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Being told your engagement is inside IR35 tends to arrive as a shock. The client issues a Status Determination Statement, the agency passes it on, and suddenly a contract you may have been working under for months or years is being treated differently for tax purposes. Before you do anything else, it is worth understanding exactly what has changed and what has not.

What an inside IR35 determination actually means

An inside IR35 determination does not mean HMRC has investigated you. It means your client has concluded, under the off-payroll working rules, that your engagement resembles employment closely enough that your income should be taxed as though you were an employee.

Under the off-payroll rules (Chapter 10 of the Income Tax (Earnings and Pensions) Act 2003), the responsibility for making this determination sits with the end client, provided that client is a medium or large company or a public sector body. If your client qualifies as a small company, the responsibility stays with your own personal service company and no Status Determination Statement is required. For the current definition of small and when it starts to apply, see our guide to the small company threshold changes and their timing.

When your client issues an inside determination, the practical consequence is that the fee-payer in the supply chain (usually the agency, or the client directly if there is no agency) must deduct income tax and National Insurance from your pay before it reaches your limited company. You receive a net payment rather than your gross contract rate. Your limited company can still receive the funds and still exists, but the income from that engagement has already been taxed at source.

What changes in practice

Your payslip. You will receive payments that look more like an employee payslip than a contractor invoice. Income tax, employee National Insurance, and the employer National Insurance and apprenticeship levy are all deducted or applied at source by the fee-payer. Most contractors in this position operate through an umbrella company, which handles the payroll mechanics.

Your take-home pay. The financial impact is real. On a typical day rate, working through an umbrella inside IR35 produces materially less take-home pay than the same rate outside IR35 through a limited company. The gap arises primarily because employer National Insurance (currently 15%) is effectively funded from your assignment rate rather than paid on top of it, and because you lose access to the dividend tax rates available to limited company contractors outside IR35. See our guide to what you actually take home inside versus outside IR35 for real figures across a range of day rates.

Your expenses. Inside IR35, you generally cannot claim business expenses against the income from that engagement in the way a limited company contractor outside IR35 can. The umbrella handles expenses within its own payroll rules, but the range of allowable claims is narrower.

Your limited company. Your company does not need to close. If you have other contracts that remain outside IR35, those still run through the limited company normally. If the inside determination affects all your current engagements, your company may be relatively dormant while you operate through an umbrella, but it remains in good standing. You will still need to file accounts and confirmation statements with Companies House while it is active.

Your pension. This one is often overlooked. Operating through an umbrella inside IR35 does not prevent pension contributions. Salary sacrifice arrangements through a compliant umbrella can meaningfully reduce the tax and National Insurance cost of pension contributions, which in turn closes some of the gap between outside and inside IR35 take-home pay. This is covered in more detail below.

What does not change

Your IR35 status applies engagement by engagement. A determination that this contract is inside IR35 does not determine the status of any other contract you hold or take on. Different engagements can legitimately produce different outcomes.

The determination also does not change your underlying employment status rights. Being taxed as though you were an employee does not make you an employee in the employment law sense. You do not acquire employment rights such as unfair dismissal protection or statutory sick pay as a result of an inside IR35 determination. The tax treatment and the legal employment relationship are separate questions.

Your options when you receive an inside determination

1. Challenge the determination

If you believe the inside determination is wrong, you have a legal right to challenge it through the client's status disagreement process. The process works as follows.

You submit a written challenge to the client, setting out specifically which aspects of the determination you believe are incorrect. A general objection is not sufficient. You need to explain, with evidence, how the working relationship differs from the assessment. Relevant evidence includes: the actual substitution arrangements in practice (not just what the contract says), documented examples of how you control your own work, evidence of financial risk you bear personally, and any other factors that point to genuine self-employment.

The client must respond within 45 days. They can either confirm the original determination or issue a revised one. Crucially, the client is not obliged to change their decision, even if your challenge is well-evidenced. There is no independent body to adjudicate the dispute at this stage, and the original determination continues to apply while the process runs. If the client fails to respond within 45 days, the tax liability may transfer to them, but this does not automatically change your status outcome.

If the client maintains the inside determination after the disagreement process, the tax treatment does not change. Your remaining options at that point are to negotiate different working arrangements, accept the determination and adapt, or walk away from the engagement.

A well-evidenced challenge is most likely to succeed where the original determination did not properly reflect how you actually work. If your working practices genuinely look like employment, a challenge based solely on contract wording is unlikely to be upheld. The leading case here is Autoclenz Ltd v Belcher [2011] UKSC 41, which established that tribunals look at the reality of the working arrangement, not just what the contract says. See our guide on IR35 working practices versus contract terms for more on this principle.

2. Negotiate a rate uplift

If the determination stands, the financial impact is a factor the client created. Some contractors successfully negotiate a higher day rate to offset the additional tax cost of the inside determination, effectively shifting some of the employer National Insurance burden back to the client. This is a commercial negotiation, not a legal right, but it is a legitimate response.

The calculation underpinning the request is straightforward: model your take-home under both scenarios, show the gap, and make the case for a rate that reflects your actual cost of engagement. The IR35 Verdict calculator produces exactly these numbers across all three scenarios, including the salary sacrifice pension case, and gives you a basis for that conversation.

3. Adapt the working arrangements

An inside determination based on genuine working practices is harder to challenge than one based on a poorly evidenced assessment. But it is sometimes possible to adapt the actual working arrangements to more clearly reflect self-employment, and then seek a revised determination.

This means looking honestly at how you work. If your client directs your day-to-day tasks, determines your hours, and you work exclusively for them on their equipment, those are factors pointing toward employment. If you can genuinely introduce more autonomy, project-based deliverables, or a real right of substitution, the factual position may change. Get the changes embedded in a new or amended contract and, where possible, documented in writing from the client before seeking a revised SDS.

This is not about papering over the reality. The Autoclenz principle means the working practices need to actually change, not just the contract wording.

4. Continue inside IR35 and mitigate where possible

If none of the above routes is viable or desirable, operating inside IR35 through a compliant umbrella is the remaining path. The financial impact is real but not always as severe as the headline gap suggests, once mitigation is applied.

The most significant mitigation available is salary sacrifice pension contributions through the umbrella. Because salary sacrifice reduces the income on which both income tax and employee National Insurance are calculated, the effective cost of pension contributions for an inside-IR35 umbrella contractor is lower than the nominal contribution amount. For contractors who would otherwise be contributing to a pension anyway, this is particularly efficient. See our detailed guide on salary sacrifice pension inside IR35 for the mechanics.

The other mitigation is ensuring you use a compliant umbrella. The umbrella market contains non-compliant operators whose arrangements can expose contractors to significant retrospective tax liability. Use an umbrella on the FCSA (Freelancer and Contractor Services Association) or HMRC's own Compliance Framework list, and confirm in writing how employer National Insurance and the apprenticeship levy are handled. Our guide to employer National Insurance on your day rate covers what compliant deductions look like and what to watch for.

Modelling the real financial impact

Before making any decision about how to respond to an inside determination, run the numbers on your specific day rate. Most contractors overestimate the impact from the headline figures and underestimate the effect of pension mitigation.

The IR35 Verdict calculator shows three scenarios side by side: outside IR35 through a limited company, inside IR35 through an umbrella, and inside IR35 through an umbrella with salary sacrifice pension contributions. The third scenario is what most contractors are actually comparing against when they consider the mitigation options available to them.

Run the calculator on your day rate before making any decisions about the engagement.

A note on blanket determinations

A common frustration for contractors is receiving an inside determination that appears identical to ones issued to other contractors in different roles, with the same wording and no engagement-specific reasoning. HMRC's guidance is explicit that blanket determinations, which classify every contractor the same way without examining individual circumstances, do not meet the reasonable care standard required of clients under the off-payroll rules.

If your SDS lacks any reasoning specific to your role and working arrangements, that is grounds for a challenge. Request the evidence and reasoning behind the determination. If the client cannot provide engagement-specific reasons, the assessment may not be valid, and the tax liability could revert to the client.

What to do if your client becomes small

From April 2027 at the earliest, and later for many depending on accounting period timing, some clients that previously issued inside determinations will reclassify as small companies under the updated Companies Act thresholds. If that happens, the off-payroll rules no longer apply to your engagement with that client. Responsibility for your IR35 assessment reverts to your own personal service company under the original IR35 rules.

This is not an automatic pass to outside IR35 treatment. You are now responsible for making a correct assessment of your own status, and that assessment must be defensible if HMRC investigates. The standards applied are the same: Ready Mixed Concrete (South East) Ltd v Minister of Pensions and National Insurance [1968] 2 QB 497 sets the three-part test, Autoclenz Ltd v Belcher [2011] UKSC 41 establishes that reality overrides paperwork, and the recent case law on the third limb (what it means to be genuinely in business on your own account) remains fully relevant. See our full guide to the small company threshold changes for the timing detail.

Frequently asked questions

Can my client issue an inside IR35 determination without explaining why? No. Under the off-payroll rules, a Status Determination Statement must include the reasons for the decision. An SDS that states only the conclusion without any reasoning is not a valid SDS, and the client may retain the tax liability as a result. You have the right to request specific reasoning.

Does an inside IR35 determination affect past years? The determination applies from the date it is issued for the current engagement. It does not retrospectively change the treatment of income you have already received and taxed correctly under outside IR35 rules in previous years or previous engagements. HMRC can investigate past years separately, but a client determination issued today is prospective.

Do I have to use an umbrella company if I'm inside IR35? Not strictly. The fee-payer must deduct and remit PAYE and National Insurance. In practice, this is most commonly handled through an umbrella company, but the mechanics can vary depending on how the supply chain is structured. Speak to your agency or client about how they intend to operate the determination.

Can I work inside IR35 through my own limited company? Your limited company can still receive payments, but the fee-payer must deduct PAYE and National Insurance before paying your company under the off-payroll rules. In most cases, agencies and clients direct inside-IR35 contractors to umbrella arrangements to simplify the payroll mechanics.

If I challenge the determination and lose, is that the end of it? The client's decision is final within the client-led disagreement process. There is no independent appeal route at this stage. If you believe you have been incorrectly taxed following an inside determination, the route thereafter is through Self Assessment and the National Insurance processes, which can be used to seek a refund of tax paid. This is a separate process and is not straightforward. Take specialist advice before pursuing it.

What happens to my limited company during an inside engagement? It continues to exist. You still need to file annual accounts and a confirmation statement with Companies House. If the company is dormant or has minimal activity, the filing obligations are simpler, but they do not disappear. If you return to outside IR35 contracting later, the company is ready to use.