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Creative agency founder reviewing IR35 status determination paperwork at desk in London office

Contractors and IR35

What Is the CEST Tool Limitation for Creative Agency Contractor Engagements?

7 min read · ·

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JW

Editorial Lead · Published 16 May 2026 · Updated 17 May 2026

Editorial content from the Agency Founder Finance team. For decisions specific to your agency, book a call.

Key takeaways

  • HMRC's CEST tool assumes binary answers for substitution, but creative contractors are often hired for their personal style, making substitution commercially unworkable.
  • CEST flags creative direction as employment-style control, failing to distinguish between client sign-off and dictating hours, location, or method.
  • The tool misinterprets long-term retainer relationships as mutuality of obligation, ignoring that both parties retain freedom to walk away.
  • HMRC claims 85% CEST accuracy across all industries, but independent research shows higher misclassification rates for creative agency engagements.
  • For creative agency contractors, a full IR35 contract review by a specialist accountant is more reliable than relying on the CEST tool alone.

HMRC's CEST Tool: What It Claims to Do

HMRC's Check Employment Status for Tax (CEST) tool is the government's own online questionnaire for determining whether a contractor falls inside or outside IR35. It asks a series of yes/no questions about how the contractor works. Then it gives you a green light or a red flag.

On paper, it sounds useful. A 12-person creative agency in Bristol Harbourside taking on a freelance brand strategist could run the tool, get an answer, and move on. In practice, the CEST tool limitation for creative agency engagements becomes obvious within the first few questions.

Creative agencies do not operate like traditional employers. Your contractors do not clock in and out. They do not use your equipment. They work on multiple client projects simultaneously, often from their own studios. The CEST tool was not built for that reality.

Why Creative Agency Contractor Engagements Break CEST

The Substitution Question

CEST asks: "Can the contractor send someone else to do the work?"

For a creative agency, this is rarely a clean yes or no. A freelance motion designer might have a trusted junior they could delegate to for a specific task. But the client specifically hired that designer for their style, their portfolio, their reputation. Substitution is theoretically possible but commercially unworkable.

CEST treats a "no" on substitution as a strong indicator of employment. But in creative work, the personal nature of the output does not mean the contractor is an employee. It means the client wants that specific person's creative vision.

This is the first major CEST tool limitation for creative agency work: the tool assumes a binary answer exists where, in reality, the answer is "it depends on the project."

The Control Question

CEST asks whether the client controls how, when, and where the contractor works. For a traditional temp worker, this is straightforward. For a creative contractor, it is almost always a mess.

Take a freelance copywriter working on a three-month brand campaign for a Manchester agency. The agency sets the tone of voice guidelines. They provide the brief. They have final sign-off on the copy. Does that mean they control the contractor?

Not necessarily. The contractor decides their working hours, their writing process, their research methods, and often pushes back on the brief if it does not work. But CEST will look at that sign-off process and flag "control" as present.

The tool does not understand the difference between creative direction (which any client provides) and employment-style control (which involves dictating hours, location, and method).

The Mutuality of Obligation Question

This is the trickiest one for agencies. CEST asks whether the client is obliged to offer work and the contractor is obliged to accept it.

Many creative agencies work on a retainer basis. A freelance developer might do 20 hours per week for the same agency for six months. There is an ongoing relationship. But there is no obligation. The retainer can be paused. The contractor can turn down individual projects within it.

CEST struggles to distinguish between a long-term working relationship and an employment contract. The tool's binary logic sees ongoing work and flags mutuality of obligation. It misses the fact that both parties retain the freedom to walk away at any point.

Real Numbers: How Often Does CEST Get It Wrong?

HMRC's own data is revealing. In 2023, CEST returned "unable to determine" in approximately 20% of cases. For the remaining 80%, HMRC claims 85% accuracy. But that figure is self-reported and applies across all industries, not specifically to creative agencies.

Independent research paints a different picture. A 2022 study by the Association of Independent Professionals and the Self-Employed (IPSE) found that CEST misclassified 1 in 5 contractors who were genuinely self-employed. The tool is particularly bad for knowledge workers, which includes virtually every creative agency contractor.

If you are running a 20-person digital agency in Soho with 8 contractors on your books, the odds are that at least one or two of them are being misclassified by CEST. That is not a risk you want to take. An HMRC investigation can go back six years. The tax bill, interest, and penalties can run into six figures.

What Happens When CEST Gets It Wrong

Let us be specific about the consequences. Say you use CEST to determine that your freelance UX designer is outside IR35. You pay them gross, no PAYE, no employer NICs. Two years later, HMRC investigates. They decide the contractor was actually inside IR35.

The liability falls on you, the agency. You owe:

  • Backdated income tax at the contractor's marginal rate (likely 40% or 45%)
  • Backdated employee NICs at 8% (the post-6 April 2024 main rate)
  • Backdated employer NICs at 15%
  • Interest on the late payments
  • Potential penalties of up to 30% of the total underpayment

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For a contractor paid £80,000 per year over two years, that is roughly £45,000 in combined tax and NICs, plus interest and penalties. That is not a theoretical risk. That is a real liability that can hit your agency's cash flow hard.

The CEST tool limitation for creative agency engagements is not an academic point. It is a direct financial exposure.

What to Use Instead of CEST for Creative Agency Contractors

The safest route is to have a qualified employment lawyer or IR35 specialist review each contractor engagement individually. This costs money upfront, typically £500 to £1,500 per determination depending on complexity. But that cost is trivial compared to the potential liability.

Your lawyer will look at the actual contract and the working practices. They will interview both the agency and the contractor. They will produce a written Status Determination Statement (SDS) that you can rely on. If HMRC later challenges it, you have a paper trail showing you took reasonable care.

Specialist IR35 Insurance

Several insurers now offer IR35 enquiry insurance. It covers the cost of defending an HMRC investigation and, in some cases, the resulting tax bill. Premiums are typically 1-2% of the contractor's annual fee. For a £60,000 contractor, that is £600 to £1,200 per year.

This is not a substitute for getting the determination right. But it is a sensible backstop, particularly for agencies with a high volume of contractor engagements.

Manual SDS Using the Three-Part Test

If you cannot afford legal advice for every contractor, you can do the determination yourself. But do not rely on CEST. Use the three-part test that the courts actually apply:

  • Personal service and substitution: Can the contractor genuinely send a substitute? Is that right written into the contract and used in practice?
  • Control: Does the agency control what, how, when, and where the contractor works? Or does the contractor control their own process?
  • Mutuality of obligation: Is there an ongoing obligation to offer and accept work? Or is each project a separate engagement?

Document your reasoning in writing. Save it alongside the contract. If HMRC ever asks, you can show them you applied the correct legal test, not a simplified online questionnaire.

Working exclusively with agency founders, we see too many agencies treat CEST as definitive. It is not. It is a directional tool at best, and a misleading one for creative work.

When CEST Might Be Acceptable

I want to be fair. CEST is not useless for every scenario. If you have a contractor who works 9-5 at your office, uses your equipment, takes direction on every task, and has no other clients, CEST will probably give the right answer. That contractor is almost certainly inside IR35 anyway.

The problem arises with the typical creative agency contractor: the specialist who works from their own studio, on their own terms, for multiple clients, and delivers a specific creative output. CEST was not designed for that person.

If your agency works primarily with contractors who operate like employees, CEST might be fine. If your agency works with genuine freelancers, specialists, and independent creatives, CEST will regularly misclassify them.

Practical Steps for Your Agency Right Now

Here is what I would do if I were running a creative agency today:

Step one: Audit every contractor engagement you currently have. List the contractor, their role, their working pattern, and how you determined their IR35 status.

Step two: For any determination that relied solely on CEST, re-do it using the three-part test or get a lawyer involved. Do this before year-end, not when HMRC writes to you.

Step three: Update your contractor contracts. Make sure they include genuine substitution clauses, even if you rarely use them. Make sure they specify that the contractor controls their own working methods. Make sure there is no obligation to offer or accept future work.

Step four: Review your contractor onboarding process. Every new contractor should receive a written SDS before they start work. That is the law for medium and large agencies under the off-payroll working rules, and it is best practice for everyone.

Step five: If your contractor mix has changed in the last 12 months, ask your accountant before year-end whether any of your determinations need revisiting. A quick review now is cheaper than an HMRC investigation later.

If you want to talk through your specific contractor engagements, get in touch with our team. We work exclusively with agency founders and we know how creative contractor relationships actually operate.

Frequently asked questions

Can I rely on CEST for all my creative agency contractors?
No. CEST is designed for straightforward employment scenarios, not creative work. It struggles with substitution, control, and mutuality of obligation questions that are common in creative agency engagements. Use it as a starting point, not a final answer. For any contractor who works independently, has their own clients, and controls their own process, get a proper legal determination instead.
What happens if HMRC disagrees with my CEST result?
If HMRC investigates and decides your contractor was inside IR35, the liability falls on you as the agency. You will owe backdated income tax, employee and employer NICs, interest, and potentially penalties up to 30% of the total underpayment. Having a CEST result in your favour does not protect you if the tool gave the wrong answer. A proper legal determination or specialist IR35 insurance is much safer.
How much does a proper IR35 determination cost for a creative agency contractor?
Expect to pay between £500 and £1,500 per contractor engagement, depending on complexity. That covers a qualified employment lawyer or IR35 specialist reviewing the contract, interviewing both parties, and issuing a written Status Determination Statement. Compared to a potential six-figure HMRC bill, it is cheap insurance.
What should I do if CEST gives an "unable to determine" result?
Treat "unable to determine" as a red flag, not a neutral outcome. It means the tool cannot handle your contractor's working arrangement. You must do a manual determination using the three-part legal test: personal service and substitution, control, and mutuality of obligation. Document your reasoning in writing. If the arrangement is complex, get legal advice before the contractor starts work.

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