Hiring a general contractor is by far the easiest route. But then a 12 X 16 building that costs $3500-$5000 to build would likely end up costing closer to $15000. It really just depends upon the individual's confidence and skill level and also how complicated of a design is needed. Another alternative is to simply hire a carpenter or two and have them build the structure while you pay them their day rate which here in S.C. is in the range of $25 to $50 per hour. Any carpenter can build a simpe A roof structure without plans. Most of the decisions that you would have to make are the same ones you would make even if you have a g.c.
One thing of note, is that many sections of the country have very aggrivating permit requirements and excessively intrusive codes. Here in the country I would ignore all of that, but hey this is rural S.C. which is often quite different from working in an incorporated city.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.