As the name suggests, the scheme only applies to the construction industry including many trades such as plumbing and heating engineers, electricians, and decorators. It does not apply to work on construction sites which is clearly not construction, such as cleaning.
Contractors must register for CIS before work begins. Whilst subcontractors do not need to register, tax will be taken at a higher rate if you are not registered so it is advised to do so.
Self-employed tradesmen who only work directly with their clients and do not provide subcontracted work for contractors nor subcontract work to others, do not need to register under the scheme.