IR35 Status Determination Letter Generator

Get a free IR35 risk score based on your working arrangements, or unlock an AI-generated Status Determination Statement letter formatted as a formal letter from your company with case law references.

IR35 determines whether a contractor is genuinely self-employed or effectively an employee for tax purposes. HMRC estimates that non-compliance with off-payroll working rules costs the Exchequer over £1.3 billion per year. Since April 2021, medium and large private sector clients have been responsible for determining IR35 status, making correct assessments critical.

A Status Determination Statement is the formal document that records the IR35 decision and the reasons behind it. Getting this wrong exposes the end client to tax liability plus penalties. The key factors are control over how work is done, the obligation of personal service, and mutuality of obligation.

Answer questions about your working arrangements to get a free IR35 risk score. The paid version generates a formal Status Determination Statement letter with case law references, formatted as a document you can issue to the contractor.

How it works

  1. Answer questions about control, substitution, and mutuality of obligation.
  2. View your IR35 risk score with an explanation of the key factors.
  3. Upgrade for an AI-generated Status Determination Statement letter with case law references.

Written by the CalcStack team · Last updated April 2026

This is a self-assessment tool. It does not constitute legal advice. Always consult a qualified IR35 specialist for formal determinations.

Your details

Working arrangements

Tick all that apply to your engagement. Each ticked item is an indicator of genuine self-employment.

Contract details

Summarise key contract terms (used for the Pro AI-generated letter).

Frequently asked questions

What is an IR35 Status Determination Statement?▾
An IR35 Status Determination Statement (SDS) is a formal document that sets out whether a contractor engagement falls inside or outside the off-payroll working rules (IR35). Since April 2021, medium and large private sector clients must issue an SDS before engaging a contractor through a Personal Service Company (PSC).
Who is responsible for determining IR35 status?▾
For medium and large private sector clients, the end client must determine IR35 status and issue a Status Determination Statement. For small private sector clients, the contractor's PSC remains responsible for making the determination. Public sector clients have been responsible since April 2017.
What is the Ready Mixed Concrete test?▾
The Ready Mixed Concrete test comes from the 1968 case Ready Mixed Concrete (South East) Ltd v Minister of Pensions and National Insurance. It established three conditions for a contract of service: the worker agrees to provide their own work and skill; the employer has a sufficient degree of control; and the other provisions are consistent with employment.
What factors determine IR35 status?▾
Key factors include: control (who decides how, when and where work is done), substitution (can you send someone else), mutuality of obligation (must work be offered and accepted), financial risk (do you bear the risk of loss), provision of equipment, part and parcel of the organisation, and the right to refuse work.
What happens if you get IR35 wrong?▾
If HMRC determines that IR35 should have applied and didn't, the fee-payer (usually the agency or client) becomes liable for the unpaid tax and National Insurance. Penalties of up to 100% of the tax due can apply, plus interest. Contractors may also face retrospective tax assessments going back several years.
Can I challenge an IR35 determination?▾
Yes. If you disagree with a client's Status Determination Statement, you have the right to make representations. The client must respond within 45 days. If you remain unsatisfied, you can escalate to the client's dispute resolution process or ultimately to a tax tribunal.
Is an IR35 letter the same as a CEST assessment?▾
No. CEST (Check Employment Status for Tax) is HMRC's free online tool. An IR35 status determination letter is a more detailed document that considers all relevant factors, references case law, and provides written reasoning. Many tax advisors recommend having a professional determination alongside CEST.
How often should IR35 status be reviewed?▾
IR35 status should be reviewed whenever the working arrangements change, when a contract is renewed or extended, or at least annually. A change in role, reporting line, or working location can affect the determination. Each engagement must be assessed individually.
Does having a substitution clause guarantee outside IR35?▾
No. A substitution clause alone is not sufficient. HMRC and tribunals look at whether the right of substitution is genuine and has been, or realistically could be, exercised. A clause that exists on paper but would never be used in practice carries little weight.
What is the difference between IR35 and off-payroll working rules?▾
They are effectively the same rules. IR35 is the informal name for the Intermediaries Legislation introduced in 2000. The off-payroll working rules refer to the 2017 and 2021 reforms that shifted responsibility for determining status from the contractor's PSC to the end client (public sector from 2017, medium/large private sector from 2021).

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