IR35 and the CEST Tool: How HMRC's Test Works
HMRC's Check Employment Status for Tax (CEST) tool is the government's official online tool for determining whether a worker falls inside or outside IR35. It is free to use, and HMRC states that it will stand behind the result — provided the information entered is accurate and the tool is used correctly. But CEST has been controversial since its launch, with contractors, tax experts, and even judges questioning its accuracy. In this guide, I will explain exactly how CEST works, where its weaknesses lie, and how to use it effectively.
What Is the CEST Tool?
CEST is a free online questionnaire available on the HMRC website. It asks a series of questions about a working engagement and, based on the answers, provides one of three outcomes: inside IR35, outside IR35, or undetermined. The tool was first launched in 2017 alongside the public sector off-payroll reforms and has been updated several times since.
CEST is designed to be used by end clients, agencies, and contractors to assess IR35 status. HMRC encourages its use, particularly for medium and large private sector clients who are responsible for making status determinations under the off-payroll working rules.
According to HMRC's CEST guidance, the tool is intended to give "an indication" of employment status for tax purposes.
How CEST Works
The tool asks questions in several categories, loosely corresponding to the established employment status tests:
Worker's duties — What the worker does, whether they have to do the work personally, and whether they can send a substitute.
Working arrangements — How much control the client has over the work, including methods, hours, and location.
Financial risk — Whether the worker risks their own money, provides equipment, or could make a loss on the engagement.
Worker's involvement — Whether the worker is treated as part of the client's organisation or operates independently.
Based on the combination of answers, CEST applies HMRC's interpretation of the law and produces a determination. If the determination is "outside IR35" and HMRC later disagrees, they have stated they will not pursue the client or worker — provided the information entered was accurate.
The Problems with CEST
Despite HMRC's support for CEST, the tool has significant limitations that contractors and clients should understand.
It Does Not Assess Mutuality of Obligation
One of the most fundamental employment status tests is mutuality of obligation (MOO). CEST does not properly assess this factor. The employment tribunals consistently consider MOO as part of the overall picture, but CEST effectively assumes it exists and moves on to other factors.
This omission has been criticised by tax professionals and was raised in several tribunal cases. The absence of MOO assessment means CEST may determine a worker is inside IR35 when a full analysis, including MOO, might reach a different conclusion.
High "Undetermined" Rate
In a significant number of cases, CEST returns an "undetermined" result, meaning it cannot decide whether the engagement is inside or outside IR35. This is unhelpful for clients and contractors who need a clear answer. An undetermined result forces them to seek additional advice or make their own judgement, which is precisely what CEST was supposed to avoid.
Binary Questions for Nuanced Situations
Employment status determination involves weighing multiple factors, each of which can exist on a spectrum. CEST reduces these nuances to yes/no questions, which can produce misleading results when the reality is more complex. For example, a question about whether you can work from any location may have the answer "mostly, but I need to attend the client site twice a month" — CEST does not accommodate this kind of nuance.
Questionable Accuracy
Several tax tribunal cases have produced results that contradicted CEST determinations. In the Atholl House Productions case, for example, the tribunal's analysis differed from what CEST would have indicated. While CEST has been updated since, the fundamental issue — that a questionnaire cannot fully replicate judicial analysis — remains.
How to Use CEST Effectively
Despite its limitations, CEST remains a useful starting point. Here is how to get the most from it:
Answer honestly and accurately. The most common mistake is answering questions based on what the contract says rather than what actually happens. CEST should reflect reality, not aspiration. If your contract says you can substitute but in practice this would never happen, answer based on the practice.
Complete it for each engagement. IR35 status is assessed per engagement, not per contractor. A new client, new contract, or significant change in working practices requires a fresh assessment.
Use it alongside other methods. CEST should not be your only assessment tool. Combine it with a review of your contract, an analysis of your working practices, and — for significant engagements — professional advice.
Save the result. CEST does not save your answers or results automatically. Print or save the result page as evidence of your assessment. If HMRC queries your status, having the CEST result demonstrates you took reasonable care in your determination.
If the result is undetermined, do not ignore it. An undetermined result means the tool cannot decide, which typically indicates a borderline case. Seek professional advice rather than guessing.
CEST vs Professional IR35 Assessment
CEST is free and quick. A professional IR35 assessment typically costs £200-500 and takes longer. Are the extra cost and time worth it?
For straightforward engagements where the answer is clearly inside or outside, CEST is usually sufficient. If you are a contractor working on a clearly defined project, with full control over your methods, a genuine substitution clause, and no integration into the client's organisation, CEST will likely confirm you are outside IR35. Similarly, if you are essentially covering a permanent role with full client control, CEST will indicate inside IR35.
For borderline cases — which many engagements are — a professional assessment provides several advantages:
It considers mutuality of obligation, which CEST does not properly address. It analyses the specific wording of your contract. It examines the nuances of your working practices. And it provides a written opinion that can serve as evidence in an HMRC investigation.
Many professional assessments also include investigation insurance, covering the costs of defending an IR35 challenge. Given that an HMRC investigation can result in backdated tax of tens of thousands of pounds, the £200-500 for a professional review is a modest investment.
For a broader understanding of how to assess your status, see my step-by-step guide to determining IR35 status. And for the key tribunal decisions that shape how IR35 is interpreted, read my article on IR35 case law.
HMRC's Commitment to CEST Results
HMRC has stated that it will stand behind CEST results, meaning they will not pursue a contractor or client who received an "outside IR35" determination from CEST — provided the information entered was accurate and complete.
This commitment is significant but comes with an important caveat: if HMRC believes the information was inaccurate, the commitment does not apply. In practice, this means that if HMRC investigates and discovers that your actual working practices differ from what you entered into CEST, the tool's result will not protect you.
This is why accuracy matters more than the result itself. An honest CEST assessment that returns "inside IR35" is far better than a manipulated one that returns "outside" but would not withstand scrutiny.
Our Recommendation
Use CEST as part of your IR35 assessment process, not as the entirety of it. It provides a useful framework and HMRC's backing of its results offers some protection. But do not rely on it blindly, particularly for engagements where the status is genuinely borderline.
Accounted's IR35 assessment tool provides an alternative assessment framework that addresses some of CEST's gaps, including a more thorough analysis of working practices.
As Contractor Calculator notes, the CEST tool should be used as guidance rather than a definitive answer.
For help managing your contracting finances beyond IR35, Accounted handles bookkeeping, tax calculations, and self-assessment filing. Visit our features page to see how I can simplify your financial admin.
Useful Resources
Penny, your AI bookkeeper, tracks your tax position in real time and flags opportunities to reduce your bill. Meet Penny →
Tax & Compliance Specialists
Our tax specialists have decades of combined experience in UK sole trader and small business taxation, MTD compliance, and HMRC submissions. All content is reviewed against current HMRC guidance before publication and updated quarterly to reflect legislative changes.
Ready to try Accounted?
Join UK sole traders who are simplifying their bookkeeping and tax.
Start your 14-day free trial