IR35: Off-Payroll Working Rules Explained

If you work with freelancers or contractors, you’ve probably heard of “IR35”. It’s a set of UK tax rules that can feel complex at first glance, but once you understand the basics, you can make confident decisions and keep your projects moving without unexpected tax bills.

This guide explains IR35 simply from a small business perspective. We’ll cover when the rules apply, how to assess a contractor’s status, the steps you need to take as a client, and practical ways to manage risk while still engaging flexible talent.

Set up your legal foundations early and you’ll be protected from day one.

What Is IR35 And Why Should Small Businesses Care?

IR35 is the informal name for the UK’s “off‑payroll working” rules. In short, it’s an anti‑avoidance tax regime that tackles “disguised employment”. If a contractor provides their personal services through a limited company (often called a “PSC”) but, in reality, they’re working like an employee, HMRC may expect income tax and National Insurance to be paid as if the person were on payroll.

These rules matter because they can shift responsibility for tax and NICs onto you (the client), especially in the public sector and for medium or large private sector clients. Even when you’re a small company and the responsibility sits with the contractor’s PSC, your working practices and paperwork can influence the status outcome-and HMRC may still look closely at the engagement if things don’t stack up.

Practically, IR35 is about two things:

• Determining whether a contractor is genuinely self‑employed for a particular engagement; and
• Ensuring the correct party accounts for PAYE and NICs when the role is “inside IR35”.

The rules currently sit in the Income Tax (Earnings and Pensions) Act 2003 (ITEPA) and have been expanded by “off‑payroll” reforms since 2017 (public sector) and 2021 (private sector).

Who Do The IR35 Rules Apply To In Practice?

IR35 applies to engagements where an individual supplies personal services to a client through an intermediary, most commonly a limited company (PSC). It does not usually apply when you hire a sole trader directly without an intermediary-but employment law and tax status still need careful thought in those scenarios.

Responsibility depends on your size and sector:

• Public sector clients: You’re responsible for deciding status and operating PAYE/NICs if the role is inside IR35.
• Medium and large private sector clients: Since April 2021, you must determine status and, if inside IR35, ensure PAYE/NICs are operated (via the “fee‑payer” in the supply chain).
• Small private sector clients: There’s a “small company” exemption. If you’re small, the contractor’s PSC remains responsible for IR35. However, you still need robust contracts and working practices, as HMRC can challenge sham arrangements.

To qualify as “small” (Companies Act thresholds), your company must meet at least two of the following for two consecutive financial years:

• Turnover of £10.2m or less
• Balance sheet total of £5.1m or less
• 50 employees or fewer

If you’re part of a group, aggregate figures can apply. When you grow beyond “small”, the responsibility for status decisions and tax can shift to you-so it’s important to monitor your size status annually.

How To Decide If A Contractor Is Inside Or Outside IR35

Status is determined on a per‑engagement basis by looking at the reality of the working relationship. The written contract matters, but HMRC puts heavy weight on actual day‑to‑day practices. The key question: if the intermediary didn’t exist, would the person be regarded as your employee for this work?

Key Status Tests (In Plain English)

• Control: Who decides how, when and where the individual works? Employees are typically under significant direction and control. A genuine contractor has more autonomy and focuses on deliverables.
• Substitution: Is there a genuine, practical right for the contractor to send a suitably qualified substitute to deliver the services, and could they actually do it in real life? A real (and exercised) right of substitution points away from employment.
• Mutuality of Obligation (MOO): Are you obliged to provide continuous work and are they obliged to accept it? Open‑ended commitments look like employment. Project‑based, outcome‑focused engagements with clear end points are safer.
• Financial Risk and In Business On Own Account: Do they provide their own equipment, carry business insurance, quote fixed prices, or correct defects at their own cost? Businesslike features indicate self‑employment.
• Integration: Are they “part and parcel” of your organisation-on your org chart, managing staff like an employee, using your HR systems, or enjoying employee‑like benefits?

No single test is decisive. You need to look at the whole picture. For a deeper dive into how UK law looks at employment and self‑employment, it’s worth reading about Employment Status.

Contract Versus Working Practices

A well‑drafted contract helps, but it must reflect reality. If your agreement says there’s a right of substitution, yet in practice the client manager insists the named individual turns up 9–5 at your premises and can’t send anyone else, HMRC will focus on the reality-not the words.

Best practice is to design the engagement around outputs (a scope of work and milestones), not “hours under supervision”. Pair this with the right paperwork-such as a clear Contractor Agreement or Consulting Agreement-and make sure managers actually follow the agreed working model.

Using Tools (CEST) And Taking “Reasonable Care”

HMRC’s CEST tool can provide an indicative outcome if you answer honestly and consistently with the engagement. However, CEST has limitations (for example, how it handles MOO), so you should take “reasonable care” by gathering facts, reviewing the contract and working practices, and documenting your analysis. A thoughtful, documented assessment goes a long way if HMRC ever asks questions.

What Are Your Responsibilities Under The Off‑Payroll Working Rules?

Your obligations vary based on size and supply chain. Here’s what to know.

Small Company Exemption

If you’re a small private sector client, the IR35 determination and PAYE/NICs obligations typically remain with the contractor’s PSC. Still, you should communicate your size status annually, maintain clean contracts, and avoid day‑to‑day practices that look like employment. If in doubt, get tailored advice.

Status Determination Statement (SDS) And Dispute Process

If you’re a public sector client or a medium/large private sector client:

• Make a status determination for each engagement and take reasonable care in doing so.
• Issue an SDS to the worker and the party you contract with (e.g. the agency), stating whether the role is inside or outside IR35 and the reasons.
• Operate a dispute process: respond within 45 days if the worker or fee‑payer challenges the determination, explaining your reasoning or changing the decision if appropriate.

Failing to take reasonable care, or failing to pass on the SDS, can shift tax liability back to you-even if you sit above an agency in the chain.

Paying Tax And NICs When Inside IR35

If the role is inside IR35 and you are the “fee‑payer” (the party paying the PSC), you must deduct PAYE income tax and employee NICs and pay employer NICs and (where applicable) the Apprenticeship Levy. The contractor’s company then receives the net payment. VAT (if charged) is usually handled separately from the deemed employment payment.

Make sure your finance team and the agency (if any) know who the fee‑payer is and how deductions will be made. Contract terms should match your IR35 workflow so there are no surprises for anyone in the chain.

Working With Agencies And Supply Chains

In multi‑party arrangements, ensure the SDS is passed down properly and that contracts clearly allocate the fee‑payer role. If the SDS doesn’t reach the fee‑payer, or an agency lower in the chain fails to comply, liability can move up the chain. Robust processes and clear contracts help prevent gaps.

If you’re hiring temporary staff via an agency, separate rules and rights can also apply, so it’s sensible to distinguish IR35 assessments from any Agency Worker Hire considerations.

Practical Ways To Manage IR35 Risk Without Stopping Your Projects

You don’t need to avoid contractors to stay compliant. With the right structure and paperwork, you can still access specialist talent quickly and lawfully.

• Define Outcomes, Not Hours: Build engagements around deliverables, milestones and acceptance criteria. Avoid blanket 9–5 on‑site requirements unless you can justify them.
• Use The Right Contracts: Put in place a tailored Contractor Agreement or Consulting Agreement with clear scope, substitution mechanics, IP ownership, data protection and liability clauses appropriate to the status you’ve determined.
• Keep Working Practices Aligned: Train line managers. If the contract allows remote, flexible delivery, don’t impose day‑to‑day employee‑style control (unless that control is necessary and reflected in your status decision).
• Make Substitution Real: If the contract includes a right to substitute, ensure your processes can accept it. Maintain a simple approval mechanism for suitably qualified substitutes.
• Protect IP Properly: Contractors often create valuable materials. Ensure clean transfer or licence of rights using appropriate IP clauses or an IP Assignment. For background on why this matters with non‑employees, see Intellectual Property.
• Cover Data Protection: If a contractor processes personal data for you, include a compliant Data Processing Agreement and ensure security expectations are clear.
• Avoid “Benefit Creep”: Don’t blur the lines with employee‑style benefits (annual leave, bonuses tied to employment, performance management systems). Keep it commercial and project‑based.
• Review Regularly: Circumstances change. Reassess status if scope, control, location or duration shift materially.

Where you decide a role is, in reality, employment, be transparent and move to an Employment Contract. It’s better to be compliant than to run a “contractor in name only” model that creates exposure.

Common IR35 Scenarios For SMEs (With Examples)

Example 1: Fixed‑Price Build With Genuine Autonomy (Likely Outside IR35)

You hire a specialist software developer via their PSC to deliver a module for a fixed fee. The scope, milestones and acceptance tests are set in the contract. They choose their hours and location, provide their own equipment, and can send a substitute (which your process accommodates). You have weekly check‑ins but don’t direct how they code.

Here, the engagement looks like business‑to‑business services. There’s a genuine right of substitution, lower control, and financial risk if defects need fixing. Assuming practice matches the paperwork, this points outside IR35.

Example 2: Day‑Rate Contractor Embedded In A Team (Potentially Inside IR35)

You bring in a day‑rate project manager for six months. They work 9–5 on‑site, report to your head of operations, attend internal appraisals, and manage two of your employees. There’s no real substitution right. Work continues to be allocated daily as business needs arise.

That looks like employment in substance (control, integration, ongoing mutual obligations). If this reflects reality, an inside‑IR35 determination is likely appropriate, with PAYE/NICs accounted for by the fee‑payer.

Example 3: Small Company Client With A PSC Contractor

You’re a small private company engaging a PSC for marketing support two days per week. Under the small company exemption, the PSC is responsible for IR35. You still ensure the contract is deliverables‑focused, with a realistic substitution right and clear IP and data terms. Even though you’re not responsible for the SDS, your practices support genuine self‑employment, reducing the risk of later challenge.

Example 4: Through An Agency (Who Is The Fee‑Payer?)

You engage via an agency supplying a PSC contractor. You’re medium‑sized, so you must issue an SDS. If the role is inside IR35, the fee‑payer (often the agency paying the PSC) should operate PAYE/NICs. Your contracts should ensure the SDS and responsibilities flow down the chain and that the correct party is making the deductions.

Records, Penalties And HMRC Checks

HMRC expects clients to take “reasonable care” in status decisions. That means documenting the facts, your analysis, the SDS, and any dispute correspondence. Keep copies of the contract, scope of work, and communications showing how the work is really delivered.

What if HMRC disagrees with your determination?

• Liability: If you fail to take reasonable care, or don’t pass the SDS correctly, liability for tax, NICs and possibly interest/penalties can shift to you as the client.
• Penalty mitigation: Demonstrating reasonable care, cooperation and good records typically reduces penalties, even where tax is due.
• Process matters: Having a consistent procedure (initial assessment, manager sign‑off, SDS issue, dispute handling) shows control and diligence.

If HMRC opens a check, don’t panic. Gather your documentation, review your processes, and get advice early. Often, the difference between a painful outcome and a manageable one is how well you can evidence your decisions.

Key Takeaways

• IR35 is about substance over form-look at how the work is really delivered, not just what the contract says.
• Your responsibilities depend on size and sector: small private clients are generally exempt from client‑side obligations; medium/large and public sector clients must determine status and (if inside) ensure PAYE/NICs are operated.
• Assess status using control, substitution, mutuality and other indicators. Keep your contract and day‑to‑day practices aligned.
• Issue a reasoned SDS (when required), pass it down the supply chain, and operate a clear 45‑day dispute process.
• Structure engagements around deliverables and outcomes and use tailored documents like a Contractor Agreement or Consulting Agreement with strong IP and data terms (including a Data Processing Agreement where needed).
• Review arrangements when circumstances change and maintain solid records to evidence “reasonable care”.
• If the role is really employment, move to an Employment Contract-compliance upfront beats remediation later.

If you’d like help assessing IR35 status, preparing a compliant scope and contracts, or setting up a practical process for your team, you can reach us at 08081347754 or team@sprintlaw.co.uk for a free, no‑obligations chat.