If you are in the foothills or mountains you need to build a bear proof coop. If you're in the flats, your biggest pests are dogs, coyote, hawks/eagles/owls and maybe raccoon.
12x12 is large and requires better building skills and engineering than a 8x8 or 8x10 coop. Both will do the job for 15-20 hens. On a 12x12, you will be buying 16' rafters which will add up quickly. You can find tons of free or cheap building supplies on CL in the FR. If you have the time, you might come across a shed that would be a simple conversion saving time and money. I have about 300' of free chain link fencing I picked up years ago.