TL;DR
On this page
A UK contractor who has billed through their own company outside IR35 for two years, then receives a client's Status Determination Statement reclassifying them as inside, is looking at a tax bill that reaches backward: PAYE and National Insurance the engagement was assumed to be free of. The 2024 offset rule softens the blow, but the real defense was built earlier, in a contract whose substitution, control, and mutuality terms point outside IR35 and match how the work is actually done. This is how that contract is structured, and what the offset rule does when it is not enough.
pro tip
Outside-IR35 status rests on three factors HMRC weighs: a genuine right of substitution, lack of control over how you work, and no mutuality of obligation. The written contract must support all three and match your actual working practices, because HMRC looks past the paper. For medium and large clients, the client decides your status and issues a determination you can challenge. If you are reclassified inside, the April 2024 offset rule reduces the client's PAYE bill by tax you already paid, but employer NICs and the Apprenticeship Levy are not offset.
The rate side of UK contracting is in the UK day rate and IR35 guide, the general contract foundation is in freelance contract essentials, and the broader tax picture is in the freelance tax guide.
The three factors that decide status · The status determination process · The April 2024 offset rule · The clauses to get right
The Three Factors That Decide Status
IR35 status turns on whether, stripped of the company in between, the relationship looks like employment. Three factors carry the most weight.
Right of substitution. A genuine right to send someone else to do the work is incompatible with employment, which is personal. Per HMRC's Employment Status Manual ESM0535, "it is the right of substitution that is important. The fact that substitution has not actually occurred during a contract is not necessarily relevant," and an unqualified right to send a substitute is "a strong pointer to self-employment." The right has to be real, not a clause the client would never honour.
Control. An employee is told how, when, and where to work; a genuine contractor controls their own method and, usually, their hours and location within the bounds of the deliverable. The less the client directs the manner of the work, the more the engagement looks outside IR35.
Mutuality of obligation. This is the most misunderstood factor. Per HMRC ESM0543, the basic requirement is that "the engager must be obliged to pay a wage or other remuneration, and the worker must be obliged to provide his or her own work or skill." Where the client is not obliged to offer ongoing work and the contractor is not obliged to accept it, the ongoing mutuality that characterises employment is absent.
The decisive point across all three: HMRC weighs the actual working relationship, not just the written words. A contract that says one thing while the engagement runs like employment will not hold.
The Status Determination Process
Since the April 2021 off-payroll reform, the decision usually is not the contractor's to make. Per GOV.UK, for medium and large clients "your client is responsible for deciding your employment status for tax," and "they should provide you with a Status Determination Statement if the rules apply."
That statement is challengeable. Per the same guidance, "all clients are required to introduce a process from April 2021 to allow you to disagree with their decision." So a contractor who receives a determination they believe is wrong, particularly a blanket inside-IR35 call applied to every contractor regardless of their actual terms, can formally dispute it through the client's process rather than simply accepting it. For small clients, the responsibility instead remains with the contractor's own intermediary, which makes the small-client exemption worth tracking.
One change is on the horizon: per Kingsbridge, HMRC has indicated the "small company" thresholds will rise from 6 April 2026 (turnover to £15m and balance sheet to £7.5m), which would move more clients into the small category and revert IR35 responsibility to those contractors' own companies. As of writing this is anticipated rather than reflected in HMRC's Employment Status Manual, so treat it as a development to watch, not settled law.
The April 2024 Offset Rule
When a contractor is reclassified inside IR35, the consequence used to be brutal and arguably unfair: the client or fee-payer owed the full PAYE and NICs, while the contractor had already paid corporation tax, dividend tax, and personal tax on the same income. HMRC, in effect, collected twice. The offset rule fixes the worst of that.
Per HMRC ESM10037, "the set-offs legislation will only apply where a trigger event occurs on or after 6 April 2024," and when HMRC "is satisfied that the conditions for a set-off have been met and makes a direction, the liability of the deemed employer will be reduced by the set-off to take account of income tax, NICs and corporation tax paid on the income from the OPW engagement by the worker and/or intermediary."
Two limits matter. First, it is not retrospective relief for old closed cases: the trigger event has to fall on or after 6 April 2024. Second, it is not a complete offset. Per RSM UK, "the engager or fee payer will still be responsible for arrears of employer NICs and, where due, apprenticeship levy." Those employer-side costs are not set off, which is why a reclassification still stings, and why clients remain cautious about engaging contractors outside IR35. The offset reduces the double-tax problem; it does not remove the reclassification risk, so the contract and working practices remain the primary defense.
The Clauses to Get Right
The contract should make the outside-IR35 position explicit on each factor, and the engagement should then run that way:
- Substitution: a genuine, not unrealistically fettered, right to provide a qualified substitute, with the contractor responsible for paying the substitute. Avoid wording that gives the client an unfettered veto over any substitute.
- Control: state that the contractor determines the manner and method of the work, and avoid fixed hours, set locations, or line-management language that reads like employment.
- No mutuality of obligation: state that the client is not obliged to offer further work and the contractor is not obliged to accept it, and that the contract is for a defined deliverable or period, not open-ended availability.
- In business on your own account: reflect financial risk, your own equipment, the ability to work for other clients, and correction of defects at your own cost.
- Working practices match the paper: the single most important point. A perfect contract paired with employee-like working practices fails, because HMRC looks at the reality.
Build the contract with these clauses in the free FreelanceDesk contract generator, and have a genuinely complex engagement reviewed by an IR35 specialist before signing.
Copy-Paste Clause Checklist
Outside-IR35 contract checklist
For the rate and day-rate context, see the UK day rate and IR35 guide; for post-reclassification invoicing, the UK invoice guide.
References
- ESM0535: Guide to Determining Status, Right of Substitution, HMRC Employment Status Manual
- ESM0543: Guide to Determining Status, Mutuality of Obligation, HMRC Employment Status Manual
- ESM10037: Off-Payroll Working, Setting Off Tax and NICs Already Paid, HMRC Employment Status Manual
- Know the Facts for Contractors: Off-Payroll Working Rules (IR35), GOV.UK
- New Rules to Offset the Risk of Using Off-Payroll Workers From 6 April 2024, RSM UK
- IR35 Small Company Threshold Changes 2026, Kingsbridge