Can you wall in the run that is attached to your prefab coop to make the whole thing the coop? Usually this will at least double your inside space. Then build a run attached to it or around it.
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I can see it now. Turn a dumpster into a chicken coop, and tie a yellow ribbon on it!
It might be cheaper, but it wouldn't necessarily be better.
Biggest downside to premade sheds is they don't have good ventilation, nor do they have decent roof overhangs to protect added windows and ventilation.
People get sick of hearing me say "add big(12-18") roof overhangs all the way around with open soffits(covered by HC)"... but it really is one of the best ways to ventilate(in conjunction with gable and/or ridge vents) and protect open windows(top hinged, of course) in summer. I'm not much for paint and cutesy coop decor, but the roof overhangs make a building look better and function better, IMO.
Can you wall in the run that is attached to your prefab coop to make the whole thing the coop? Usually this will at least double your inside space. Then build a run attached to it or around it.