You might look through this. It's in the Articles section above. Many of them have decent plans.
https://www.backyardchickens.com/articles/categories/large-coops.20/
As mentioned, if you can get plans for a shed you can modify that into a coop. You can probably find detailed plans online but you might have to pay for them. That might be plans or a book. I'd think it is worth a look. Home Depot and Lowe's often have books on building sheds available, not sure if you can find the size you want. If you can manage it with things as they are, your librarian can often find a book you could check out that would work really well. You might call and see what you can do there.
Instead of 10 x 12 I'd consider an 8 x 16. Most building materials come in standard 4' or 8' dimensions, if you can base your plans on that you can avoid a lot of cutting and waste. That size it has to be a walk-in, make sure it is tall enough for you to stand up in it. I think an 8x16 is more usable anyway than a more square building.
I personally like a single sloped roof but in Colorado (thanks for providing that bit of info) you might want a steeper gable roof for snow. You can still do it with a single sloped roof, but a gable roof will give you nice overhang on both sides so you can put required ventilation at the top of your side walls.
To convert a shed into a coop, you need roosts, nests, a pop door, and ventilation. None of those are really that hard to do but there may be little tricks depending on your materials. If you need help on specifics for those just ask.