Probably less than twice as hard as it was for me to build my 4 x 8 coop by myself. (I'm 56 years old, not slim nor athletic, and have never built anything but a wonky dog house out of scrap wood about 30 years ago.)
Hint: Get someone to help you hold up walls while you secure them, and to help you put on the roof.
I built mine in several weekends, after having built an A-frame coop last year. What I did wrong with that first coop, I learned NOT to do with the traditional, rectangular building.
Remember, it's Measure Twice, cut ONCE, not the other way 'round.
..a little bit more seriously, there are a whole lot of great ideas in the Coops section here on BYC. Head over that way and look through 'em to see what you could manage.