The day rate is not the cost
Where an engagement falls inside the employment or off-payroll rules the practice carries employer National Insurance on top of the rate — 15% above a £5,000 secondary threshold. That total is the figure to compare against an employed optometrist, and it is the comparison most practices never actually make: a day rate against a salary is not like for like.
And status is established, not assumed
Control over how and when the work is done, a genuine right of substitution, who carries financial risk, and mutuality of obligation beyond the individual booking. HMRC looks at how comparable people were treated across the practice rather than at one engagement alone, so consistency matters as much as the individual answer.
There is no sector safe harbour, and the nearest one was withdrawn
This is worth knowing before someone tells you otherwise. HMRC's Employment Status Manual contains no page on locum optometrists or opticians at all. The closest analogue was ESM4270 on locum pharmacists, and that was withdrawn with effect from 30 June 2023, redirecting to the general principles at ESM0500 and the CEST tool.
So there is no concession and no sector-specific position to rely on. Status is determined on general principles, on the facts of each engagement. Anyone telling you HMRC accepts locum optometrists as self-employed is citing guidance that no longer exists.
Why most practices are outside the off-payroll rules anyway
The small-client exemption. Where a worker provides services to a small client outside the public sector, the worker's own intermediary decides status and applies the rules — not you. For medium and large private-sector clients it flips: the client determines status and the fee-payer operates PAYE, employee National Insurance and employer National Insurance.
Because most independent practices are small companies, the locum's personal service company remains responsible under the original Chapter 8 rules, and the practice has no status-determination obligation and no PAYE exposure. A practice inside a large group is a different question and needs testing on its own facts.
The size test itself moved on 6 April 2025: turnover above £15 million and balance sheet total above £7.5 million, with the 50-employee limit unchanged, and an entity is medium or large only if it meets the criteria for two consecutive financial years. Because of how the transitional provision works, the earliest tax year a client's size can change under the new thresholds is 2027/28. Some medium-sized groups will drop out of the regime, but not before then.
If the locum trades through a company
Two things changed the arithmetic for 2026/27. Dividend rates rose on 6 April 2026 to 10.75% ordinary and 35.75% upper, which makes the salary-versus-dividend calculation materially different from last year and is the single most useful conversation to have with a locum client this year. And a company whose only employee is also its sole director cannot claim the £10,500 Employment Allowance, so the National Insurance saving people assume is there frequently is not.