IR35 (UK)

IR35 is the headline UK rule that catches contractors who work through a personal company but operate effectively like employees. If a contract is inside IR35, income tax and National Insurance are deducted at source as if the worker were employed. Status is decided by who controls the work, whether the contractor can substitute, and how integrated they are into the client. Most freelance side hustles for UK individuals are clearly outside IR35, but always check current HMRC guidance for medium and large client work.