For general information on building in Aspen, check out this site. It’s a great idea to familiarize yourself with the currently adopted building and energy codes. You’ll also want to read up on planning and zoning. The City of Aspen has a municipal code and Title 8 has to do with buildings. Read about it here.
Aspen is in Pitkin County so be sure and check out the resources on that page!
Meanwhile, read this interesting article about Transferable Development Rights in the Aspen Times.