Does IR35 apply if my client is a small business?

If your end client qualifies as a small company under the Companies Act size thresholds (based on turnover, balance sheet total, and employee numbers), the responsibility for determining your IR35 status stays with you and your own limited company, rather than the client. The underlying IR35 rules themselves still apply, it's only the responsibility for assessing and accounting for status that shifts. Since the small company thresholds can change and client size can vary year to year, it's worth checking your client's status and getting an accountant to confirm which rules apply to your engagement.