IR35 Rules In The UK: How Off‑Payroll Working Really Works

Hiring contractors can be a smart, flexible way to grow your business without taking on the cost and commitment of permanent staff.

But if you get a contractor’s status wrong under the UK’s IR35 rules (the “off‑payroll working” regime), you could face unexpected PAYE income tax and NICs, penalties and interest - not to mention disruption to your projects.

In this guide, we break down IR35 in plain English from a small business perspective. You’ll understand how IR35 works, what the law requires, when you’re responsible for tax, and the practical steps to stay compliant while still working with great talent.

What Is IR35 And How Does It Work For Businesses?

IR35 is a set of UK tax rules that tackle “disguised employment”. In short, if a contractor provides their services through a personal service company (PSC) or another intermediary, but in reality works like your employee, HMRC may treat the engagement as “inside IR35”. When that happens, PAYE income tax and employer/employee National Insurance Contributions (NICs) are due as if the contractor were on payroll.

The law sits mainly in the Income Tax (Earnings and Pensions) Act 2003 (ITEPA) - Chapter 8 (original IR35) and Chapter 10 (off‑payroll working in the public sector and medium/large private sector). Finance Acts in 2017 and 2021 extended who carries the tax risk.

Here’s what changed and why it matters to you:

• Public sector since April 2017: The client (you) must assess status and, if inside IR35, ensure PAYE/NICs are operated by the “fee‑payer”.
• Private sector since April 2021: The same rules apply to medium and large private businesses. Small companies are exempt from these off‑payroll rules (the PSC stays responsible), but you still need to understand IR35 if you grow or if you’re not truly “small”.

Under Chapter 10, three roles matter in a supply chain:

• The client (end user): the organisation receiving the contractor’s services.
• The fee‑payer: the party that pays the PSC (could be you or an agency).
• The intermediary: usually the contractor’s PSC.

If an engagement is “inside IR35”, the fee‑payer must account for PAYE and NICs on the contractor’s fees (excluding VAT) and pay employer NICs and (where applicable) the Apprenticeship Levy.

Does The Small Company Exemption Apply To You?

The off‑payroll IR35 rules don’t apply to “small” clients in the private sector. If you’re small, the contractor’s own PSC is responsible for determining IR35 and paying any tax due under Chapter 8.

You’re “small” if you meet two of these criteria for two consecutive financial years (Companies Act 2006 thresholds):

• Annual turnover not more than £10.2 million
• Balance sheet total not more than £5.1 million
• Average employees not more than 50

Unincorporated businesses assess only their turnover threshold. If you are part of a group, size is assessed on a group basis. If you cease to be small, the off‑payroll rules apply from the start of the next tax year (6 April), so plan ahead.

Even if you’re small today, make it a habit to document your size assessment annually and to include IR35 compliance in your growth planning. Crossing the threshold without updating processes is a common (and costly) mistake.

How To Decide If An Engagement Is Inside Or Outside IR35

What are the IR35 rules actually testing? HMRC and the courts look at the hypothetical contract between the contractor and your business to decide if it looks like employment. It’s about the real working relationship, not just what your written contract says.

Key factors that influence status include:

• Control: Who decides how, when and where the work is done? The more day‑to‑day control you exercise, the more it looks like employment.
• Personal service and substitution: Is the contractor required to do the work personally, or can they send a suitably qualified substitute that you can’t unreasonably refuse? A genuine, workable right of substitution supports “outside IR35”.
• Mutuality of obligation (MOO): Are you obliged to offer work and are they obliged to accept it on an ongoing basis? Continuous obligation points to employment‑type status.
• Financial risk: Do they bear a risk of loss or extra cost (e.g. needing to fix defects at their own expense, or charging a fixed price for deliverables)? Genuine business risk supports “outside”.
• Equipment and integration: Do they use their own equipment and systems, advertise to the market, and remain clearly separate from your internal teams? Being “part and parcel” of your organisation suggests employment.
• Exclusivity: Can they work for multiple clients at the same time? Exclusivity and set hours are employment‑like.

A strong contract helps, but HMRC will always look at reality. For example, a substitution clause that’s never allowed in practice won’t carry much weight.

HMRC’s CEST tool can be a useful indicator, but it’s not infallible. You still need to take “reasonable care” when making each determination and record your reasoning.

It’s also important to separate tax status from employment law status. A contractor can be “outside IR35” for tax but still be a “worker” with certain rights. If you’re unsure how someone should be classified overall, review the common employment status tests and get tailored advice.

Your IR35 Responsibilities As The Client (Off‑Payroll Working)

If the off‑payroll rules apply to you (public sector or medium/large private sector), you have specific obligations for each engagement:

1) Make A Status Determination Statement (SDS)

• Assess the contractor’s status taking reasonable care.
• Issue a written Status Determination Statement to the contractor and to the next party in the chain (e.g. your agency), setting out your decision and the reasons.
• Keep records of your analysis and the evidence you relied on.

2) Operate The Disagreement Process

• Have a clear, documented process to handle challenges to your SDS.
• Respond within a reasonable timeframe (typically within 45 days) with a reasoned outcome and update the SDS if you change your view.

3) Ensure PAYE Is Operated If Inside IR35

• If you’re the fee‑payer, deduct PAYE income tax and employee NICs from the contractor’s fees (excluding VAT), and pay employer NICs and any Apprenticeship Levy.
• If an agency is the fee‑payer, make sure they understand the SDS and are operating PAYE correctly. Contractual indemnities can help allocate risk, but they won’t protect you if you fail to meet your own obligations.

4) Manage The Supply Chain

• Do due diligence on agencies and umbrella companies - non‑compliance upstream can drag you into HMRC disputes.
• Use robust contracts that reflect the actual working practices and include IR35‑related warranties and audit rights.

5) Revisit Determinations If Things Change

• IR35 status is not “set and forget”. If the scope, control, substitution arrangements or working practices change, reassess and reissue the SDS.

Reasonable care matters. If you blanket‑assess everyone as inside or outside IR35 without looking at the facts, HMRC can treat you as having failed to take reasonable care - potentially shifting tax liability to you even when you’re not the fee‑payer.

Practical Steps To Build An IR35‑Friendly Contractor Model

Want to keep working with contractors while staying on the right side of the IR35 law? Focus on aligning both your paperwork and your day‑to‑day practices.

Step 1: Map Your Contractor Population

• List all engagements supplied via PSCs or intermediaries (direct and through agencies).
• Identify which business units use contractors, what they do, where they work, and who manages them.
• Confirm whether the small company exemption applies. If not, prioritise high‑risk roles for early review.

Step 2: Define Roles As Projects And Deliverables

• Structure scopes around outputs (milestones, deliverables, fixed price or capped T&M) rather than ongoing duties.
• Limit day‑to‑day control: focus on “what” is delivered, not “how, when and where”.
• Avoid blanket exclusivity and fixed working hours unless genuinely necessary.

Step 3: Draft Contracts That Match Reality

• Use a well‑drafted Contractor Agreement or Consulting Agreement that sets out deliverables, a genuine right of substitution, limited control, ability to work for others, and contractor‑borne risks (e.g. rectification of defects).
• Agree clear IP ownership and licensing terms - with contractors, IP doesn’t automatically belong to you unless the contract says so. See how to structure IP with contractors in our guide on IP and independent contractors.
• Where the contractor processes personal data for you, include a compliant Data Processing Agreement and ensure your Privacy Policy reflects the relationship.
• Use a Non‑Disclosure Agreement where appropriate to protect confidential information.

Step 4: Put Substitution Into Practice

• If your contract includes a substitution right, make sure it’s workable in real life. For example, set out a simple notification process and qualification criteria for substitutes.
• If a substitute is used, keep records - it’s powerful evidence that the engagement is a business‑to‑business relationship.

Step 5: Build A Status Assessment Process

• Create a standard IR35 questionnaire that covers control, substitution, MOO and financial risk.
• Train hiring managers on the basics of IR35 rules and when to involve finance/legal.
• Document each decision in an SDS and store it with your contracts.

Step 6: Align Your Day‑To‑Day Practices

• Keep contractors separate from employees in org charts and comms where possible.
• Avoid granting benefits reserved for employees (e.g. paid annual leave, sick pay, or staff perks) to PSC contractors.
• Where sensible, allow remote or off‑site working and flexible scheduling.

Step 7: Review Overseas Arrangements

• IR35 focuses on UK tax status, but international engagements raise extra questions (tax residence, PE risk, data transfer rules). If you use offshore talent, read our guide to engaging overseas contractors and get tailored advice.

Working With Agencies, Umbrella Companies And The Fee‑Payer Role

Many small businesses hire contractors via agencies or umbrella companies. This can be helpful, but it doesn’t remove your IR35 responsibilities.

• You still need to issue the SDS to the contractor and to the next party in the chain.
• Make sure your contracts with agencies include clear IR35 clauses: passing on the SDS, operating PAYE when required, indemnities for failures, cooperation with audits, and prompt information sharing.
• Check who the “fee‑payer” is. If the agency pays the PSC, they’re usually the fee‑payer. If you pay the PSC directly, you are.
• Be cautious of arrangements promising “higher take‑home pay” through aggressive umbrella schemes. HMRC routinely challenges non‑compliant models.

What Happens If You Get IR35 Wrong?

If HMRC finds that an engagement was inside IR35 and PAYE/NICs weren’t accounted for, the fee‑payer can be liable for:

• Unpaid PAYE income tax
• Employee and employer NICs
• Apprenticeship Levy (if applicable)
• Interest
• Penalties (which increase if you didn’t take reasonable care)

In some cases, liability can be transferred up the chain - for example, if the fee‑payer can’t be traced or didn’t receive the SDS due to failures higher up. This is why having documented processes and solid contracts across the chain is essential.

Remember also that IR35 is a tax test. Separately, if someone you treat as a contractor is, in law, an employee or worker, you could face employment law claims. Keeping your contractor model clean and consistent reduces both tax and employment risk.

FAQs: Common IR35 Questions From Small Businesses

What Does “Inside IR35” Mean In Practice?

It means the engagement is treated like employment for tax. The fee‑payer must operate PAYE and NICs on payments to the PSC (excluding VAT). It doesn’t automatically grant employment rights, but it’s a red flag to check whether the person is actually a “worker” or employee for employment law purposes.

Can We Just Put Everyone On A Blanket “Outside IR35” Determination?

No. The IR35 regulation requires reasonable care on a case‑by‑case basis. Blanket decisions (in or out) risk penalties and can shift liability to you. Build a repeatable, documented assessment process instead.

Is A Substitution Clause A Silver Bullet?

It helps, but only if genuine and workable. If the clause exists on paper but would never be allowed in practice, HMRC will give it little weight.

Do Day Rates Automatically Mean “Outside IR35”?

No. Payment method is just one factor. Control, personal service, MOO and financial risk carry more weight.

Should We Use HMRC’s CEST Tool?

It’s one input. You can use it to inform your decision, but record the facts you feed in, keep your own reasoning, and don’t rely on CEST where your scenario doesn’t fit neatly into the questions.

What About Data Protection When Working With Contractors?

If contractors handle personal data for you, include a proper Data Processing Agreement and ensure your Privacy Policy and internal processes comply with UK GDPR and the Data Protection Act 2018.

Essential Documents To Support IR35‑Compliant Contracting

• Contractor Agreement/Consulting Agreement: Clear statement of services, deliverables, substitution, control, ability to work for others, and financial risk allocation. Consider a Contractors Agreement or a bespoke Consulting Agreement.
• IP And Confidentiality: Express IP assignment/licence, moral rights waivers where needed, and strong confidentiality provisions. You can also use a standalone NDA during pre‑contract discussions.
• SDS Template And Process: A repeatable status assessment workflow with reasoned decisions and a disagreement policy.
• Data Protection Paperwork: A Data Processing Agreement where a contractor acts as your processor, and an up‑to‑date Privacy Policy.
• Agency Terms: IR35 warranties, cooperation clauses, audit rights and indemnities if there’s a supply chain.
• Status Playbook For Managers: Practical guidance so hiring managers don’t inadvertently create employment‑like conditions day‑to‑day.

Getting documents right is only half the story - your working practices must match what’s written. Avoid generic templates and ensure your contracts reflect how the relationship will actually run.

Key Takeaways

• IR35 is about substance over form: if a PSC contractor works like an employee, IR35 tax rules can apply and PAYE/NICs may be due.
• Public sector and medium/large private clients must issue an SDS with reasonable care and ensure PAYE is operated when engagements are inside IR35.
• Small private companies are exempt from the off‑payroll regime, but you should assess your size annually and prepare for the rules if you grow.
• Status depends on control, substitution, mutuality of obligation, financial risk, equipment and integration - and on real‑world practices, not just the contract.
• Build a repeatable process: map contractors, design deliverable‑based scopes, use robust contracts, document SDS decisions, and manage supply‑chain risk.
• Protect your wider legal position too: lock down IP ownership with contractors, include data protection terms, and keep your Privacy Policy and Data Processing Agreement in order.
• If you’re unsure whether a role is employment, worker or contractor, revisit the employment status tests and get tailored advice before someone starts.

If you’d like help assessing IR35, drafting a Contractor Agreement or building your off‑payroll process, our team can help you get protected from day one. You can reach us at 08081347754 or team@sprintlaw.co.uk for a free, no‑obligations chat.