IR35 is one of the most misunderstood areas of UK tax and employment status. The core question is straightforward. Would the contractor look like an employee if the limited company was removed from the arrangement? Everything else follows from that.
This guide sets out what IR35 means in practice, what HMRC looks at when deciding status, and what a good determination process looks like for both clients and contractors.
What is IR35?
IR35 is UK tax legislation that decides whether a contractor is genuinely self-employed or should be taxed as an employee.
If the contractor would look like an employee without the limited company in between, the engagement is inside IR35. If the arrangement looks like a genuine business supplying a service, it is outside IR35.
The purpose is simple. HMRC wants people doing employee work to pay employee taxes, regardless of how they invoice.
What is the difference between inside and outside IR35?
Inside IR35 means the engagement is treated as employment for tax purposes. Income Tax and employee National Insurance are deducted at source, and the fee-payer pays employer NI on top.
Outside IR35 means the contractor is treated as a genuine business. They invoice through their limited company and manage their own tax affairs.
The financial gap between the two is significant once Income Tax, National Insurance, employer NI, and any umbrella margin are accounted for. Getting the status wrong therefore has a material impact on the contractor’s take-home pay and the client’s cost base.
Who decides IR35 status?
Since April 2021, the end-client decides status in the private sector, unless the client is a small business.
The client must:
- Take reasonable care in making the decision
- Issue a Status Determination Statement (SDS) explaining the reasoning
- Share the SDS with the contractor and any agency in the supply chain
- Respond within 45 days if the contractor challenges the decision
If the client fails to take reasonable care, the tax liability can transfer to them.
For small private-sector clients, the contractor’s own intermediary still handles the determination under the original 2000 rules.
What does HMRC look at when deciding IR35 status?
There is no single test. HMRC and the tax tribunals weigh several overlapping factors drawn from decades of employment case law.
The five that matter most are:
- Personal service and substitution – is the contractor required to do the work personally?
- Control – how much does the client dictate what is done, how, when, and where?
- Mutuality of obligation – is there an ongoing expectation of offering and accepting work?
- Financial risk – does the contractor carry commercial exposure like a business would?
- Integration – is the contractor treated as part of the client’s organisation?
No factor alone settles the question. A weak position on one test can be balanced by a strong position on others. The overall picture matters more than any single indicator.
What is the substitution test?
A genuine contractor should not be personally tied to delivering the work.
If the contract allows the contractor to send a suitably qualified substitute at their own cost, and the client cannot refuse without a proper business reason, the engagement leans towards outside IR35.
The right must be real. HMRC and the tribunals look through clauses that exist on paper but would never be permitted in practice.
What counts as control in IR35?
Control looks at how much say the client has over:
- What the work is
- How it is done
- When it is done
- Where it is done
Every engagement has some oversight. Clients can reasonably expect deadlines, security policies, and quality standards to be followed.
The question is whether the contractor is being managed like an employee, or trusted to deliver an agreed outcome using their own expertise. The more freedom over the how, the stronger the outside IR35 position.
What is mutuality of obligation?
Mutuality of obligation looks at whether both parties expect the relationship to continue.
An employment relationship assumes the employer will keep offering work and the employee will keep accepting it.
A genuine contractor engagement is typically project-based, with no expectation that further work must be offered or accepted once the assignment ends.
What is financial risk in IR35?
An independent business normally accepts some commercial exposure. Common indicators of genuine financial risk include:
- Correcting mistakes at their own cost
- Holding professional indemnity or other business insurance
- Investing in their own equipment or software
- Quoting a fixed price for a piece of work
- Managing their own business expenses
Not every contractor will carry every risk. What matters is whether there is genuine evidence of business behaviour rather than payment for time.
What counts as integration into the business?
Integration looks at whether the contractor has become part of the client’s organisation in the way an employee would. Common indicators include:
- Receiving employee benefits
- Managing permanent members of staff
- Following the same HR procedures as employees
- Appearing on internal systems as though they were staff
- Being introduced externally as part of the team rather than as a supplier
Attending a team meeting or using a client laptop does not automatically place someone inside IR35. It is the overall pattern that counts.
Does the contract or the working practice matter more?
Working practice matters more. A well-written contract will not save an engagement where the reality tells a different story.
HMRC and the tax tribunals consistently place greater weight on how the arrangement actually operates than on what the paperwork says.
If the contract gives the contractor control but a manager directs their daily activities, the outside IR35 position weakens quickly.
The lesson is not to write weaker contracts. It is to make sure the contract, the SDS, and the working practices tell the same story from day one, and to revisit the position if the role changes during the assignment.
What happens if you get an IR35 determination wrong?
The client can be liable for unpaid Income Tax, employer and employee National Insurance, the Apprenticeship Levy, penalties, and interest.
The scale of the risk is real. HMRC’s reform of the off-payroll rules in the private and voluntary sectors generated £4.2 billion in additional tax and National Insurance between October 2019 and March 2023 (Source: HMRC, Update to the impacts of the 2021 off-payroll working rules reform in the private and voluntary sectors, 2023).
The public sector, running on equivalent rules since 2017, shows how easily things go wrong. The National Audit Office reported that central government departments and agencies owed HMRC £263 million in 2020 to 2021 alone as a result of incorrect administration of the off-payroll rules (Source: National Audit Office, Investigation into the implementation of IR35 tax reforms, February 2022).
If government departments with in-house tax teams can misapply these rules on that scale, private-sector businesses without those resources should take the risk seriously.
Can you apply the same IR35 decision to every contractor?
No. Blanket determinations do not meet HMRC’s reasonable care requirement.
Every contractor is different. Their role, contract, and working practices must be considered individually.
Copying a decision across every engagement, or refusing to consider outside IR35 as a matter of policy, is not compliance. It is the absence of a decision, and it carries both regulatory and commercial risk.
How does IR35 affect hiring interim talent?
This is where many businesses miss the wider picture.
A large proportion of senior interim professionals in finance, transformation, technology, and project delivery work through personal service companies by choice.
When a business closes the door to outside IR35 engagements as a blanket policy, three things tend to happen:
- Access to experienced interim talent narrows sharply
- Day rates rise, because contractors need higher gross pay to reach comparable take-home
- Time-to-hire on critical roles slows down
Contractors have adapted too. The best ones now read a determination process the same way a candidate reads an interview.
Clients who treat status determination as a considered, contract-by-contract exercise attract stronger interim talent than those who default to inside IR35 on day one.
What does a good IR35 determination process look like?
The businesses that handle IR35 well share a small number of habits:
- They agree the shape of the engagement before the contractor starts
- They involve the hiring manager in the determination, not just HR or finance
- They document their reasoning clearly in the Status Determination Statement
- They treat inside and outside IR35 as two legitimate options, chosen on the facts
- They review the position if the role changes materially
- They train hiring managers on what they can and cannot do without undermining the SDS
Reasonable care is not a single act at onboarding. It is a series of deliberate choices across the life of the engagement.
How can Four Squared Recruitment help?
Four Squared Recruitment works with clients across the Midlands and beyond on interim and contract assignments. We help hiring businesses structure roles, design engagements, and think through status determinations with the working relationship, not just the contract, in mind.
If you would like a second view on a Status Determination Statement, or want to talk through how your contracting arrangements are affecting your ability to attract the right people, please feel free to get in contact to discuss your recruitment needs or to explore how we can support your interim and contract requirements.
Written by Melanie Poulter, Interim Services Manager, Four Squared Recruitment
Contact Details
DD: 0121 669 0050
M: 07854 254647
E: melanie.poulter@four-squared.com
This article is intended as general information only and does not constitute tax, legal, or employment advice. IR35 determinations depend on the specific facts of each engagement, and the position can change as legislation and case law evolve. Businesses and contractors should seek advice from a suitably qualified professional based on their own circumstances before acting on any of the information above.
