If you run an agency in the UK and engage contractors, you need to understand the difference between inside IR35 and outside IR35. Get it wrong, and HMRC can come after your agency for unpaid tax, National Insurance, and penalties going back years. That liability can easily run into six figures for a medium-sized agency with a handful of contractors.

The core question is simple: is your contractor genuinely self-employed, or are they effectively an employee in disguise? The answer determines who pays the tax. For outside IR35 engagements, the contractor's limited company handles their own tax. For inside IR35 engagements, your agency (or the fee payer) must operate PAYE and deduct tax and National Insurance at source.

This guide covers the inside outside IR35 agency rules for 2025/26, the tests HMRC uses, and the practical steps you need to take as an agency founder. We work exclusively with agency founders at Agency Founder Finance, and this is the single most common compliance question we get from agencies scaling with contractors.

What Does Inside IR35 Mean for Your Agency?

When a contractor is inside IR35, HMRC treats them as a deemed employee for tax purposes. Your agency must deduct income tax and National Insurance from their payments, just as you would for a permanent employee. The contractor's limited company invoices you, but the net amount after deductions is treated as employment income.

Here is what that looks like in practice for a 12-person digital agency based in Manchester's Northern Quarter, billing £800k per year with three contractors:

  • Your agency operates PAYE on the contractor's payments. You deduct income tax and employee NI from each invoice.
  • You pay employer NI at 13.8% on top of the gross amount. This adds roughly £6,900 per year for a contractor earning £50,000.
  • You must issue a payslip and report through RTI (Real Time Information) each month.
  • The contractor's take-home pay drops significantly compared to an outside IR35 arrangement. A contractor billing £500 per day outside IR35 might take home roughly £400. Inside IR35, that same day rate nets to around £290 after all deductions.
  • You must include the contractor in your agency's pension auto-enrolment if they meet the criteria, though most contractors opt out.

The fee payer is the entity that pays the contractor. For most agencies, that is your agency itself. If you use an umbrella company, the umbrella becomes the fee payer, but your agency still carries responsibility for getting the status determination right.

What Does Outside IR35 Mean for Your Agency?

When a contractor is outside IR35, they are genuinely self-employed. Their limited company invoices your agency, and they handle their own tax through salary and dividends. Your agency does not deduct PAYE or NI. You pay the gross invoice amount, and the contractor's company accounts for corporation tax at 19% or 25% depending on profit level, plus dividend tax on what they draw personally.

For your agency, outside IR35 engagements are simpler administratively. You do not run payroll. You do not pay employer NI. You do not issue P60s or P45s. But you still carry a compliance responsibility: you must issue a Status Determination Statement (SDS) confirming the decision, and you must pass that SDS down the supply chain.

A 15-person creative agency in Bristol Harbourside we work with runs four contractors outside IR35. The agency saves roughly £22,000 per year in employer NI compared to employing those people directly. The contractors prefer it because they take home more and have more flexibility on how they structure their tax affairs.

The Key Tests: How HMRC Determines IR35 Status

HMRC does not just guess. They apply case law tests developed over decades of tribunals. The three most important factors are substitution, control, and mutuality of obligation. Understanding these tests is essential for any agency founder trying to determine the inside outside IR35 agency status of their contractors.

Substitution

Can the contractor send someone else to do the work? A genuine contractor has a right of substitution. If your contract says the contractor must personally perform the work, that points towards inside IR35. If the contractor can send a suitably qualified replacement, that points towards outside IR35.

Realistically, most agency engagements require the specific person. A client hires a particular developer because of their skills. Substitution clauses exist on paper but rarely get exercised. HMRC knows this. A substitution clause that is never used carries less weight than one that is genuinely available and occasionally used.

Control

Who decides how, when, and where the work is done? If your agency or your client tells the contractor what hours to work, what tools to use, and how to do the job, that points towards inside IR35. If the contractor decides their own schedule, uses their own equipment, and determines their own methods, that points towards outside IR35.

For agencies, control is often the trickiest factor. Your client needs the contractor to be available during business hours for meetings. They need the work to fit with the rest of the team. But that is not the same as control. A contractor who sets their own hours but agrees to attend a weekly 10am stand-up is still outside IR35 if they control the rest of their schedule.

Mutuality of Obligation

Is your agency obliged to offer work, and is the contractor obliged to accept it? For a permanent employee, the answer is yes on both sides. For a genuine contractor, each engagement is separate. Your agency does not have to offer more work after the current project ends. The contractor does not have to accept the next project.

This is where fixed-term retainers can cause problems. If you have a contractor on a 12-month retainer with notice periods and guaranteed minimum hours, that looks a lot like employment. Short project-based engagements with clear deliverables and no ongoing obligation are safer.

The CEST Tool: Useful but Not Definitive

HMRC provides the Check Employment Status for Tax (CEST) tool online. You answer a series of questions about the engagement, and it gives you a determination of inside or outside IR35. The tool is free and provides some legal protection if HMRC later challenges your decision, provided you answered honestly and accurately.

But CEST has limitations. It works well for straightforward cases but struggles with complex arrangements. It does not handle scenarios where the contractor works through their own limited company but also provides services to other clients. It does not account for the full range of case law factors.

As ICAEW qualified accountants, we recommend using CEST as a starting point, not a final answer. If the tool says outside IR35 but the engagement looks borderline, get a professional IR35 review. The cost of a review is typically £300-£800 per contractor. The cost of getting it wrong is far higher.

What Happens If You Get the Status Wrong?

HMRC can investigate your agency's contractor arrangements going back up to six years if they suspect deliberate non-compliance. The consequences include:

  • Backdated tax and NI on all payments made to contractors who should have been inside IR35. This includes both employee NI and employer NI.
  • Interest on the unpaid amounts from the date each payment was due.
  • Penalties of up to 100% of the tax due in cases of deliberate non-compliance.
  • Reputational damage if HMRC names your agency in a public notice.

We have seen a 20-person SEO agency in Soho hit with a £140,000 tax bill after HMRC determined their six contractors were all inside IR35. The agency had been treating them as outside IR35 for three years. The contractors themselves were not penalised, because the liability falls on the fee payer, which was the agency.

Practical Steps for Agency Founders

Here is what you should do today to manage your IR35 risk:

  1. Review every contractor engagement you currently have. Go through each contract and working arrangement against the three tests: substitution, control, and mutuality of obligation.
  2. Run each engagement through CEST and document the results. Keep screenshots or PDFs of the output.
  3. Issue a Status Determination Statement for every contractor, even if you believe they are outside IR35. The SDS must state your decision and the reasons for it. You must pass this to the contractor and to any agency in the supply chain above you.
  4. Review your contracts. Standard employment-style contracts will get you into trouble. Your contractor agreements should reflect genuine self-employment: right of substitution, no guaranteed hours, no holiday or sick pay, the contractor provides their own equipment.
  5. Separate your contractors from your employees. Do not invite contractors to team social events. Do not give them company email addresses. Do not include them in performance reviews. The more you treat them like employees, the harder it is to argue they are outside IR35.
  6. Get professional advice on borderline cases. If you are unsure, pay for a proper IR35 review. Our ICAEW qualified team can review your contractor arrangements and give you a clear recommendation.

IR35 and Your Agency's Financial Planning

The inside outside IR35 agency decision affects your cash flow and your contractor's willingness to work with you. Outside IR35 contractors typically demand lower day rates because they keep more of their earnings. Inside IR35 contractors need higher rates to compensate for the tax hit. Your agency's gross margin on contractor-delivered projects will differ depending on which status applies.

Factor IR35 status into your project pricing. If you know a contractor is inside IR35, budget for employer NI at 13.8% on top of their rate, plus the administrative cost of running payroll. If they are outside IR35, you avoid those costs but need to ensure your contracts and working practices support the status.

For agencies planning an exit, contractor status matters. Potential buyers will scrutinise your contractor arrangements during due diligence. A clean IR35 record with proper SDS documentation makes your agency more attractive. A history of treating contractors as outside IR35 without proper evidence is a red flag that can kill a deal or reduce your valuation.

What About Umbrella Companies?

Some agencies use umbrella companies to handle contractor payroll. The contractor joins the umbrella company as an employee, and the umbrella handles PAYE, NI, and pension. This removes the payroll burden from your agency, but it does not remove your responsibility for getting the status determination right.

If HMRC determines your contractor was inside IR35 and the umbrella company did not operate PAYE correctly, HMRC can still come after your agency as the fee payer. Do not assume that using an umbrella company absolves you of IR35 responsibility. It does not.

IR35 and International Contractors

If your agency engages contractors based outside the UK, IR35 rules still apply if the work is performed in the UK or if the contractor is UK tax resident. For contractors based in the UAE or other jurisdictions, the rules around employment status and tax residence are different. You should take specific advice on cross-border engagements.

We work with several agencies that have contractors in Dubai. Those contractors are typically outside IR35 because they are genuinely running their own businesses from a different jurisdiction, but the status determination still needs to be documented properly. Our team can advise on digital agency contractor arrangements specifically.

The Bottom Line

The difference between inside and outside IR35 is not about what you call the arrangement. It is about the reality of how the contractor works. HMRC looks at the substance, not the labels. A contract that says "self-employed" means nothing if the contractor turns up at your office every day, uses your equipment, and takes instructions from your client's project manager.

If your contractor mix has changed in the last 12 months, ask your accountant before year-end. Review every engagement. Issue SDS documents. And if you are unsure, get professional advice. The cost of a review is trivial compared to the cost of an HMRC investigation.

For agency founders who want to structure their contractor arrangements properly, get in touch with our team. We help agencies across the UK and UAE navigate IR35, contractor compliance, and tax-efficient structures.