When an inside determination does not have to be the final word

An inside IR35 determination can arrive by letter, by email, or embedded in an agency's paperwork, and it can feel like a fait accompli. The fee-payer starts deducting PAYE, the payments into your limited company drop sharply, and the assumption is that nothing can be done. That assumption is wrong.

Under ITEPA 2003 s.61T, every contractor has a formal statutory right to challenge an inside Status Determination Statement (SDS). The client-led disagreement process requires the client to consider your representations and respond within 45 days. A well-prepared challenge, built on solid evidence and a clear analysis of the status tests, can produce a revised outside SDS or at least a more defensible position if the matter ever reaches HMRC.

This guide covers the practical mechanics: what makes an SDS valid or vulnerable, how to build your evidence file, how to write an effective representation, what the 45-day process involves, and why blanket determinations are a particularly strong attack line. For a full explanation of what an SDS is, how it is issued, and why it matters, see our guide to the Status Determination Statement. For the underlying status tests the challenge turns on, see what IR35 is and how it works.

Step 1: understand what makes an SDS valid (and when it is not)

Before challenging a determination, check whether the SDS meets the legal minimum. An SDS that does not is already invalid, which changes the tactical picture significantly.

Under ITEPA 2003 s.61NA and HMRC's guidance at ESM10013, a valid SDS must do three things:

  • State the client's conclusion (inside or outside IR35).
  • Give the reasons for that conclusion, addressing the relevant employment-status factors.
  • Be issued by the client having taken reasonable care.

The SDS must also be passed to the worker and to the next party in the supply chain (usually the agency). If it is not passed down, it is treated as if it had never been issued, and the liability stays with the client until it is.

An SDS that simply states "inside IR35" with no further analysis is unlikely to satisfy the reasoned-conclusion requirement. A statement that cuts and pastes generic employment indicators without engaging with the specifics of your engagement similarly lacks the reasoning the legislation requires. HMRC's guidance at ESM10014 sets out what reasonable care looks like: the client must genuinely assess the employment-status question for each engagement using the correct legal framework, not apply a blanket policy.

If the SDS you have received is evidently cursory or generic, note that in your representation. An invalid SDS does not just weaken the client's position on the merits; under the off-payroll framework, a failure of reasonable care moves the deemed-employer PAYE liability from the fee-payer back to the client itself.

Step 2: build your evidence file

The disagreement process turns on the underlying employment-status question: would the engagement, as a hypothetical direct arrangement, be one of employment or self-employment? The three tests at the heart of that question are personal service and substitution, control, and mutuality of obligation, assessed on the whole picture of working practices. Your evidence file needs to address each.

The written contract

Obtain the full engagement contract, including any schedules or statements of work. Check what it says about substitution (does it give you an unfettered right to send a substitute at your own cost, with the client unable to unreasonably refuse?), control (does the client direct how, when and where work is done, or does it specify only the outcome?), and ongoing obligation (is there an obligation to offer and accept further work beyond this assignment, or is each project a discrete, standalone piece of work?). Contract wording is a starting point, not a conclusion: HMRC and tribunals look at whether the wording reflects the reality of how the engagement works.

Working-practices records

Working practices consistently outweigh contract wording. Gather any records that demonstrate professional autonomy in practice: correspondence that shows you set your own hours or method of working, records of equipment you supply yourself, evidence of professional indemnity insurance or your own business bank account, correspondence showing you worked across multiple clients simultaneously during the engagement, or records of times you declined work or renegotiated scope without penalty. If you have exercised the substitution right, document that thoroughly. If you have not exercised it but it is genuine and unfettered, document the circumstances that confirm a substitute would be accepted.

CEST tool results

Run HMRC's Check Employment Status for Tax (CEST) tool on the engagement, answering every question accurately and consistently with how the engagement actually operates. If the result is outside IR35, include a printout of the completed CEST with your representation. HMRC has confirmed it will stand behind a CEST result where the inputs are accurate, consistent with the working practices, in line with the guidance, and there is no avoidance intent. That confirmation carries real weight with a client reviewing your challenge. Do not inflate or shade your answers to obtain an outside result; a CEST that does not reflect the real working practices is worthless and will undermine your credibility on everything else. CEST is a useful supporting document, not a substitute for a full assessment.

Previous determinations and correspondence

If the same engagement was previously assessed as outside IR35, gather that SDS and the reasoning behind it. A change of conclusion without a corresponding change in working practices is a strong indicator that the new inside determination lacks sufficient reasoning or was produced without individual assessment. Also gather any correspondence with the client or agency about the scope of work, the way deliverables were specified, or any previous discussions about IR35 status.

Step 3: identify your strongest arguments

With your evidence file assembled, map the arguments. The most powerful challenges usually fall into one or more of the following categories.

Procedural invalidity

If the SDS lacks reasons, was not passed to you properly, or was issued without individual assessment of your engagement, the SDS may be invalid on its face. An invalid SDS means the liability sits with the client under the Chapter 10 framework. Put the procedural point clearly and separately at the top of your representation before engaging with the substantive status question.

Blanket determination as a failure of reasonable care

A client that issues the same inside IR35 conclusion across an entire category of contractors, a whole project team, or all PSC suppliers without assessing each engagement individually is very likely failing the reasonable-care requirement. The off-payroll framework requires the SDS to relate to the specific engagement. A blanket approach cannot, by definition, satisfy that requirement.

If you have reason to believe your inside determination was part of a blanket exercise (perhaps every contractor on your project received the same letter, or the agency told you the client was applying a blanket policy), say so explicitly in your representation. This is the firm's consistent position: blanket determinations are bad practice and legally risky for the client, and they give the contractor a strong basis for challenge independently of the underlying status tests. A client facing that argument has two options: abandon the blanket approach and conduct individual assessments, or risk the liability moving back to it.

Factual misapplication of the status tests

Where the SDS gives reasons but those reasons are factually wrong or misapply the case law, challenge them specifically. For example, if the SDS states the client has control over how you work but your records show you determine your own method and tools, address that directly. If the SDS ignores a genuine and documented substitution right, point to it. Work through the status tests methodically and refute each incorrect finding with your evidence.

Failure to consider the whole picture

The employment-status question is not resolved by any single factor. As the Supreme Court confirmed in Professional Game Match Officials Ltd v HMRC [2024] UKSC 29, even where mutuality of obligation and control are both present, the tribunal must stand back and consider the whole picture: is this, viewed in the round, a contract of employment, and is the contractor in business on their own account? If the SDS focuses exclusively on one or two factors that point to inside IR35 and ignores multiple factors pointing the other way, that selectivity is itself an argument against reasonable care having been taken.

Step 4: write the representation

The representation is a formal business document. It should be structured, factual, and specific. Avoid emotional language and focus on the evidence. A clear, well-organised representation is more persuasive than a lengthy general complaint.

A workable structure is:

  1. Opening statement: identify the SDS by reference, its date, and the engagement it covers. State that you are formally exercising your rights under ITEPA 2003 s.61T to challenge the determination.
  2. Procedural points (if any): set out any invalidity arguments first, clearly separated from the substantive analysis.
  3. Factual background: describe the engagement briefly, covering the scope of work, how you were engaged, and the key working-practice facts relevant to status.
  4. Status test analysis: address each of the three main tests in turn, citing your evidence. For control: what autonomy do you exercise? For substitution: what does the contract provide, and what does the working practice confirm? For mutuality: is there any ongoing obligation beyond the assignment itself?
  5. The whole picture: stand back and explain why, taken as a whole, the engagement looks like an independent professional relationship rather than employment. Factors like multiple clients, financial risk, your own equipment, and the absence of the normal incidents of employment all belong here.
  6. Supporting documents: list the attachments, which should include the contract, any CEST printout, and the working-practices records you have gathered.
  7. Request: conclude by asking the client to either issue a new outside SDS or to confirm in writing the reasons why the inside conclusion is maintained.

Address the representation to the decision-maker at the client, not to the agency. Under the off-payroll framework, the determination is the client's responsibility, and the disagreement process runs between you and the client. Send it with a clear date record (email delivery confirmation or recorded post) so you can demonstrate when the 45-day clock started.

Step 5: understand the 45-day process

Once the client receives your representations, the 45-day clock runs under ITEPA 2003 s.61T. Within that period, the client must:

  • Consider your representations.
  • Either confirm the original SDS, giving its reasons for doing so, or issue a new SDS with a revised conclusion.

If the client confirms the original inside determination, it must explain why. A confirmation that simply repeats the original conclusion without engaging with your representations is itself evidence of a failure to take reasonable care in the review process. A well-evidenced representation puts the client in a difficult position: engaging with it properly requires either accepting the outside case or producing a more detailed and robust inside analysis that is harder for HMRC to fault.

If the client does not respond within 45 days, it is treated as having failed to comply with its off-payroll obligations. The debt-transfer provisions under the off-payroll framework can then be engaged, moving the PAYE exposure to the client rather than the fee-payer. In practice, most medium and large clients will respond within the deadline; they are aware of the liability consequences of not doing so. Keep a date-stamped copy of your representation to start the clock clearly.

What happens if the challenge succeeds

If the client issues a new outside SDS, the fee-payer should stop operating PAYE from the date of the new determination. Payments made before the new SDS was issued will generally not be reprocessed retrospectively. Any tax already deducted in the same tax year can potentially be reclaimed through your self-assessment return, but this is a matter for your accountant to model based on your overall income position for the year. The key practical point is that the new SDS governs future payments, so filing your challenge early in the engagement maximises the benefit of a successful outcome.

What happens if the challenge fails

If the client confirms the inside determination after considering your representations, the PAYE position continues unchanged for this engagement. You can still keep the evidence file because it supports a future challenge if circumstances change, an engagement-extension assessment is required, or you want specialist support in engaging with the client further. It also provides documentary protection if HMRC enquires into the PSC at a later date. The process does not bar you from seeking professional advice, commissioning a specialist contract review, or raising the matter through other channels if you believe the determination is wrong.

The connection between an SDS challenge and HMRC compliance risk

There is a wider compliance picture beyond the disagreement process itself. A successful challenge produces an outside SDS, which is a primary piece of defence documentation if HMRC later opens a compliance check on the client, the fee-payer, or your PSC. An inside determination that you have challenged on the record, even one that was confirmed, creates a paper trail that demonstrates you took the status question seriously and engaged with the evidence properly.

That matters because the off-payroll framework includes a debt-transfer mechanism that can, in certain circumstances, move the liability up or down the supply chain depending on where the failure of compliance occurred. A contractor whose PSC has a well-documented challenge history is in a much stronger position if a later compliance check raises questions about the period of the inside determination.

The off-payroll working rules that sit behind the SDS process are complex, and the interaction between the debt-transfer provisions, the set-off rules from April 2024, and the disagreement process can become technical quickly. Professional support at this stage is worth the cost.

When to get professional help

Some situations benefit from specialist input before you file your representation:

  • The inside determination is based on a CEST result that you believe was answered incorrectly or inconsistently with your working practices.
  • The engagement is a high-value, long-running contract where the difference in net pay between inside and outside is substantial.
  • The client is resistant or the agency is unhelpful in clarifying the basis for the determination.
  • You suspect the inside determination is part of a blanket exercise affecting many contractors and the client is likely to refuse to engage individually without legal pressure.
  • You have received a compliance notice from HMRC in connection with the same engagement, in which case the disagreement process and the compliance investigation may interact in ways that require careful management.

A specialist IR35 status review covers both the working-practices analysis and the contract, producing a written assessment that directly supports your representation. It also gives you a clear view of whether the challenge is strong or marginal before you file it, which avoids the risk of a weakly-argued representation being dismissed and making the client's position harder to shift later.

Key deadlines and mechanics: a summary

Stage Legal basis Deadline / detail
Client issues SDS ITEPA 2003 s.61NA Must include reasons and be passed to worker and next party in chain
Worker files representations ITEPA 2003 s.61T No statutory deadline on the worker; file promptly to maximise the period covered by any revised SDS
Client responds ITEPA 2003 s.61T 45 days from receiving the representations; confirms original or issues new SDS
PAYE obligation ITEPA Chapter 10 Continues unchanged during the disagreement process; stops (for that engagement) only if an outside SDS is issued
Client fails to respond Off-payroll debt-transfer rules Non-compliance; PAYE liability can transfer to the client under the Chapter 10 framework
Invalid SDS (no reasons / no reasonable care) ITEPA 2003 s.61NA; ESM10013 SDS is void; client becomes deemed employer liable for the PAYE/NIC

Practical steps before your next engagement

The disagreement process is easier to run when the evidence already exists. Three habits, adopted from the start of every engagement, make a material difference if a challenge becomes necessary later.

First, document working practices as they happen rather than trying to reconstruct them after an inside SDS arrives. Brief notes or emails that record how you exercised professional autonomy, what equipment you supplied, and how deliverables were agreed are far more persuasive than a retrospective account.

Second, keep a copy of every SDS you receive, together with the reasoning. If the reasoning is thin or absent, raise that with the client and the agency at the point of receipt rather than waiting for a disagreement to arise. A contemporaneous note that the client failed to provide reasons is useful evidence later.

Third, have your engagement contract reviewed by a specialist before you sign. A contract that accurately reflects genuine self-employment working practices gives you a stronger starting point for any future challenge than one drafted on default employment-style terms. See our guide to the IR35 contract review checklist for what to look for.

Challenging an inside determination is a legitimate, statutory process and, where the facts support it, a commercially sensible one. The key is preparation: the contractor who arrives at the disagreement process with an organised evidence file, a clear analysis of the status tests, and a well-structured representation is in a fundamentally different position from one who sends an informal email of complaint. Take the process seriously and it can produce a real change in outcome.

If you would like a specialist assessment of your engagement to support a challenge, or if you want to understand your IR35 position before the next contract is signed, our IR35 status review service is the place to start. We review both the written contract and the working practices to give you a complete picture.