IR35 is off-payroll working legislation designed to prevent 'disguised employment'. If a role is 'Inside', it's treated as employment for tax purposes. If 'Outside', it's a true business-to-business relationship.

Inside IR35 usually requires an Umbrella company to handle PAYE, while Outside IR35 is typically managed through your own Limited Company (LTD).

Outside IR35 is significantly more tax-efficient as you can take a low salary and the rest as dividends, while avoiding Employer National Insurance.

Since April 2021, the end-client is usually responsible for determining IR35 status, unless they are a 'small' company.