Anti-avoidance tax rules for contractors working through intermediaries (usually their own limited company). IR35 asks: if you worked for the client directly, would you be an employee? If yes ("inside IR35"), you are taxed broadly like an employee — Income Tax and National Insurance deducted at source. If no ("outside IR35"), your company is taxed as a genuine business. The rules have existed since 2000; the off-payroll reforms of 2017 (public sector) and 2021 (private sector) moved the decision to the client.

It depends on the client's size. For medium and large private-sector clients (and all public-sector bodies), the client must assess your status and issue a Status Determination Statement; the fee-payer deducts tax if you are inside. For small private-sector clients (meeting two of: turnover ≤ £10.2m, balance sheet ≤ £5.1m, ≤ 50 employees), you decide — and you carry the risk if HMRC disagrees. Wholly overseas clients with no UK presence: you decide too.

HMRC's free Check Employment Status for Tax tool at gov.uk/guidance/check-employment-status-for-tax. It asks structured questions about the contract and working practices, then returns "employed for tax purposes", "self-employed for tax purposes", or "unable to make a determination". HMRC says it will stand by the result provided the answers were accurate. Run it for every contract you are unsure about and keep a copy of your answers — it is the single most useful IR35 action a contractor can take.

Because employees cannot send someone else to do their job. A genuine, unfettered right to send a qualified substitute — actually exercised or credibly available — is the clearest marker of a business relationship rather than personal service. It carries the heaviest weight in this questionnaire (3 points) for that reason. A substitution clause that exists on paper but would never be honoured in practice will not survive HMRC scrutiny — the reality must match the contract.

The ongoing two-way obligation: the client must offer work, and you must accept it. Project-by-project engagements with no obligation between projects point outside IR35; a rolling arrangement where work is continuously offered and expected to be accepted points inside. Notice periods, exclusivity expectations, and "we'll keep you busy" assurances all feed this test — which is why the questionnaire asks about it directly.

No — but it is a risk flag. Single-client dependence suggests the relationship functions like employment, especially combined with time-based pay and integration. Many genuinely outside contractors have one major client at a time; what matters is whether you could take others, actively market, and bear business risk. The questionnaire weights it lightly (1 point) — it is supporting evidence, not decisive alone.

They can recover the tax that should have been deducted, with interest — and penalties can apply. Since 6 April 2024, where HMRC pursues a deemed employer, it can set off tax and National Insurance already paid against the bill. Investigations can reach back several years, so the liability compounds. This is why borderline and inside-leaning results demand professional advice, not hope.

No — and that distinction matters. This page simplifies CEST-style logic into 9 questions for self-assessment; it is not HMRC's tool, HMRC has never seen your answers, and the result has no standing in any dispute. CEST itself asks more questions with branching logic. Use this questionnaire to understand your position, then run the real CEST tool and keep its result.

Genuinely, yes — artificially, no. Real changes count: a real substitution right, project-based deliverables, your own equipment, multiple clients, genuine financial risk. Paper changes that contradict daily reality do not — HMRC investigates working practices, interviews both sides, and disregards sham clauses. Change the reality first; the paperwork follows.

The client's written IR35 decision, required for medium/large engagers. It must state the determination and the reasons. You have the right to challenge it — the client must respond within 45 days. If you disagree with an inside determination, challenge it with evidence (this questionnaire's flags are a starting point for that conversation), and consider independent review.

The rules still apply to UK tax residents; the question is who decides. A wholly overseas client with no UK presence falls outside the off-payroll decision rules, so the contractor (your company) is responsible for the assessment and the liability. Overseas clients do not exempt you from IR35 — they move the responsibility to you.

Three steps, in order. (1) Run HMRC's official CEST tool and save the result. (2) If borderline or inside-leaning — or the contract is high value — get a professional IR35 contract review from a qualified accountant or specialist. (3) Keep evidence of your actual working practices (emails showing autonomy, substitution correspondence, multiple client invoices). Then re-check whenever the engagement changes — status is per-engagement, not per-company.