Hundreds of thousands of IR35 status determinations have been made using HMRC’s CEST tool since 2017. CEST is not legally binding. It has known structural weaknesses acknowledged by HMRC itself. Relying on it without understanding those weaknesses has exposed contractors and engagers to successful HMRC challenge.
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What CEST is and why HMRC built it
HMRC’s Check Employment Status for Tax (CEST) tool is a free online questionnaire, available on the GOV.UK website, designed to help workers, engagers and agencies determine whether a specific engagement falls inside or outside IR35. It was launched in 2017 to coincide with the public-sector reform of the off-payroll rules and updated following criticism in 2019 and again in 2023.
HMRC built CEST in response to a practical problem: the employment status tests developed in case law over 60 years are complex, fact-specific and often uncertain in their application. Engagers faced with a large contractor population needed a scalable tool to produce Status Determination Statements (SDS). CEST was intended to fill that gap, providing a straightforward digital interface that distils the legal tests into a series of structured questions.
The tool produces one of three outputs: “inside IR35,” “outside IR35,” or “unable to determine.” HMRC has stated that where answers to CEST are accurate, it will stand behind the result.
CEST’s four question areas
CEST works through a branching questionnaire organised around four broad areas:
- Personal service: whether the worker must perform the work personally or can send a substitute and whether a substitution right has been exercised.
- Control: who decides what work is done, how it is done and when and where it takes place. CEST asks about the presence of supervision and direction by the client.
- Financial risk: whether the worker bears genuine financial risk, for example, by pricing work at a fixed fee, bearing the cost of corrections or providing their own equipment.
- Worker’s circumstances: whether the worker has other clients, whether they are integrated into the client’s organisation (e.g. have a company email address) and whether the engagement looks like employment from a broader perspective.
The tool does not require answering all questions, the branching logic means certain answers lead to an early conclusion. This design means CEST can produce an “outside IR35” result without working through all four areas.
The mutuality of obligation gap
The most significant and widely criticised structural weakness in CEST is its inadequate treatment of mutuality of obligation (MOO). MOO is a threshold test for employment: for a contract of employment to exist, there must be mutual obligations, an obligation on the engager to provide work and an obligation on the worker to accept and perform it.
CEST effectively assumes that MOO is present whenever there is a contract in place and asks no questions designed to test whether the mutual obligations present are those characteristic of employment rather than a series of individual engagements. This approach has been criticised by:
- The House of Lords Economic Affairs Committee, in its 2020 report on IR35 reforms, which noted that CEST “does not consider mutuality of obligation”;
- The Chartered Institute of Taxation (CIOT), which has consistently flagged the MOO gap in its technical submissions to HMRC;
- The ICAEW Tax Faculty; and
- Various First-Tier Tribunal judges, who have noted that CEST outputs do not assist where MOO is in issue.
HMRC has acknowledged the MOO gap in its own technical guidance, stating that CEST takes MOO as given. The consequence is that for engagements where MOO is genuinely absent and therefore where outside-IR35 status should follow as a matter of law, CEST may produce an “inside IR35” result that overestimates the worker’s exposure.
Over-reliance on substitution
A second structural weakness is CEST’s approach to personal service and substitution. The tool asks whether the worker can send a substitute and whether the client would have the right to reject a substitute for reasons beyond the substitute’s skills. For many contractors, the answer to the latter question is “yes”, the client retains an identity-based veto and CEST then moves towards an inside-IR35 conclusion on this branch, regardless of the strength of other outside-IR35 indicators.
Tribunals approach substitution differently. They look at whether the right is genuine and unfettered in practice, what the parties’ reasonable expectations were and whether the right has been used. A contractor who has genuinely worked across multiple projects without supervision, with financial risk and with multiple simultaneous clients may well be outside IR35 even where the client has an identity-based approval right over substitutes, but CEST’s binary logic does not accommodate that nuance.
Legal standing of CEST
CEST is not legislation. It has no statutory basis. HMRC’s commitment to honour CEST results is a policy position, not a legal obligation and it is subject to the condition that the answers given were accurate.
The First-Tier Tribunal (Tax) has made clear in multiple decisions that it does not treat a CEST output as determinative. The tribunal applies the employment status tests as established in case law to the evidence before it. An “outside IR35” CEST result that conflicts with the evidence will be disregarded; an “inside IR35” result does not preclude the taxpayer from arguing outside status before the tribunal on properly marshalled evidence.
The practical implication is that relying on CEST alone, without independent review of the contracts and working practices, creates a false sense of security. For engagements where the factual matrix is borderline or where significant sums are at stake, independent legal review of employment status is the appropriate standard of care.
Cases where CEST outcomes diverged from tribunal findings
A number of IR35 cases have proceeded to tribunal in circumstances where the engager or contractor had a CEST result (or an equivalent determination) that suggested outside-IR35 status. In those cases, the tribunal independently applied the employment status tests and reached a different conclusion, inside IR35, based on the evidence of actual working practices.
The cases of Christa Ackroyd Media Ltd v HMRC [2019], RALC Consulting Ltd v HMRC [2023] and Kickabout Productions Ltd v HMRC [2022] EWCA Civ 29 all illustrate how a tribunal will look behind any documentary tool or label and assess the reality of the relationship. In none of these cases did a favourable status tool output assist the taxpayer. The key findings in each case turned on the factual evidence of how the engagement operated day to day.
These outcomes underscore the risk of treating CEST as the end of the analysis rather than the start of it.
What to do instead
For contractors and engagers who have relied on CEST and now face HMRC scrutiny or who want to ensure their current compliance position is defensible, the appropriate steps are:
- Independent status review: a qualified tax adviser reviews the full factual matrix, contracts, working practices, financial arrangements and any communications that reveal the reality of the relationship and provides a written opinion on status. Unlike CEST, this opinion can engage with MOO, substitution nuance and sector-specific case law.
- Contract review: ensuring the written contract accurately reflects genuinely outside-IR35 working practices. Where the contract and practices diverge, the contract should be updated or the practices addressed.
- Working practice documentation: contemporaneous records (project agreements, invoices to multiple clients, substitution correspondence, evidence of financial risk) are the most persuasive evidence available to a tribunal.
If you are facing an HMRC enquiry that has drawn on CEST outputs, see our IR35 defence strategy guide and our complete IR35 guide. For engager compliance, see PAYE audits and the off-payroll working rules guide.
Frequently asked questions
Is CEST legally binding?
No. CEST is not legally binding on HMRC or on any tribunal or court. HMRC has stated that it will stand behind a CEST result only if the answers given to the questions were accurate and honest. If HMRC subsequently finds that the answers were wrong or that working practices differ from what was described, it can challenge the determination regardless of what CEST produced. The First-Tier Tribunal has consistently refused to treat a CEST output as determinative.
What happens if CEST gives an “unable to determine” result?
An “unable to determine” result means CEST cannot reach a conclusion on the information provided, typically because the answers fall into genuinely borderline territory. In this situation, the engager cannot use CEST to fulfil its SDS obligation and must make its own determination through other means, such as an independent status review or legal advice. Issuing an SDS that simply records “unable to determine” is not a valid status determination under Chapter 10 ITEPA 2003.
Can I rely on a CEST result if HMRC later investigates?
With caution. HMRC will honour a CEST “outside IR35” result only if: (1) the answers given accurately reflected the true facts; (2) those facts remain unchanged during the period under investigation; and (3) the version of CEST used at the time produced the result. If HMRC concludes that the answers were inaccurate or that working practices differed from what was described, it will disregard the CEST output entirely. Engagers should retain contemporaneous records of the answers given so they can demonstrate accuracy if challenged.
Does CEST test mutuality of obligation?
Not adequately. CEST assumes that mutuality of obligation exists whenever there is a contract in place and does not ask the questions needed to test whether mutual obligations characteristic of employment are present. This was acknowledged by HMRC itself in evidence to the House of Lords Economic Affairs Committee. Critics including the CIOT and ICAEW have consistently noted this as CEST’s most significant structural weakness.