyard barbecue in my back yard," it's easy to hear (if it's spoken aloud) the difference in emphasis between adjective and noun. The emphasis is on the first syllable of the compound "backyard," but equal on the two words of the noun "back yard." The same is true of backseat and back seat, makeover and make over, start-up and start up, etc.

I have permitted copy editors to mush together the two words of my nouns on occasion: the one I remember most clearly was when I spoke of someone in the back seat (which the c.e. changed to backseat) of a car; that was the first time I'd ever seen anyone compound the noun, and as a quick glance at my then-new dictionary seemed to establish that it was now correct, I let it stand.

But although it's now had even more years to settle into even more dictionaries, I'd argue the point these days. Complaisance can be taken too far. Yes, it's a backseat driver. But he's sitting in the back seat.