CEST: what it is, how it works, and why you shouldn't rely on it alone

What CEST actually does

CEST is an automated questionnaire that assesses the key factors HMRC uses to determine employment status for tax purposes. It asks a series of questions about the working arrangement:

• Right of substitution: Can someone else do the work instead of you?
• Control: Who controls how and when the work is done?
• Mutuality of obligation: Are both parties obliged to each other (more on this below)?
• Financial risk: Do you bear financial risk? Do you provide equipment?
• Exclusivity: Are you required to work exclusively for the client?

Based on the answers, CEST returns one of three results:

• Inside IR35: The engagement is likely to be treated as employment for tax purposes.
• Outside IR35: The engagement is likely to be treated as self-employment.
• Undetermined: CEST cannot reach a conclusion based on the inputs provided.

The undetermined outcome — returned in a significant minority of cases — means CEST cannot reach a conclusion and the parties must assess status by other means.

What CEST was updated in 2024

HMRC updated CEST in 2024 in response to evolving case law and feedback. The changes included:

• Rewording questions to better reflect case law intent and court decisions
• Adding new questions to improve the quality of captured information
• Publishing the underlying logic for greater transparency

These were substantive improvements. However, the fundamental architecture of the tool hasn't changed, and the criticisms that have followed CEST since its launch in 2017 have not been fully resolved.

The core problem with CEST

Two issues stand out as genuinely problematic:

1. HMRC only backs results if inputs are 100% accurate

This is the critical limitation. If a contractor or client answers questions based on the written contract rather than actual working practices — or misunderstands what a question is asking — CEST's result offers no protection. HMRC can (and does) disregard a CEST outcome in compliance checks if it believes the inputs were inaccurate.

In practical terms: a CEST "outside IR35" result is only as good as the accuracy of the information entered. If you tick "you have full right of substitution" when the contract says substitution is permitted but the client has never actually allowed it, your CEST result is worthless.

2. Mutuality of obligation is not properly assessed

Mutuality of obligation (MOO) is one of the three key tests courts use to assess employment status. Yet CEST does not include it as a standalone question. HMRC's reasoning: the existence of any contract implies some MOO. Employment law specialists and recent case law — particularly the PGMOL case — have repeatedly questioned this assumption.

This is not a minor quibble. MOO has been central to several tribunal decisions and remains a live area of debate.

When CEST results have been challenged

High-profile cases have exposed CEST's limitations in real-world scenarios. The Gary Lineker employment dispute involved CEST assessments. More significantly, the PGMOL case — which reached the Supreme Court in 2024 — highlighted gaps in CEST's handling of mutuality of obligation and control.

The key takeaway from these cases: courts assess the full factual matrix, not a CEST printout. CEST is a useful starting point, not a defence.

How contractors should use CEST

CEST can be useful — but only if you use it correctly:

• Run it honestly, based on actual working practices — not the contract. Answer every question as it applies in reality, not as it's written down.
• If you get "undetermined," don't ignore it. Seek specialist advice. Undetermined means CEST cannot assess your position — you'll need a deeper analysis.
• Keep a copy of your CEST result with the inputs recorded. If HMRC later questions your status, having a contemporaneous CEST assessment (even if they don't accept it) shows you took reasonable steps.
• Use it alongside a well-drafted contract and a review of actual practices. All three need to align. If your contract says one thing, your CEST answers say another, and your working practices show a third, you have a problem.
• If your CEST result is outside IR35 but your working practices are inside, the CEST result is worthless. The tool only works if the reality matches the inputs.

What CEST can't replace

CEST is one input. It is not a substitute for:

• A properly drafted outside IR35 contract — reviewed by a specialist, with clear clauses on substitution, control, and financial risk.
• Documented working practices that reflect the contract. If your contract says you have substitution rights but you've never used them, and the client has never offered that option, you're vulnerable.
• An accountant who understands contractor tax — one who can flag structural risks, spot misalignments between contract and practice, and maintain the records needed if HMRC ever investigates.

AutoBooks clients operate with all of this in place. CEST is one input among several, not a substitute for proper setup.