How do I know if I'm a CIS contractor, a subcontractor, or both?

You're a contractor under CIS if you pay other businesses or individuals for construction work, and a subcontractor if you're the one being paid for construction work by another contractor — many building businesses are both at different times. Work done directly for a private homeowner who isn't themselves a contractor generally falls outside CIS, but work you do for another construction business or a contractor further up the chain usually does fall within it. Because the rules depend on exactly who's paying whom and what the work involves, it's worth checking your specific arrangements with an accountant.