IR35 for Employers: Off-Payroll Working Rules Explained

If your business engages contractors through personal service companies, you need to understand IR35. Getting the off-payroll working rules wrong can result in significant tax liabilities, penalties, and reputational damage. This guide explains what IR35 means for employers in the UK, who is responsible for making status determinations, and how to manage your compliance obligations.

Last updated: April 2026

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IR35 Employer Guide UK: What the Off-Payroll Rules Actually Mean

IR35 refers to the Intermediaries Legislation, originally introduced in April 2000. The name comes from the Inland Revenue press release that announced it. In practical terms, IR35 determines whether a contractor working through an intermediary (usually a limited company or personal service company) should be taxed as an employee.

The core question: if the contractor were engaged directly, without the intermediary, would the relationship look like employment? If yes, the engagement falls "inside IR35" and the worker should be taxed as an employee. If no, the contractor continues to be paid gross through their limited company.

The rules exist to prevent "disguised employment", where someone works in a way that is functionally identical to employment but avoids PAYE tax and National Insurance Contributions (NICs) by operating through a company.

Why Employers Need to Care

Before April 2021, contractors working through their own limited companies determined their own IR35 status. That changed with the off-payroll working rules reform. For medium and large businesses, the responsibility shifted to the end client (the employer, in most cases).

This means you are legally responsible for:

• Assessing whether each contractor engagement falls inside or outside IR35
• Issuing a Status Determination Statement (SDS) for each engagement
• Communicating that determination to the contractor and the fee-payer (the agency, if one is involved)
• Deducting PAYE tax and NICs if the engagement is inside IR35
• Maintaining records of your reasoning

If you fail to make a determination or make one without taking reasonable care, HMRC treats the engagement as inside IR35 by default and holds you liable for the unpaid tax.

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Who Is Responsible? Medium and Large Business Rules

The off-payroll working rules for medium and large businesses came into force in April 2021, after being delayed by one year due to the pandemic. They apply to any private sector organisation that meets two or more of the following criteria in the relevant financial year:

• Annual turnover of more than £10.2 million
• Balance sheet total of more than £5.1 million
• More than 50 employees

If your business meets two or more of those thresholds, you are the "client" under the rules and must take responsibility for IR35 status determinations.

The Small Business Exemption

If your business does not meet two of the three thresholds, the original IR35 rules still apply: the contractor remains responsible for assessing their own status and accounting for the correct tax.

However, this is not a free pass. If HMRC believes a contractor engaged by a small business is incorrectly self-assessing as outside IR35, they can and do investigate the contractor directly. As a small business, you should still understand the basics of IR35 to ensure you are not unknowingly creating arrangements that look like disguised employment.

The size test applies to the end client, not the agency. If a large business engages a contractor through a recruitment agency, the large business is still responsible for the status determination, even if the agency handles the payments.

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How to Determine IR35 Status

HMRC looks at several factors when assessing whether an engagement is inside or outside IR35. No single factor is decisive on its own. It is the overall picture that matters. The three most important tests are:

1. Personal Service (Substitution)

Can the contractor send a qualified substitute to do the work in their place? If the contractor must perform the work personally and cannot provide a replacement, this points towards employment. A genuine right of substitution is one of the strongest indicators of an outside-IR35 engagement. Paper clauses that would never be exercised carry little weight.

2. Control

Does the client control how, when, and where the work is done? If the contractor decides their own methods, hours, and location, this suggests a genuine business-to-business relationship. If the client dictates specific hours, requires on-site work alongside employees, and manages the contractor like a member of staff, this points towards employment.

3. Mutuality of Obligation (MOO)

Is there an ongoing obligation for the client to provide work and the contractor to accept it? In a genuine contractor relationship, the engagement should be project-based or task-based, with no expectation of continuous work beyond the current assignment. Rolling extensions with no clear project boundaries look more like employment.

Other Relevant Factors

• Financial risk: Does the contractor bear genuine financial risk, invest in their own equipment, and carry professional indemnity insurance?
• Part and parcel: Is the contractor integrated into the business with a company email, attending staff meetings, or appearing on org charts?
• Exclusivity: Does the contractor actually work for other clients simultaneously?
• Payment structure: Fixed fee for deliverables, or a day rate that mirrors a salary?

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The Status Determination Statement (SDS)

For medium and large businesses, you must issue an SDS for every contractor engagement. It must:

• State whether the engagement is inside or outside IR35
• Provide the reasons for reaching that conclusion
• Be given to the contractor and the fee-payer before work begins

You should keep records of all determinations and the evidence you relied on.

Taking Reasonable Care

HMRC expects "reasonable care" in status determinations. This means considering each engagement individually, gathering information about actual working practices (not just the contract), using HMRC's CEST tool or specialist advice, documenting your reasoning, and reviewing determinations when circumstances change.

Blanket assessments are the opposite of reasonable care. Assessing every contractor as "outside IR35" to keep them happy, or as "inside IR35" to avoid risk, without examining the facts will land you in trouble.

If HMRC finds you did not take reasonable care, the tax liability transfers to you as the end client: you pay the unpaid PAYE and NICs, plus potential interest and penalties.

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HMRC's CEST Tool: Useful but Imperfect

HMRC provides the Check Employment Status for Tax (CEST) tool as a free online resource at gov.uk/guidance/check-employment-status-for-tax. It asks questions about the engagement and returns a determination of "employed for tax purposes" (inside IR35), "self-employed for tax purposes" (outside IR35), or "unable to determine."

Benefits: Free, quick, and HMRC will stand behind the result if the information entered was accurate and reflects actual working practices.

Limitations: It does not fully assess mutuality of obligation, which is critical in many engagements. It can return "unable to determine" in borderline cases, which does not help you. It relies entirely on the accuracy of the answers you provide. And it does not consider the overall picture in the way a tribunal or tax adviser would.

CEST is a useful starting point, but it should not be your only tool. For borderline cases or high-value engagements, consider specialist advice from an employment status adviser or tax professional.

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What Happens If You Get IR35 Wrong

Tax liability. If HMRC reclassifies an engagement, the unpaid income tax (PAYE), employee NICs, and employer NICs all become due. For medium and large businesses that failed to take reasonable care, the full liability falls on the end client.

Penalties and interest. HMRC can charge interest on late payment, plus penalties for carelessness or deliberate non-compliance, and penalties for failure to operate PAYE correctly.

Reputational risk. Experienced contractors avoid businesses known for blanket inside-IR35 determinations without proper assessment. Getting this wrong can make it harder to attract quality contractors.

Contractor disagreements. If a contractor challenges your determination, you must respond within 45 days through a formal disagreement process. You are legally required to have this process in place. Failing to respond means the obligation to deduct tax transfers back to you as the client.

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Practical Steps for Compliance

Before Engagement

1. Confirm your size status against the three thresholds. If you are close to the boundary, get your accountant to confirm.
2. Assess each engagement individually. Speak to the hiring manager about actual working practices, not just the procurement team.
3. Use CEST as a starting point. If it returns "unable to determine," seek specialist advice.
4. Issue the SDS to the contractor and any agency before work begins.
5. Set up your disagreement process so contractors can challenge determinations.

During the Engagement

6. Monitor working practices. Status can change over time. A contractor brought in for a specific project who starts attending team meetings, reporting to a manager, and working fixed hours may need a fresh assessment.
7. Review determinations at every contract renewal, or at least annually.
8. Keep records. HMRC can investigate IR35 determinations years after the fact. Maintain records of your assessments, the evidence you considered, and your reasoning.

Contractual Hygiene

Your contracts should reflect the reality of the working arrangement. Make sure your employment contracts and contractor agreements are clearly distinguished. Key provisions for contractor engagements include:

• A genuine right of substitution
• No mutuality of obligation beyond the specific project
• Contractor control over how, when, and where work is performed
• Payment linked to deliverables, not time
• Contractor responsibility for their own tax, insurance, and equipment

Contracts matter, but HMRC looks at the reality of the working relationship. If your contract says the contractor can send a substitute but in practice they never could, the clause carries little weight.

If you are hiring employees directly, make sure you understand what must be in a UK employment contract and how to write one that is compliant. And regardless of IR35 status, ensure right to work checks are handled correctly from the start.

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Common Mistakes Employers Make

Blanket determinations. Assessing all contractors the same way without considering individual circumstances. This is the opposite of reasonable care and is exactly what HMRC looks for.

Relying on contracts alone. A well-drafted contract is important, but if the day-to-day reality contradicts it, the contract is worthless for IR35 purposes.

Ignoring the disagreement process. Not having a formal process for contractors to challenge determinations is a compliance failure in itself.

Not reviewing when things change. A contractor who starts a three-month project and is still there two years later, embedded in your team, needs a fresh assessment.

Confusing the small business exemption with immunity. Being a small business means the contractor handles their own status, but it does not mean you have zero risk.

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Review Your Contractor Arrangements

HMRC's enforcement activity around IR35 has increased steadily since the 2021 reforms, and the costs of getting it wrong are significant. If you engage contractors and are unsure whether your determinations, contracts, and processes are compliant, now is the time to act.

Review your contractor agreements and employment documents with the EmployerKit Audit. From £49. Visit employerkit.com/tools/employerkit-audit.

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Frequently Asked Questions

Q: What does IR35 mean for employers in simple terms?

A: IR35 determines whether a contractor working through a limited company should be taxed as an employee. If your business is medium or large (meets two of three size thresholds), you are responsible for assessing each contractor's status and deducting tax if the engagement is inside IR35. Small businesses leave the determination to the contractor.

Q: What happens if I do not make an IR35 status determination?

A: If you are a medium or large business and fail to issue a Status Determination Statement, the engagement is treated as inside IR35 by default. You become liable for the unpaid PAYE tax and NICs, plus potential penalties and interest.

Q: Can I rely on HMRC's CEST tool for IR35 determinations?

A: CEST is a useful starting point and HMRC will stand by the result if your answers are accurate. However, it has limitations: it does not fully assess mutuality of obligation and can return inconclusive results in borderline cases. Use it as a first step, not your only tool.

Q: Does the small business exemption mean I have no IR35 risk?

A: No. The contractor is responsible for their own assessment, but HMRC can still investigate the contractor and question the arrangement. You should ensure your engagements reflect genuine self-employment.

Q: How often should I review IR35 status determinations?

A: At every contract renewal and whenever the nature of the engagement changes significantly. If a contractor takes on new responsibilities or shifts from project work to an ongoing support role, a fresh assessment is needed. Annual reviews are a sensible minimum.

Q: What is the difference between IR35 and the off-payroll working rules?

A: They are the same legislation. "IR35" refers to the original 2000 rules where contractors self-assess. "Off-payroll working rules" refers to the 2017 (public sector) and 2021 (private sector) reforms that shifted responsibility to the end client. The underlying employment status test is identical.