If I were going to build a coop, this is what I'd do.
1. How many birds? Multiply that by at least 4 sf. No, 6. Nope! 8 sf, because I probably will get more. In other words, don't scrimp on space.
2. Do I have a building that would meet that criteria, even if it needs some repair? If so, is it in a good spot?
3. If no on #2, what is my absolute budget? Take 10% off that because it's bound to run over, and I like wiggle room. Don't forget to include the run when thinking about cost.
4. If I'm going to build something, is there something I can add on to, or do I have to start from scratch?
5. Think about where the coop will be. Consider the run, how far from the house, drainage, trees (shade or lack of), proximity to fences and neighbors.
6. Read everything I can find on BYC about coops. There are some VERY inventive coops built by BYCers. Not many people can drop $3000 on the coop of their dreams, and with lumber costs INSANE right now, people have found ways to build with recycled and "found" materials.