What a Good HR Department Structure for 100 Employees Looks Like  

At 100 employees, informal HR arrangements can stop being enough. Recruitment, absence, performance management, employee relations and compliance can quickly become too much for an office manager or finance team to handle alongside their core responsibilities. 

For UK employers, the HR team size for 100 employees depends on workforce complexity, growth plans and the level of HR support already available. Understanding your HR staff requirements for 100 employees is about having the right expertise in place, rather than simply increasing headcount. 

The regulatory environment also makes structured HR increasingly important. From 1 January 2027, the qualifying period for ordinary unfair dismissal protection is due to fall to six months, while the existing cap on compensatory awards is due to be removed, as outlined in the UK Government’s guidance on unfair dismissal rights. 

The right HR department structure for 100 employees should match the needs of the business. This guide covers practical options, including a lean small business HR structure, typical roles and salary costs, and when to hire an HR manager. 

For the wider growth journey, see our guide to how to structure your HR department as your business grows. 

By the end, you will know which role to hire first and how to build your HR function as your business grows. 

Key Takeaways 

• Most 100-employee UK businesses run one to two dedicated HR staff, usually an HR Manager supported by an HR Administrator or HR Officer. 

• A workable HR to employee ratio at this size sits between 1 to 50 and 1 to 100. Complexity matters far more than raw headcount. 

• Budget for the full employment cost, not just salary, including employer National Insurance, pension contributions and benefits. 

• For most 100-person businesses, an HR Manager is the more practical senior hire, while an HR Director is better suited to organisations with greater strategic, board-level or transformation needs. 

• Keep employee relations, recruitment and manager coaching in-house. Payroll, occupational health and complex legal advice are cheaper bought in. 

Contents 

1. What Good HR Department Structure for 100 Employees Looks Like 

2. How Many HR Staff Do You Need for 100 Employees 

3. The Core Roles in a 100-Person HR Team 

4. Three HR Structure Models to Choose From 

5. When to Hire an HR Manager and Who Comes Next 

6. What to Keep In-House and What to Outsource 

7. Building Your Structure, a Five-Step Framework 

8. Conclusion 

What Good HR Department Structure for 100 Employees Looks Like 

For many UK businesses with 100 employees, a practical HR structure starts with one experienced HR Manager supported by an HR Administrator or part-time HR Advisor. The HR Manager typically reports to the Managing Director, Chief Executive or Finance Director, depending on the organisation’s structure. Payroll may sit within Finance or be outsourced, while specialist services such as occupational health and employment law advice can be bought in as required. 

The exact HR team size for 100 employees will vary by workforce complexity, growth plans, employee relations needs and the systems already in place. A well-configured HR system can reduce administrative workload, automate routine processes and give managers greater self-service capability. But three things should be in place: 

• Clear ownership. One named person sets policy, holds the risk register and answers to the board. Splitting responsibility between finance, operations and line managers can leave important gaps. 

• Enough capacity. At a 15% annual turnover rate, a 100-person workforce could generate around 15 recruitment campaigns a year, alongside absence, family leave, performance and employee relations matters. 

• A clear escalation route. Managers need access to appropriate HR support before an employee issue develops into a formal grievance or employment dispute. 

At this size, HR cannot be treated as a side responsibility. Employers also need sufficient capacity to keep pace with changing employment requirements. From 6 April 2026, employers must retain adequate records showing compliance with specified annual-leave and holiday-pay obligations for six years. From 1 October 2026, the time limit for bringing many employment tribunal claims increased from three months to six months, subject to claim-specific and transitional rules. From 30 October 2026, employers will need to take all reasonable steps to prevent sexual harassment and must not permit harassment of employees by third parties such as customers or clients. ACAS Employment Rights Act 2025 provides the latest implementation timetable. 

The goal is not to build the largest HR team. It is to build enough internal capability to manage people effectively, with specialist support available when the business needs it. 

How Many HR Staff Do You Need for 100 Employees 

For many UK businesses at this size, one to two dedicated HR staff is a practical starting point. One experienced HR Manager may be sufficient for a stable, single-site business with straightforward people needs. Additional support becomes more relevant where there is rapid growth, multiple sites, shift or unionised workforces, or frequent employee relations activity. 

The 1:100 ratio is a useful starting point, not a staffing formula. Large organisations may have shared service centres and specialist teams supporting that ratio, resources that smaller businesses do not necessarily have. Based on our experience recruiting HR leaders across the UK, we generally see a working range of one HR professional for every 50 to 100 employees, depending on workforce complexity. See our guide to the ideal HR to employee ratio for more context. 

Use these factors to assess where your business sits: 

Factor	One HR person is enough	You need two or more
Growth rate	Headcount stable or growing under 10% a year	Growing 25% or more or acquiring
Sites	Single site, one location	Multiple sites or field-based teams
Workforce mix	Salaried, office-based, low turnover	Shift work, seasonal labour, turnover above 20%
Employee relations	One or two live cases a year	Regular grievances, disciplinaries or long-term absence
Systems	Established HR system with manager self-service	Spreadsheets, paper files, manual payroll input
Change activity	Business as usual	Restructure, acquisition or major system implementation

If most of your answers sit in the right-hand column, plan for additional HR capacity rather than waiting for the workload to become unmanageable. For the earlier growth stage, see our guide to HR structure for companies with 50 employees. 

The Core Roles in a 100-Person HR Team 

A 100-person business does not need every HR role in-house. It needs clear ownership of the work that matters most. An HR Administrator handles day-to-day transactions and employee records. An HR Officer or HR Advisor manages employee casework and supports line managers, while an HR Manager takes ownership of policy, recruitment strategy and senior reporting. As the workforce grows beyond around 150 employees, a Head of HR or HR Director may be appropriate where the business needs greater strategic leadership, sometimes instead of an HR Manager. An HR Business Partner may also be recruited at this stage, depending on the organisation’s structure and needs. Specialist areas such as employment law and occupational health can be outsourced, while payroll may either sit within Finance or be outsourced. 

The small business HR structure should reflect the workload, not simply the size of the workforce. Combining too many administrative tasks with senior HR responsibilities can quickly reduce the value of an experienced hire. 

Role	What the role owns	Typical UK salary outside London
HR Administrator	Contracts, onboarding paperwork, records, payroll input, absence logging	£28,000 to £32,000
HR Officer	First-line queries, straightforward casework, recruitment coordination, reporting	£32,000 to £38,000
HR Advisor	Disciplinaries, grievances, absence management, manager coaching	£35,000 to £45,000
HR Manager	Policy, employee relations risk, recruitment strategy, board reporting	£45,000 to £65,000
HR Business Partner	Commercial partnering with a division, site or function	£52,000 to £68,000
HR Director, fractional	Governance, people strategy, board assurance, one to two days a month	£700 to £1,500 a day

Salary levels vary by location and experience. London and the South East can command a premium of around 10% to 20%, while employers should also budget for employer National Insurance, pension contributions and benefits. 

For a closer breakdown of responsibilities, see our guide to the essential roles in a small HR team. 

Three HR Structure Models to Choose From 

For a 100-person UK business, three HR structures cover most practical needs: a solo HR Manager, an HR Manager with administrative support or an HR Manager plus HR Advisor with fractional HR Director oversight. The right model depends on workforce complexity, growth plans, employee relations activity and the level of management capability already in place. 

Model One: The Solo Generalist 

One experienced HR Manager reports to the Managing Director or Chief Executive and owns the full HR function. This can work for a stable, single-site business with capable line managers and relatively low employee relations activity. 

The trade-off is capacity. There is no cover during annual leave, sickness or resignation, and routine administration can take time away from higher-value HR work. 

Model Two: HR Manager Plus HR Administrator 

For many growing businesses at 100 employees, this provides the most practical balance between HR expertise and day-to-day capacity. The HR Administrator handles contracts, onboarding, records and payroll input, allowing the HR Manager to focus on employee relations, recruitment, policy and senior stakeholder support. 

In our experience, separating these responsibilities can help shorten hiring timelines, with some businesses seeing reductions of two to four weeks as interview scheduling and offer management become easier to manage. 

Model Three: HR Manager Plus HR Advisor With Fractional Director Oversight 

This suits private equity backed businesses, multi-site operations and anyone planning a restructure or acquisition. The HR Advisor adds capacity for employee relations casework and manager support, while a fractional HR Director can provide senior-level people strategy and board advice. 

It provides greater strategic depth without requiring a permanent HR Director appointment. The key is to define responsibilities clearly so that the HR Manager, Advisor and fractional Director are not duplicating each other’s work. 

Model	Team	Annual salary cost	Best suited to	Main weakness
Solo generalist	1 HR Manager	£45,000 to £65,000	Stable single-site businesses with mature managers	No cover, senior time lost to administration
Manager plus administrator	HR Manager and HR Administrator	£73,000 to £97,000	Most growing businesses at 100 staff	Limited employee relations depth
Manager, advisor and fractional director	HR Manager, HR Advisor, fractional HR Director	£80,000 to £110,000 plus day rate	Private equity backed, multi-site or transforming businesses	Highest cost, needs clear role boundaries

The right structure is the one that gives your business enough capacity to manage today’s workload while leaving room for what comes next. 

When to Hire an HR Manager and Who Comes Next 

Hire an HR Manager once employee relations casework, recruitment volume or compliance risk exceeds what an outsourced or part-time arrangement can absorb. Many businesses reach this point at around 100 employees, but workload and organisational complexity should guide the decision, not headcount alone. 

Five signs indicate that the timing may be right: 

1. Managers are handling dismissals without a consistent, documented process. 

2. You have faced a grievance or a tribunal threat in the last 12 months. 

3. Standard vacancies regularly take more than 10 weeks to fill. 

4. Your leadership team spends more than half a day a week on people administration. 

5. An investor, acquirer or major customer is asking for evidence of people governance. 

The next hire should address the biggest operational gap left once the HR Manager takes ownership. An HR Administrator can take on transactional work, while an HR Advisor adds greater casework capacity. Where the business needs board-level people strategy, a fractional or permanent HR Director may be the next step. 

Whether you start with a manager or a director is a genuine question at this size, and the answer depends on what the board needs rather than on headcount. We explore the key considerations in Should You Hire an HR Director or HR Manager First. 

What to Keep In-House and What to Outsource 

Keep work that relies on relationships, context and day-to-day judgement in-house. Outsource work that is transactional, specialist or required only occasionally. For a 100-person business, this approach can provide access to specialist expertise without building every capability into the permanent HR team. 

• Keep in-house: Employee relations casework, recruitment and selection, onboarding, manager coaching, performance management and employee engagement. 

• Buy in:  Occupational health, high-level employment law advice, health and safety support and specialist reward benchmarking. Payroll can either be outsourced or managed by Finance. 

• Decide case by case: Learning and development delivery, HR system implementation, and senior or executive recruitment. 

The important distinction is ownership. An external provider can advise on a complex employment law issue or process payroll, but someone inside the business should remain accountable for the employee experience and the resulting decision. 

Th CIPD provides guidance and resources on HR outsourcing and the use of external specialists. Outsourcing should extend your HR capability, not replace internal ownership of important people decisions. 

Building Your Structure, a Five-Step Framework 

The right HR structure should be built around the work your business needs done, not a generic employee-to-HR ratio. Start with the workload, identify where risk and capacity gaps sit, then build the team around those findings. 

For a 100-person business, this five-step framework provides a practical way to decide what to hire, when to hire it and where external support may be more appropriate. 

1. Audit the work  

Track every people task for four weeks, who does it and how long it takes. This can help leadership teams identify the time spent on HR activities across finance, operations and the Managing Director’s role. 

2. Assess your exposure  

List current employee relations issues, policy gaps and compliance deadlines, then assess the potential operational and financial impact of each gap. This gives senior leadership a clearer basis for deciding where dedicated HR expertise is needed. 

3. Choose your model  

Use the HR structure models above alongside your plans for the next 12 to 18 months. Consider workforce growth, locations, turnover, employee relations activity, management capability and any planned restructuring or acquisition. Choose the structure your business is moving towards, not simply the one that matches today’s headcount. 

4. Sequence the hires 

Once the structure is clear, decide which capability is missing first. In many businesses, that means appointing an experienced HR lead before adding administrative or advisory support. The first hire should close the biggest capability gap, not simply absorb the easiest tasks to delegate. Once the senior HR role is established, its holder can help define the responsibilities and profile of subsequent hires. 

5. Set a review trigger 

HR structures should evolve as the business changes. Agree the headcount or event that forces a structure review, such as 150 employees, a second site or an acquisition. Do not wait for the HR function to become overwhelmed before reviewing its capacity. A predefined trigger makes workforce planning a regular management decision rather than a reaction to a crisis. 

Three Mistakes We See Repeatedly 

• Promoting an office manager into HR without the right expertise or support. Good intentions do not survive when employee relations or discrimination issues arise. 

• Hiring two junior people instead of one experienced HR professional. The administrative workload gets covered, but the business can still be left without the judgement needed for complex people issues. 

• Keeping HR under the Finance Director indefinitely. It works administratively but it can limit HR’s influence over workforce planning, management capability and wider people decisions. 

Conclusion 

For most UK businesses with 100 employees, one to two dedicated HR professionals provide a practical starting point. An experienced HR Manager can lead the function, with administrative or advisory support added as workload and complexity increase. Payroll, employment law and other specialist services can be outsourced where appropriate. 

The right structure depends on more than headcount. Growth plans, workforce complexity, employee relations activity and management capability should all inform the level of HR support you put in place. 

Start with the biggest capability gap, then build the function around the work that follows. Review the structure as the business grows or enters significant change, such as rapid expansion, a new site or an acquisition. 

For more on structuring the function at other stages, including 50-employee and 250-plus businesses, see our complete guide to how to structure your HR department as your business grows. 

Not sure whether your 100-person business needs an HR Manager, HR Advisor or additional support? Our specialist HR recruiters can help you identify the right capability for your current structure and growth plans.  

This is general guidance, not legal advice. For case-specific employment law decisions, consult the Advisory, Conciliation and Arbitration Service or qualified legal counsel.