If a client has told you that you are inside IR35, or you are trying to work out your own status before signing a new contract, you need to understand what that determination actually means for you - not in abstract tax terms, but in terms of what you take home, how you work, and what your options are.
IR35 - more precisely, Chapter 8 of the Income Tax (Earnings and Pensions) Act 2003, and its related off-payroll working rules in Chapter 10 - is HMRC's framework for deciding whether a contractor who works through a limited company (often called a personal service company, or PSC) should be taxed like an employee. The determination has real financial consequences. This guide explains both statuses, how to assess yours, and what to do if you disagree with a client's decision.
Inside IR35 vs Outside IR35: The Core Difference in Plain English
The simplest way to think about it: outside IR35 means HMRC accepts you are genuinely self-employed for tax purposes. Inside IR35 means HMRC considers you to be, in substance, an employee of your client - even though you are contracting through a limited company.
The rules exist because HMRC identified a pattern where workers were delivering what looked like permanent, employee-style work through limited companies, then paying themselves in dividends rather than salary - legitimately reducing their Income Tax and National Insurance contributions. IR35 was designed to close that gap.
Personal Service Company (PSC)
A PSC is a limited company set up and operated by a single contractor, through which they invoice clients and receive payment. The contractor is typically the sole director and shareholder. IR35 applies specifically to this structure.
The distinction matters enormously in practice. Outside IR35, you receive your full contract fee into your company and can manage your income tax-efficiently - taking a salary and dividends, making pension contributions, and deducting legitimate business expenses. Note that the annual dividend allowance is currently £500 (significantly reduced from prior years), and as of April 2026 the basic rate of dividend tax is 10.75% and the higher rate is 35.75% - both up 2 percentage points from the previous year.
The tax efficiency of this approach should be reviewed against current rates with an accountant. Inside IR35, income tax and National Insurance are deducted at source before you see the money, much like an employee's payslip.
What Being Inside IR35 Means for Your Tax and Take-Home Pay
When you are inside IR35, the fee paid to your company is treated as a deemed salary. Income Tax and National Insurance Contributions (NICs) are calculated on that income in the same way they would be for an employee. Employer's NICs are also due - and in practice, those costs come out of the same contract rate.
The result is a substantially lower effective take-home compared to an equivalent outside IR35 contract at the same day rate. The exact difference depends on your rate, your circumstances, and the contract structure - but the gap is significant enough that many contractors negotiate higher rates for inside IR35 engagements to compensate.
A critical misconception to clear up
Being inside IR35 for tax purposes does not make you an employee in employment law terms. You do not automatically gain employment rights - sick pay, holiday pay, unfair dismissal protection - by being determined inside IR35. Tax treatment and employment status are assessed under separate legal frameworks.
Who makes the inside IR35 determination depends on who your client is. Since April 2021, medium and large private sector organisations (and all public sector bodies) have been responsible for assessing the status of contractors they engage.
Note that the definition of 'small' (and therefore exempt) companies is changing: raised financial thresholds - turnover no more than £15m, balance sheet total no more than £7.5m, no more than 50 employees - apply to accounting periods starting on or after 6 April 2025, meaning some organisations previously classed as medium will progressively move out of scope from April 2026 onwards.
If your client is a small company under these thresholds, the responsibility for determining your own IR35 status still sits with you.
What Being Outside IR35 Means - and What You Must Be Able to Demonstrate
Outside IR35 is not just a label you choose. It is a status you must be able to substantiate if HMRC or a client asks. The working arrangements that exist in practice - not just what your contract says on paper - are what matter.
Outside IR35 contractors typically have genuine flexibility: they can work for multiple clients, send a substitute to carry out the work if they are unavailable, are not directed in how or when they work in the way an employee would be, and are engaged for a specific deliverable rather than indefinite availability.
Your contract should reflect the genuine nature of the engagement - not be a template designed to tick IR35 boxes while the working reality tells a different story
You should be able to show you have (or could realistically exercise) a right of substitution - sending someone else to do the work in your place
You should not be subject to day-to-day supervision and control in the way an employee would be
There should be no expectation of ongoing, open-ended work once a project ends - mutuality of obligation should be limited
HMRC looks at the totality of the relationship. A well-drafted contract helps, but it cannot override working arrangements that point clearly in the other direction.
How to Use HMRC's CEST Tool to Assess Your Status
HMRC's Check Employment Status for Tax (CEST) tool is the starting point for most IR35 assessments. It asks a structured set of questions about your working arrangements and returns a determination: employed, self-employed, or unable to determine.
CEST is useful as a structured self-assessment prompt - it forces you to think carefully about substitution, control, and mutuality of obligation in a methodical way. But there are real limitations you need to understand before treating its output as your final answer.
Go to the HMRC CEST tool on GOV.UK and work through every question carefully - answer based on how the engagement actually works, not how you wish it worked
Save or print your result - HMRC states it will stand by CEST determinations where the tool has been used correctly and the information entered accurately reflects actual working arrangements, but this is not an absolute guarantee. HMRC can and has challenged CEST results during investigations where it believes inputs did not reflect reality, so treat the result as one part of a broader compliance record rather than definitive protection.
If the result is 'unable to determine', that is not a pass. It means the facts are genuinely borderline and you should seek specialist IR35 advice before proceeding
Run the assessment again if working arrangements change - a new contract or a change in how you work could change the determination
CEST is an indication, not a guarantee
HMRC's commitment to stand by CEST results is conditional on the answers being accurate. If you answer the questions in the way you would like them to be rather than how they actually are, the determination offers no protection.
CEST was historically criticised for not properly addressing mutuality of obligation (MOO), but the April 2025 update introduced an explicit MOO question, removing the previous blanket assumption.
Nonetheless, critics note that the underlying decision logic remains largely unchanged, and CEST should be used alongside professional advice rather than as a standalone compliance tool.For high-value or complex contracts, a specialist IR35 review is worth considering.
The Three Key Tests HMRC Uses to Determine IR35 Status
HMRC's IR35 assessment comes down to three primary tests. Understanding these helps you evaluate your own position - and respond intelligently if a client's determination surprises you.
The Three IR35 Status Tests
Substitution
Can you send a suitably qualified substitute to carry out the work in your place, without the client's personal approval of that individual? A genuine, unrestricted right of substitution is a strong indicator of self-employment. If the client is paying for you specifically - your skills, your presence, your personal output - that points toward inside IR35.
Control
Does the client control what you do, how you do it, where you work, and when? Employee-style supervision and direction is a strong indicator of inside IR35. Outside IR35 contractors typically control their own methods and schedule, delivering an agreed outcome rather than following a manager's day-to-day instructions.
Mutuality of Obligation
Is there an expectation on both sides that work will continue to be offered and accepted beyond any specific project? Employees typically have an ongoing obligation to turn up and be paid; employers have an obligation to provide work. The closer your arrangement resembles that ongoing mutual expectation, the more it points toward inside IR35.
No single test is conclusive on its own. HMRC and tribunals look at the overall picture. A contract that scores well on substitution but fails completely on control and mutuality is still likely to be inside IR35.
What to Do If a Client Says You Are Inside IR35 and You Disagree
When a medium or large client issues a Status Determination Statement (SDS) placing you inside IR35, you have a formal right to dispute it. The client is legally required to have a status disagreement process in place and must respond to your challenge within 45 days.
Before you dispute, do the work. Run CEST yourself with honest answers. If the result is outside IR35 and you can demonstrate why your working arrangements genuinely support that, you have grounds for a formal challenge. If CEST returns inside IR35 when you answer accurately, the honest conclusion is that the client's determination is probably correct.
Request the client's Status Determination Statement in writing - they are legally required to provide one with their reasoning
Complete CEST yourself, answering every question based on actual working arrangements
Identify the specific points where your assessment and the client's differ - substitution rights, control arrangements, project scope
Submit a written disagreement to the client, referencing the SDS and explaining your reasons clearly
Consider engaging an IR35 specialist for contracts where the financial stakes justify the cost - specialist reviewers can assess your contract and working practices, identify risks, and support a formal dispute if one is warranted
Before disputing, check the contract reflects reality
One common reason contractors lose IR35 disputes is that their contract says the right things but the working arrangements tell a different story. If you are disputing an inside IR35 determination, make sure the way you actually work - not just the written contract - genuinely supports an outside IR35 position.
Umbrella Company vs Limited Company: Which Makes More Sense Inside IR35?
If a contract is genuinely inside IR35, operating through your limited company still works - but the tax efficiency advantage largely disappears. The deemed salary calculation means you are taxed in a broadly similar way to an employee.
Running a limited company has ongoing costs: accountancy fees, Companies House filing (the annual confirmation statement fee is currently £50 for digital filing, and digital incorporation now costs £100 - both increased significantly in 2024 and again in February 2026), and admin time. For a contract that is clearly inside IR35, those costs may not be justified.
An umbrella company is an employer that employs contractors directly, processes payroll, and handles employer responsibilities. For inside IR35 work, an umbrella company simplifies the process considerably - you are paid as an employee of the umbrella, with tax and NICs handled automatically. The umbrella takes a margin for this service, typically a fixed weekly or monthly fee.
The right answer depends on your broader situation. If you have a mix of inside and outside IR35 contracts, keeping your limited company active makes sense - you need it for the outside IR35 work. If all your current contracts are inside IR35 and you do not foresee that changing, the umbrella route is often the more practical and cost-effective option.
Umbrella companies are not all equal
The UK umbrella market has a small number of non-compliant operators who promote tax avoidance schemes. These schemes are illegal. Under rules in force from 6 April 2026, where an umbrella fails to remit PAYE and National Insurance correctly, primary liability falls on the recruitment agency in the supply chain - or on the end client where no agency is involved - rather than on the contractor.
Contractors who knowingly participate in avoidance schemes may still face personal liability, but the new joint and several liability rules are specifically designed to protect workers from unexpected tax bills caused by non-compliant umbrellas.
Prefer FCSA-accredited or Professional Passport-approved umbrella companies, but be aware that these accreditations are not a statutory defence under the new rules — always verify current accreditation status directly and check HMRC's published list of named tax avoidance schemes.
If you are unsure which structure suits your current contract mix, a contractor-specialist accountant can model the net take-home under both options based on your actual rates - that comparison is worth having before you commit to either route.
Illustrative example - based on a common UK founder scenario, not a specific documented case
A freelance IT developer in her third year of contracting through a limited company is offered a 6-month engagement with a large financial services firm. The client issues an SDS placing her inside IR35. She runs CEST herself, answering honestly - the role involves dedicated desk space at the client's offices, fixed hours, and daily stand-ups with the client's permanent team. CEST returns 'employed'. She accepts the determination, negotiates a higher day rate to partially offset the additional tax cost, and uses an FCSA-accredited umbrella company for the duration of the contract - keeping her limited company dormant but registered for when she returns to outside IR35 work.
Cut Through the Noise - Get the BGE Newsletter
Get Practical Guidance You Can Use This Week
Ready to cut through the noise? Join the BGE newsletter for practical guidance, tool recommendations, and real-world insights for UK founders and business owners - delivered weekly to your inbox. No fluff, no spam, unsubscribe any time.
BGE newsletter
