If you contract through your own limited company, one HMRC questionnaire can change how much tax you pay on a piece of work. It's called CEST, and the answer it gives sits at the heart of every IR35 decision.
The trouble is, most contractors run it without really understanding what it's testing, who's meant to fill it in, or how much weight the result actually carries. Get any of that wrong and you can end up with a determination that doesn't hold up.
This guide explains what the CEST tool is, how it decides whether you're "inside" or "outside" IR35, what changed in the 2025 update, and where the tool can leave you exposed. It's written for contractors and the businesses that engage them.
What is the CEST tool?
CEST stands for Check Employment Status for Tax. It's a free online questionnaire from HMRC that decides whether a working arrangement looks more like employment or genuine self-employment for tax purposes.
You answer a series of questions about a specific engagement, and the tool returns HMRC's view of the status. It can assess two related things: whether a worker is employed or self-employed, and whether the off-payroll working rules, better known as IR35, apply to a contract.
The tool is anonymous. It doesn't ask for your name or your client's name, and it doesn't report anything to HMRC. You answer the questions, you get a result, and you keep the printout as your record.
It's worth being clear on what IR35 is, because that's the decision CEST is really driving. IR35 is the set of rules that asks a simple question: if you stripped away your limited company and looked at how you actually work day to day, would you look like an employee of your client? If yes, you're "inside" IR35 and should be taxed broadly like one. If no, you're "outside" and taxed as a genuine business.
How does CEST decide your IR35 status?
CEST works through a structured set of questions about the engagement. According to HMRC's guidance, you'll need details about:
- the contract terms
- the worker's responsibilities
- who decides what work needs doing
- who controls how, when and where the work is done
- how the worker is paid
- whether the worker gets corporate benefits or has expenses reimbursed
Behind those questions sit the long-standing employment-status tests that the courts use. The three that matter most are:
- Personal service and substitution. Do you have to do the work yourself, or can you genuinely send a qualified substitute in your place? A real, unrestricted right to substitute points towards self-employment.
- Control. How much say does the client have over what you do, and how, when and where you do it? Heavy control points towards employment.
- Mutuality of obligation. Is the client obliged to offer work and are you obliged to accept it? An ongoing obligation on both sides looks like employment.
CEST returns one of six possible outcomes:
| CEST outcome | What it means |
|---|---|
| Self-employed | Off-payroll rules don't apply to this engagement |
| Employed | The engagement has the hallmarks of employment for tax |
| IR35 rules do not apply | You're outside IR35 for this contract |
| IR35 rules apply | You're inside IR35 for this contract |
| Unable to determine (employment status) | The facts are too finely balanced to decide |
| Unable to determine (IR35) | The facts are too finely balanced to decide |
That last pair matters. CEST can and does come back undetermined, and when it does, it gives you no practical direction at all. HMRC's own usage data, released under Freedom of Information requests, has shown an undetermined rate of around one in five (roughly 19% to 20%) across recent years, so it's far from a rare edge case.
What does inside or outside IR35 actually cost?
The status decision isn't academic. It changes who deducts tax and how much lands in your pocket.
If a contract is outside IR35, your company is paid gross. You then decide how to draw the money, typically a mix of a modest salary and dividends, which is usually more tax-efficient than pure employment income.
If a contract is inside IR35, the fee is treated as employment income. Income Tax and employee National Insurance are deducted before you see it, and an employer National Insurance charge applies on top.
Here's a simplified illustration of why the label matters so much.
Illustrative example: Sam, a contractor weighing one contract
Sam runs a personal service company and takes on a 12-month contract worth £60,000 in fees for the work. Sam wants to understand the rough income-tax shape of an inside-IR35 versus outside-IR35 outcome. These are illustrative figures using 2025/26 rates and bands, ignoring the personal allowance taper, pension contributions, the employment allowance and other reliefs, to show the direction of travel rather than a precise tax bill.
If the contract is inside IR35, the £60,000 is broadly treated as employment income in Sam's hands. Using the 2025/26 thresholds, the first £12,570 is covered by the personal allowance and taxed at 0%. The next £37,700 falls in the basic-rate band at 20%, which is £7,540. The remaining £9,730 (from £50,270 up to £60,000) is taxed at the 40% higher rate, which is £3,892. That's roughly £11,432 of Income Tax, before any National Insurance.
If the contract is outside IR35, Sam can split the income. Say Sam takes a £12,570 salary (covered by the personal allowance) and the rest as dividends. The first £500 of dividends is covered by the dividend allowance at 0%. Dividends are taxed after non-dividend income, so part falls in the basic-rate band at the 8.75% dividend ordinary rate and part in the higher-rate band at the 33.75% dividend upper rate. The headline dividend rates of 8.75% and 33.75% are materially lower than 20% and 40% on the same slices, which is exactly why an outside-IR35 result is usually worth more after tax (and why HMRC scrutinises it).
The point isn't the exact pounds, which depend on your full circumstances, your corporation tax position and how you actually draw the money. The point is that the CEST result can swing your take-home by thousands a year. To model your own position properly, try our dividend tax calculator and take-home pay calculator.
All figures here use the 2025/26 rates: a personal allowance of £12,570, a basic-rate band of £37,700, the 40% higher rate, a dividend allowance of £500, and the 8.75% and 33.75% dividend rates.
Who is supposed to use CEST, you or your client?
This trips up a lot of contractors. Since the off-payroll rules were extended, the answer depends on who you're working for.
- Public sector clients, and medium or large private-sector clients: the client decides your status. They run the assessment (often using CEST), and they must give you a Status Determination Statement (SDS) setting out the result and their reasons.
- Small private-sector clients: the responsibility sits with your own company. You assess the contract and you carry the risk if you get it wrong.
So if you're contracting for a large business, you may receive a CEST-based determination rather than running it yourself. You're entitled to see the reasoning and to challenge it through the client's disagreement process if you think it's wrong.
If you're working through a small client, the determination is on you, which makes understanding CEST, and not just blindly trusting it, far more important. If you're new to all this, our guide to working as a limited-company contractor and our dedicated IR35 contractor support page walk through the wider picture.
Will HMRC stand by the CEST result?
This is the reassuring part, with a catch. HMRC's guidance says:
HMRC will stand by all determinations given by the tool, as long as the information you give remains accurate and in accordance with our guidance.
In plain terms, a CEST result is only as good as the answers you fed it. HMRC's commitment falls away if:
- the information you gave was inaccurate, or
- a contract or working arrangement has changed since you ran it.
That's the heart of the risk. If your answers paint a rosier picture than your real working practices, an "outside IR35" result gives you false comfort. HMRC isn't bound by a determination built on answers that don't match reality, and an enquiry will look at how you actually worked, not just what the tool printed.
There's also a subtler trap: an "unable to determine" outcome isn't a determination at all. It doesn't give you HMRC's backing either way, so you can't lean on it for protection.
What changed in the 2025 CEST update?
HMRC refreshed the CEST tool with effect from 30 April 2025. The headline changes were:
- A new opening question on whether a contract exists, reflecting the PGMOL Supreme Court decision on employment status.
- Clearer guidance on personal service and substitution, sharpening the focus on whether a genuine, unrestricted right of substitution exists.
- Updated guidance on financial risk and simpler wording throughout to make the questions more accessible.
- Publication of the CEST decision matrix, which shows the full set of routes through the tool and how many lead to each outcome.
Two things are widely misunderstood about this update. First, HMRC has confirmed the changes are about clarity and wording, not the underlying logic. If you got a defensible "outside IR35" result before 30 April 2025 and nothing about your engagement has changed, you don't need to re-run the tool just because the interface looks different.
Second, the refresh didn't fix the tool's deeper limitations, which is where the next section comes in.
What are the limits of the CEST tool?
CEST is a useful starting point, but it's widely regarded as too blunt to rely on alone for a genuinely borderline contract. The recurring criticisms are worth knowing:
- It oversimplifies mutuality of obligation. Although the 2025 version asks about a contract existing, employment-status specialists argue the tool still doesn't properly weigh mutuality of obligation the way a tribunal would.
- It can't see the whole picture. Tribunals decide status by standing back and looking at the entire working relationship. CEST answers a fixed set of questions and can't read nuance or context.
- "Unable to determine" is common. When the facts are finely balanced, the very cases where you most need an answer, CEST often gives none.
- It's only as honest as the inputs. A contractor keen on an "outside" result can, consciously or not, answer optimistically. The tool can't check your answers against your day-to-day reality.
- The wrong person often fills it in. It's frequently completed by someone who doesn't know the true contractual terms or how the work is actually carried out.
None of this means you should ignore CEST. It means you should treat the result as one piece of evidence, supported by a contract that reflects reality, working practices that match the contract, and, for anything borderline, a professional review.
How to use CEST properly: a step-by-step
- Read the contract first. Have the written terms in front of you and check they match how the work really happens. If they don't, fix the working practices or the contract before you rely on any result.
- Get the right person to answer. Whoever runs CEST must genuinely know the engagement, the substitution rights, the level of control and how you're paid.
- Read HMRC's guidance as you go. Don't skip the explanatory notes on personal service and financial risk. They change how you should answer.
- Answer honestly, not optimistically. A favourable result built on shaky answers protects nobody.
- Save and date the result. Print or save the PDF with the answers shown. That's your evidence the determination was reached in good faith on a given date.
- Re-run it when things change. A new contract, a changed role or different working practices can flip the status. Re-test rather than assume.
- Get a second opinion on borderline cases. If the result is "unable to determine", or the contract value and length make the risk material, have a specialist review it before you commit.
Frequently asked questions
Is the CEST tool legally binding on HMRC?
Not automatically. HMRC says it will stand by a CEST result, but only while the information you gave is accurate and matches HMRC's guidance. If your answers don't reflect how you really work, or the arrangement changes, HMRC isn't bound by the result.
What does an "unable to determine" CEST result mean?
It means the facts you entered were too finely balanced for the tool to decide. It is not a determination, so it doesn't give you HMRC's backing either way. In that situation you should get a specialist employment-status review rather than guess.
Do I have to re-run CEST after the 2025 update?
No, not just because the tool was updated. HMRC has confirmed the April 2025 changes affect the wording and guidance, not the underlying logic. If you have a defensible result and your engagement hasn't changed, you don't need to re-test.
Who decides my IR35 status, me or my client?
For public sector clients and medium or large private-sector clients, the client decides and must issue a Status Determination Statement. For small private-sector clients, the responsibility sits with your own limited company.
Is CEST enough on its own to prove I'm outside IR35?
It's a strong starting point but rarely enough by itself for a borderline contract. You also need a contract that reflects reality, working practices that match it, and, for anything finely balanced, a professional review.
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Talk to a Zmartly accountant about your IR35 status
A CEST result is only as solid as the contract and working practices behind it. If you're facing a borderline determination, an "unable to determine" outcome, or you simply want certainty before you sign, we can review the engagement and tell you where you really stand.
Book a free 20-minute call with a Zmartly accountant and get your IR35 position checked properly. Explore our tax advisory services or visit our IR35 contractor hub to get started.
