Local authorities across the UK rely on locum social workers to maintain service continuity, manage spikes in demand, and cover permanent vacancies. Yet engaging off-payroll workers through intermediaries introduces a specific set of tax compliance obligations that too many commissioning teams overlook until a problem arises. The IR35 rules for social work locums are not a peripheral concern; they are a core governance issue that directly affects budget forecasting, financial risk, and the legal relationship between the authority, its agency partners, and the locum.
Failing to apply the off-payroll working rules correctly can lead to HMRC investigations, backdated tax liabilities, and penalties. For local authorities in England, Scotland, and Wales, the responsibility for determining IR35 status sits squarely with the public sector body acting as the end client. This article sets out what commissioners, HR teams, and finance officers need to know to make informed, defensible decisions.
Understanding the IR35 rules for social work locums
The off-payroll working rules, commonly referred to as IR35, were introduced to tackle what HMRC terms "disguised employment". They apply when an individual provides their services to a client through an intermediary, typically a personal service company, but would be regarded as an employee if they were engaged directly. For social work locums, the intermediary is often a limited company owned by the worker, and the agency sits between the locum and the local authority.
Since April 2017, public sector bodies that engage workers through intermediaries must determine whether the assignment falls inside or outside IR35. The local authority, as the fee payer, carries the liability for any underpaid tax and National Insurance contributions if it fails to take reasonable care in making that determination. This is not a matter of contractual convenience; it is a statutory obligation under Chapter 10 of the Income Tax (Earnings and Pensions) Act 2003.
An inside IR35 determination means the locum is treated as a deemed employee for tax purposes. The local authority, or the agency on its behalf, must deduct income tax and employee National Insurance contributions from the payments made to the intermediary, and must also account for employer National Insurance contributions and the apprenticeship levy where applicable. An outside IR35 determination means the locum can continue to invoice through their limited company, and the authority does not operate PAYE or pay employer NICs.
The outcome matters because it changes how the total cost of a locum sits within the authority's budget. Getting it wrong is not simply an administrative error; it can result in significant financial adjustments that have to be absorbed within already pressured children's and adults' services budgets.
Key factors that determine IR35 status for social workers
HMRC uses three main tests to assess employment status for tax purposes. Local authorities must consider these in the specific context of social work locum assignments.
Control. Does the local authority dictate how, when, and where the locum works? If a team manager allocates cases, sets deadlines for assessments, requires the locum to attend weekly supervision, and expects them to follow the authority's procedures and recording systems, that points strongly towards a relationship of control. A locum who is genuinely self-employed would typically have autonomy over their workflow and would not be subject to the same level of day-to-day direction as a permanent employee.
Substitution. Can the locum send someone else to perform the work, or is the contract for the personal service of that named individual? In social work, the reality is that the local authority has engaged a specific professional, often after a thorough vetting process. A substitution clause in a contract is rarely honoured in practice because the authority needs continuity and the individual must have the correct qualifications, DBS clearance, and knowledge of the caseload. If personal service is required, the substitution test fails and points towards employment.
Mutuality of obligation. Is there an ongoing obligation for the authority to offer work and for the locum to accept it? Most locum assignments are booked for a defined period or a specific piece of work, such as a service review or maternity cover, and there is no expectation of future work beyond that assignment. This lack of mutuality can support an outside IR35 determination. However, if a locum has been working continuously for the same authority for two years, with repeated extensions, the working relationship may begin to resemble permanent employment.
Other factors include whether the locum bears financial risk, provides their own equipment, and whether the parties intended to create a contract for services rather than a contract of service. In social work, the practical arrangements often override what is written in the contract, so local authorities must assess the actual day-to-day working practices, not just the paperwork.
Inside IR35 vs outside IR35: what it means for your budget
The distinction between inside and outside IR35 has direct consequences for the total cost to the authority and for the locum's net pay.
When a locum is inside IR35, the agency or the authority must process the payments through payroll. The amount paid to the locum's limited company is treated as deemed employment income. Income tax and employee National Insurance contributions are deducted from that amount. On top of this, the fee payer also pays employer National Insurance contributions at 13.8% and, for large employers, the apprenticeship levy at 0.5%. These employer costs are additional to the agreed contractor rate and must be budgeted for. What looks like a competitive hourly rate on paper can, after adding employer NICs and levy, become considerably more expensive than the headline figure suggests.
When a locum is outside IR35, the authority pays the agreed rate and no employer NICs are due. The locum's limited company handles its own tax affairs. For the authority, the cost is simpler to forecast, and the overall spend may be lower even if the hourly rate appears higher than an inside IR35 equivalent. However, outside determinations require robust evidence that the working arrangement genuinely reflects self-employment, and the authority must be prepared to defend that position if challenged.
Commissioning managers should work with finance colleagues to model the true cost of each assignment rather than comparing headline rates alone. An inside IR35 assignment may offer more predictable compliance but will carry additional payroll costs that need to be reflected in the budget. An outside IR35 assignment may appear more cost-effective but places a greater compliance burden on the authority to demonstrate that the determination is correct.
How to conduct an IR35 assessment for locum social work roles
Local authorities should follow a structured process when determining IR35 status for each locum engagement.
Start with HMRC's Check Employment Status for Tax (CEST) tool. It is not perfect, and it sometimes produces indeterminate results, but it provides a useful baseline and demonstrates that the authority has used HMRC's own recommended starting point. Enter the information honestly and in detail, reflecting the actual working practices rather than an idealised version of the arrangement.
Gather evidence from multiple sources. Obtain the contract between the agency and the locum, but do not rely on it exclusively. Speak to the team manager who will supervise the locum and ask specific questions about control: who sets the working hours, who allocates cases, who reviews the work, and what level of autonomy the locum will have. Review the practicalities of substitution: would the authority genuinely accept a replacement, or is the engagement personal to one named individual? Record the expected duration of the assignment and whether there is any mutuality of obligation beyond the current engagement.
Once you have reached a determination, you must produce a Status Determination Statement (SDS). The SDS must state whether the assignment is inside or outside IR35 and give the reasons for that conclusion. It must be provided to the agency and, through the agency, to the worker. Keep a copy of the SDS and all supporting evidence on file. If HMRC ever questions the determination, the authority will need to show that it took reasonable care and did not simply apply a blanket assumption.
Finally, review determinations regularly. The IR35 status of a locum assignment is not static. If the scope of the role changes, if the assignment extends beyond its original term, or if supervisory arrangements tighten, the original determination may no longer be valid. Build a review process into your locum management procedures, ideally at the point of each extension or after 12 months.
Common pitfalls in IR35 compliance for social work locums
Even experienced local authorities can fall into traps when applying the off-payroll rules. Recognising these pitfalls can help you avoid costly errors.
One of the most frequent mistakes is assuming that all locums are automatically outside IR35 because they are temporary. Temporary status, in itself, is not determinative. A locum who works full-time hours, under the same level of supervision as a permanent social worker, and who is fully integrated into the team, will almost certainly be inside IR35 regardless of the contract label.
Another pitfall is failing to look beyond the written contract. If the contract states that the locum has a right of substitution but the authority would not, in practice, accept an unfamiliar replacement without a full vetting process, HMRC will give weight to the reality on the ground. The actual working practices always take precedence over the contractual terms.
Local authorities sometimes fail to update the SDS when an assignment changes. A locum who initially worked on a discrete project with high autonomy may, over time, be absorbed into the day-to-day operational management of the team. If a team manager begins to exercise greater control, the original determination can become incorrect. The authority must revisit the SDS at that point, otherwise it risks being non-compliant.
A final common error is treating IR35 as a purely HR or finance issue. Line managers who commission locums often provide the information that feeds into the determination, but they may not understand the significance of their answers. Without proper training, a manager might describe the role as "just like a permanent social worker" without realising this points directly towards an inside IR35 finding.
Best practices for managing IR35 with agency social workers
A local authority that manages locum social workers effectively integrates IR35 compliance into its everyday commissioning processes. Several practical steps can make this easier.
Work with specialist recruitment agencies that understand both the social work sector and the IR35 rules. An agency that provides transparent, well-documented determinations and contracts that align with the actual working arrangements reduces the burden on the authority. When evaluating agency partners, look for those that do not use off-the-shelf contract templates but instead tailor the engagement to reflect the reality of each assignment. Our guide on evaluating social work recruitment agencies for local authorities sets out what to look for in a partner that takes compliance seriously.
Train hiring managers and team leaders on the basics of IR35. They do not need to become tax experts, but they do need to understand how their answers to questions about control, substitution, and mutuality shape the determination. A simple checklist and a short briefing session can prevent inaccurate data from entering the assessment process.
Carry out regular audits of live locum assignments. At least once a year, or at the point of any extension, review whether the working practices still align with the original SDS. Pay particular attention to locums who have been with the authority for more than 12 months, because the longer the engagement continues, the more likely it becomes that mutuality of obligation has crept in.
Keep meticulous records. For every locum assignment, maintain a file that contains the SDS, the supporting evidence, the CEST output, any correspondence with the agency, and notes of any review conversations. In the event of an HMRC enquiry, the quality of your documentation will be the difference between a clean outcome and a disruptive, expensive investigation.
Finally, maintain an open dialogue with your agency partners. If the nature of the role changes, inform the agency immediately so that the determination can be reviewed. A collaborative relationship built on transparency benefits everyone: the authority stays compliant, the agency avoids disputes, and the locum understands their tax position with clarity.
If your local authority needs a trusted partner to manage IR35-compliant locum social work placements, contact Vantis Workforce Solutions today. Our team understands the regulatory demands of the sector and works to ensure every determination is robust and transparent. Visit our contact page to discuss your requirements. For more information about our specialist social work recruitment, see our social work recruitment services.
Frequently asked questions
What is IR35 and how does it affect social work locums? IR35 is the shorthand for the off-payroll working rules that apply when an individual provides services through an intermediary, such as a limited company. If a locum social worker would be treated as an employee if engaged directly, the assignment is inside IR35, and the local authority, or the agency on its behalf, must deduct income tax and National Insurance contributions and account for employer costs.
How do I determine if a locum social worker is inside or outside IR35? The determination must be based on the actual working practices, not just the contract. Consider the degree of control the authority exercises, whether the locum is required to perform the work personally, and whether there is an ongoing mutual obligation beyond the current assignment. HMRC's CEST tool is a starting point, but it should be supplemented with evidence gathered from the team manager and a careful review of the day-to-day arrangements.
What are the financial implications of IR35 for my local authority? An inside IR35 assignment means the authority or agency must pay employer National Insurance contributions and, where applicable, the apprenticeship levy on top of the worker's gross pay. This increases the total cost compared to the headline rate. An outside IR35 assignment avoids these additional employer costs, but the authority must have robust evidence that the arrangement genuinely reflects self-employment. Commissioners should model the full cost of each assignment rather than comparing hourly rates in isolation.
What tools can I use to assess IR35 status? HMRC provides the Check Employment Status for Tax (CEST) tool, which is designed to help public sector bodies determine IR35 status. It is a useful initial step, but it should be used alongside direct evidence of working practices. The output should be retained as part of your compliance records alongside the Status Determination Statement.
What are common mistakes local authorities make with IR35? Common errors include assuming all locums are outside IR35 simply because they are temporary, failing to review actual working practices as opposed to contractual terms, not updating the determination when an assignment extends or changes, and not training line managers to describe the role accurately. Each of these can lead to incorrect determinations and potential HMRC liabilities.