How to Use HMRC's CEST Tool and What the Result Means

What CEST is and what it is not

HMRC's Check Employment Status for Tax (CEST) tool is a questionnaire that works through the key employment-status factors for a specific engagement and produces one of three outputs: inside IR35, outside IR35, or unable to determine. It was updated on 30 April 2025, adding a dedicated mutuality of obligation (MOO) question and an upfront gate that requires users to confirm a contract exists before proceeding. The underlying technical logic and the weighting applied to substitution, control and financial risk were not changed by that update.

Used properly, CEST serves two purposes. It is a first screen: a structured way to identify obvious inside or outside conclusions quickly. And it is an audit document: a record that shows HMRC and, where relevant, a tribunal that the status question was turned to consciously and the answers were documented. Those are genuine and useful functions. What CEST is not is a legal guarantee of status, a substitute for looking at the actual working arrangement, or a complete treatment of the case law.

Understanding what IR35 is and how the tests work is essential context before running the tool. CEST tests the same status factors the courts use, but it applies them through a fixed questionnaire rather than a tribunal's multi-factorial judgment, and there are areas, mutuality of obligation in particular, where the tool's logic does not map cleanly onto the case-law position. Those gaps matter for the quality of the result you get.

The stand-behind promise: what it means and where it stops

HMRC has said it will stand behind a CEST result where four conditions are met: the user answers every question accurately; the answers are consistent with the actual working practices, not just the contract; the tool is used in line with HMRC's guidance; and there is no tax-avoidance arrangement in place. Where those conditions are met, HMRC will not challenge a status determination that is consistent with the CEST result. That is a meaningful assurance and a significant reason to run the tool rather than ignore it.

But the promise has real limits, and it is important to understand them precisely rather than treating the assurance as wider than it is.

First, it does not bind an employment tribunal. A tribunal considering an IR35 dispute applies the case law directly; it is not obliged to reach the same conclusion as CEST. Where HMRC accepts a CEST result but a worker (or, more unusually, a client) challenges status through an employment or tax tribunal, the tribunal will make its own assessment.

Second, the promise applies to how you actually work, not what the contract says. A CEST result that is built on accurate contract language but inaccurate answers about the day-to-day reality of the engagement gives no protection. If HMRC investigates and finds the working practices do not match the tool inputs, HMRC is released from the stand-behind position entirely. The protection is only as good as the honesty of the answers.

Third, where CEST returns an "outside" result but the engagement is later found to be inside on a full assessment, the client or fee-payer under Chapter 10 may face PAYE/NIC liability. The CEST result matters for reasonable care (see below), but it does not prevent an outside-IR35 conclusion from being overturned on the full facts.

Which chapter applies and who should run CEST

Before running the tool it is worth being clear on which legislative chapter applies, because that determines who has the responsibility to determine status in the first place.

Under Chapter 10 of Part 2 ITEPA 2003 (the off-payroll working rules), which applies where the end client is a medium or large company, the end client determines status and must issue a Status Determination Statement (SDS) with reasons. The client, or a specialist running CEST on the client's behalf, is the right person to complete the questionnaire. The contractor running CEST themselves is a useful preliminary step and produces a record they can compare against the client's SDS, but a contractor-completed run is not the SDS and does not satisfy the client's obligation.

Under Chapter 8, which applies where the client is small or overseas with no UK connection, the PSC assesses its own status. CEST is a legitimate self-assessment tool in that setting, and the result supports the PSC's own record.

The small-company exemption matters here. A client is small (and therefore leaves the PSC responsible under Chapter 8) if it meets two or more of: turnover not more than £15m, balance sheet total not more than £7.5m, and not more than 50 employees, under Companies Act 2006 s.382 as applied by ITEPA 2003 s.60A. The thresholds were raised for financial years beginning on or after 6 April 2025, but because of the relevant-financial-year lag and the two-consecutive-years rule, the earliest a previously medium client can drop out of Chapter 10 scope is 6 April 2027. Until a client confirms its status, the safer assumption for 2026/27 is that a medium or large client is still in scope.

Walking through the CEST questionnaire

The CEST questionnaire has several distinct sections. Understanding what each section is testing helps you give accurate answers rather than answers that look favourable on paper but do not reflect reality.

Section 1: Is there a contract?

The April 2025 update added an upfront gate asking whether a contract exists for the work. This needs to be answered honestly. Where no formal written contract has been signed, that itself is a risk indicator worth addressing: a written contract that sets out the terms of the engagement supports the broader status record. CEST will not proceed to the substantive questions without a positive answer to this gate.

Section 2: Why has the worker been hired?

This section asks whether the worker is hired to complete a defined task or project, or whether the engagement is open-ended with rolling work. A project-based, time-limited engagement with a specific deliverable is a stronger outside-IR35 indicator than a rolling arrangement with no defined end point. Answer based on how the engagement actually works. If the contract says "project" but in practice the contractor arrives daily and picks up whatever work needs doing, answer for the reality.

Section 3: Substitution and exclusivity

CEST asks whether the worker must do the work personally, or whether they can send a substitute. A genuine, unfettered right to substitute is a strong outside pointer; a clause that requires client approval or that has never been used and was never intended to be used carries little weight. Answer based on whether a substitute has actually been sent, or could realistically be sent without client refusal, not on whether the contract contains a substitution clause. The case law is consistent on this: working practices over contract wording.

Section 4: Control

Control is tested in two dimensions: what work is done and how it is done. CEST asks whether the client sets the hours and location, whether it decides how the work is performed, and whether the worker can work for other clients simultaneously. A contractor who works specified hours at the client's premises, follows the client's methods and cannot take on other work without permission is in a strongly employment-like position on control. A contractor who agrees what to deliver, decides when and how to do the work, and takes on multiple clients simultaneously looks much more like a business operating on its own account.

Answer each question for the actual engagement. If the contract gives flexibility on hours but in practice the contractor always works 9 to 5 at the client's office, the working practice is what matters.

Section 5: Financial risk

CEST asks whether the worker bears any financial risk: fixing a price and absorbing cost overruns, providing equipment at their own cost, correcting defective work at their own expense, or facing financial loss if the contract is terminated early. These are indicators of business operation rather than employment. Many contractors bear minimal financial risk (they bill time, they do not fix a project cost, the client provides all equipment), and those answers are appropriate where they reflect the reality, even if they point in an employment direction.

Section 6: Mutuality of obligation (April 2025 addition)

The dedicated MOO question added in April 2025 asks about the obligations on both sides during and between assignments. This section covers whether the client is obliged to offer work and whether the worker is obliged to accept it. Understanding what this question is actually testing, and where the tool's treatment of it differs from the case law, is important enough to warrant its own section below.

Mutuality of obligation: where CEST and the case law diverge

Mutuality of obligation has been a contested area in IR35 status cases for many years, and the Supreme Court's decision in Professional Game Match Officials Ltd v HMRC [2024] UKSC 29 (16 September 2024) clarified the position in a way that has direct implications for how CEST results should be read.

The Supreme Court confirmed in PGMOL that MOO can exist within a single engagement once work is accepted: once a match official accepted a specific appointment, there were obligations on both sides for that engagement. The absence of an umbrella obligation to offer or accept further work between assignments does not by itself take a contractor outside IR35. MOO is a necessary precondition (you cannot have employment without some mutual obligation), but it is not sufficient: even where MOO exists, the tribunal must still assess whether, on the whole picture, the contract is one of employment.

CEST's treatment of MOO is narrower than that case-law position. The tool's question focuses primarily on whether there is an ongoing obligation to offer and accept work between assignments. Where a contractor correctly answers that the client is not obliged to offer further work after one assignment ends, the tool may weight this as an outside indicator. But PGMOL makes clear that the absence of a between-assignments obligation is not dispositive: MOO within the individual engagement is what the courts are looking at, and that typically exists whenever work is commissioned and performed.

In practical terms: a CEST "outside" result driven partly by MOO answers should not be read as a reliable outside-IR35 conclusion on that ground. A tribunal would assess MOO differently, and correctly, under PGMOL. The broader substitution, control and financial-risk picture is what carries the weight.

Reading the three possible results

"Outside IR35"

An outside result means that, on the answers given, the tool concludes the engagement looks more like self-employment than employment. This is positive evidence. HMRC will stand behind it where the four conditions above are met. It is supporting evidence, not a guarantee. An outside CEST result backed by consistent contract terms, consistent working practices and a contemporaneous documentary record is a much stronger position than a CEST result that was run once and then filed away while the working practices drifted in an employment direction.

For this result to be meaningful on MOO, note the caveat above: the outside conclusion may partly rest on MOO answers that a tribunal would not accept as dispositive under PGMOL. The substitution and control sections carry the real evidential weight.

"Inside IR35"

An inside result means the tool concludes the engagement looks more like employment on the facts entered. Under Chapter 10, this informs the client's SDS and the fee-payer will operate PAYE and NIC on the payments to the PSC. Under Chapter 8, the PSC will need to operate the deemed employment payment calculation at year-end.

An inside CEST result is worth reviewing carefully. If the answers reflect the actual working practices and the result is inside, the engagement almost certainly is inside and the tax consequences follow. If the result is inside because the answers entered do not fully reflect the flexibility and autonomy that the contractor actually has, it is worth revisiting the answers before treating the result as final. But never change answers to get a preferred result: that is precisely the working-practice gap that destroys the stand-behind protection and exposes both the contractor and the client.

"Unable to determine"

An unable-to-determine result means the tool cannot give a clear conclusion: the factors are genuinely mixed and the questionnaire cannot resolve them. This is not a neutral or mildly positive result. It means the engagement is genuinely borderline and should be treated with particular care. Under Chapter 10, the end client still needs to issue a valid SDS, and an unable-to-determine CEST result does not satisfy that obligation. Specialist professional assessment is most important in exactly these cases.

Building a documentation record that actually protects

The CEST result is only one layer of a proper IR35 audit trail. The documentation record that actually provides protection covers four components.

The CEST printout

Save a PDF or screenshot of the completed CEST questionnaire, including every answer and the final result. Label it clearly with the engagement name, the contract start date, and the date the tool was run. If the contract is renewed or the working practices change materially, run it again and save a new record. A single CEST run at the start of a two-year engagement gives progressively weaker protection as the engagement evolves.

The written contract

The contract should reflect the actual terms under which the work operates. Key provisions to review: whether the right to substitute is genuine (not fettered by client approval that would in practice always be refused); whether the control terms match how the work actually happens; and whether the engagement is project-specific rather than open-ended. A contract that says one thing while the reality is another is worse than a contract that is honest about the terms and then supported by evidence that those terms are real.

Working-practices evidence

This is the layer most contractors underinvest in, and it is the one that matters most in an investigation. Relevant evidence includes: correspondence showing the client does not direct how the work is done; project deliverables and invoices (rather than timesheets that look like payslips); records of substitute use or formal offers of substitution; evidence of concurrent clients; documentation that the contractor provided their own equipment; and records of having corrected defective work at their own cost.

None of this needs to be elaborate. An email trail showing a contractor disagreed with the client's suggested approach and proceeded differently, or an invoice for a specific project deliverable rather than a daily-rate timesheet, is practical evidence. The test is whether the documentary record, taken as a whole, supports the CEST answers.

Professional review

For a new outside-IR35 engagement, particularly with a medium or large client under Chapter 10, a professional contract and working-practices review is the most important step after the initial CEST run. CEST tests structure; a specialist review tests substance. The two together give the most robust position. If you are assessing a contract before signing, the review should happen at that stage, not after the engagement has been running for six months.

Our IR35 status review service covers exactly this: reviewing the written terms and the proposed working practices together, not just validating a CEST result.

Common mistakes that undermine a CEST result

A CEST result can be undermined in several ways, most of which come down to the gap between the tool inputs and the working reality.

Completing CEST from the contract rather than the working practices. A contract may say the contractor controls their own hours, but if in practice they are expected to be on-site from 8.30 to 5.30 Monday to Friday, the contract wording is not the reality. Entering the contract terms rather than the actual working arrangement produces a result that provides no protection if HMRC investigates.

Running CEST once and not revisiting it. Working practices evolve over long engagements. A contractor who ran CEST at month one of a two-year engagement and has not revisited it has a record that may not reflect the current position. For engagements that extend, re-run and re-document.

Treating an "outside" result as an end point. The result is a first screen. It should prompt the next steps (contract review, working-practices documentation, professional advice on borderline points) rather than close the question. A CEST result filed and forgotten is not an audit trail.

Anchoring on the MOO answer. As noted above, the April 2025 MOO question does not fully reflect the PGMOL position. An outside result that rests substantially on the MOO answer (for example, because the contractor correctly answered that the client is not obliged to offer further work between assignments) may not hold up if the engagement is assessed on the full case-law position. The substitution and control sections are the more reliable indicators of the tool's accuracy.

CEST in the context of a Chapter 10 SDS and the disagreement process

Where the client is medium or large, the formal obligation is not to run CEST but to issue a Status Determination Statement with the reasons for the conclusion and to pass it to both the worker and the next party in the chain. CEST is a widely used tool for informing that conclusion, but using CEST is not the same as issuing a valid SDS.

A valid SDS under ITEPA 2003 s.61NA must set out the conclusion (inside or outside) and the reasons for it, taken with reasonable care. Using CEST honestly and thoroughly is evidence of reasonable care. Pasting the CEST output into an SDS without articulating why the answers are correct for this specific engagement falls short of what a reasoned SDS requires.

If the client issues an inside-IR35 SDS (whether CEST-informed or otherwise) and the contractor disagrees, the client-led disagreement process under ITEPA 2003 s.61T allows the contractor to raise a written representation. The client must respond within 45 days, either confirming the original determination with reasons or issuing a revised SDS. The process is client-led (the client makes the final call), but the contractor is entitled to individual consideration. A blanket determination applied across a category of contractors without individual assessment is very likely to be a failure of reasonable care, which can invalidate the SDS and leave the liability with the client.

Where CEST fits in a complete IR35 approach

CEST tests the same status factors the courts use, but applies them through a fixed questionnaire rather than a tribunal's multi-factorial judgment. The foundational case law from Ready Mixed Concrete (South East) Ltd v Minister of Pensions [1968] 2 QB 497, confirmed and developed through Atholl House [2022] EWCA Civ 501 and PGMOL, requires a whole-picture assessment that a questionnaire cannot fully replicate. CEST approximates it; it does not replace it. The status tests in full are covered in our guide to what IR35 is.

Those assessed as outside who want to protect that position through their working practices will find our guide on protecting an outside-IR35 status sets out what actually makes the difference in practice. Where a client has issued an inside determination and the contractor wants to understand the options, the guides on being inside IR35 and on challenging an inside determination cover the practical steps.

The status tests are the same across sectors, but the facts that drive CEST answers vary by type of work. IT contractors delivering a defined software module tend to have strong substitution and financial-risk answers, but where the client dictates the technology stack and daily standup attendance, those are employment-pointing control answers regardless of the contract's autonomy language. Engineering contractors on client sites face the location-and-hours dimension of control more directly. Whatever the sector, the principle is the same: answer for the reality of the engagement. The specialist accountants for IT contractors and engineering contractors can help structure the CEST run and the supporting documentation.

After CEST: the next steps

A CEST outside result is the beginning of a defensible position, not the end. Review the contract against the CEST answers: are the substitution, control and financial-risk terms consistent with what was entered? If the contract is silent on substitution or includes fettered approval clauses, address that before the engagement starts. Build the working-practices documentation record from day one rather than trying to reconstruct it if an investigation arises later. For an engagement with a medium or large client, ensure the client's SDS reflects an outside conclusion with reasons and that you hold a copy.

If CEST returns an unable-to-determine result, or the engagement has features (heavy client control, no genuine substitution, long continuous tenure) that point toward employment, seek specialist advice before proceeding rather than hoping the tool reaches a different conclusion on a second run.

For a review of your contract and working practices, our contractor accountancy services include IR35 status assessments that go beyond what any tool can offer.