IR35 inside vs outside: the contractor's definitive guide for 2026

IR35 — officially the off-payroll working rules — determines whether a contractor working through a personal service company should be taxed as an employee. If HMRC decides that a contractor is effectively an employee of their end client (but for the presence of an intermediary company), it applies employment taxes to their income. The financial difference between being inside and outside IR35 is significant: a typical contractor earning £100,000 per year can pay £15,000 to £25,000 more in tax and National Insurance if they are incorrectly classified as outside IR35.

Since April 2021, the responsibility for determining IR35 status has rested with the client organisation for medium and large businesses. Under the off-payroll working rules, the client issues a Status Determination Statement (SDS) declaring whether an engagement is inside or outside IR35. If inside, the fee-payer (usually the client or agency) deducts PAYE and National Insurance before paying the contractor's company. If outside, the contractor's company receives the gross fee and is responsible for its own tax affairs.

Three factors dominate IR35 status determinations, drawing on decades of case law. The first is substitution: can the contractor send a substitute to do the work, and would the client accept that substitute without involvement from the named contractor? A genuine right of substitution — not just a theoretical one buried in a contract — strongly points towards self-employment and outside IR35. If the client insists on the specific individual, the substitution argument is weakened.

The second factor is control. Does the client direct how, when, and where the work is done — or does the contractor have genuine autonomy over method and schedule? An employee is typically subject to the employer's direction. A genuinely self-employed contractor delivers a defined output and has discretion over how they achieve it. Contractors who are embedded in a client's management structure, attend mandatory all-hands meetings, and work fixed client hours look very much like employees to HMRC.

The third factor is mutuality of obligation. Is the client obliged to offer work, and is the contractor obliged to accept it? Employment involves this reciprocal obligation. A genuine contractor relationship does not — the contractor takes on a defined project, completes it, and the relationship ends. Contractors who have been on rolling engagements with the same client for years, with regular renewals and no defined project end, face a harder mutuality-of-obligation argument.

HMRC's Check Employment Status for Tax (CEST) tool is the most widely used first-line assessment. Its output is not infallible — HMRC has lost IR35 cases where CEST returned an inside determination, and courts have ruled contractors outside IR35 in cases CEST might have called inside. But the tool has improved significantly, and clients who use it properly and follow its output have a degree of protection if their determination is later challenged. If CEST returns 'unable to determine', that is a strong signal to get specialist advice before issuing the SDS.

Being placed inside IR35 is not necessarily the end of the world, but it changes the economics of the engagement. The deemed employer deducts PAYE and employee National Insurance from the contract rate. The contractor's company still pays employer National Insurance and the Apprenticeship Levy on those deemed payments. The contractor company then receives a net payment, having borne the full employment cost. The result: the effective take-home on an inside-IR35 engagement is materially lower than an outside engagement at the same headline rate. Contractors should factor this into their rate negotiation — day rates need to rise to compensate for the increased tax burden, or the engagement may no longer be commercially viable.

If your client has determined you are inside IR35 and you believe that determination is wrong, you have a right to challenge it through the client's status disagreement process. The client must respond within 45 days with either a new SDS or written reasons why they are maintaining the original determination. You cannot override a client determination unilaterally, but a well-prepared challenge — with supporting evidence on substitution, control, and mutuality — has succeeded in many cases. HMRC can also be challenged directly, but that route is longer and more expensive.

At Stertha Advisory, we advise contractors, their personal service companies, and clients making status determinations. Whether you need a contract review, help with a status challenge, or guidance on extracting profits from your PSC tax-efficiently in 2026, we can help. Book a consultation.