Conducting Workplace Investigations: A Practical Guide for HR

A grievance is raised, a harassment complaint lands on your desk, or a manager flags a possible disciplinary breach. Before any hearing, warning or dismissal can be fair, someone has to establish the facts — calmly, impartially and without prejudging the outcome. That is what a workplace investigation is for. Get it wrong and even a genuine case of misconduct can unravel at an employment tribunal on procedural grounds. This guide sets out how HR, managers and investigators can run a workplace investigation that stands up to scrutiny, in line with the Acas Code of Practice on disciplinary and grievance procedures.

When Does an Investigation Need to Happen?

Not every complaint needs a full formal investigation, but most disciplinary and grievance matters do. Acas guidance is clear that the amount of investigation required will depend on the nature and seriousness of the allegation — a minor timekeeping issue needs far less than an allegation of bullying, harassment or fraud. What matters is that some reasonable investigation takes place before a decision is made. Skipping this step, or treating it as a formality once a manager has already decided the outcome, is one of the most common reasons a dismissal or disciplinary sanction is later found unfair.

Planning the Investigation: Scope, Timeline and Investigator

A good investigation starts with a plan, not an interview. Before speaking to anyone, set out:

• The allegation or issue, written clearly and specifically — vague terms of reference lead to a vague, challengeable report.
• The scope — what is, and is not, being investigated. Scope creep (chasing unrelated issues that surface along the way) delays outcomes and can feel like a fishing expedition to those involved.
• Who will investigate. Acas guidance recommends using someone who was not involved in the events in question and, wherever the organisation is large enough, someone different from whoever will later hear any disciplinary or grievance case. In smaller workplaces this separation is not always possible, but any conflict of interest should be identified and managed.
• A realistic timeline. The Acas Code does not set a fixed deadline, but it does expect matters to be dealt with promptly and without unreasonable delay. A rough target — and keeping the complainant and respondent updated if it slips — helps manage anxiety on all sides and reduces the risk of the process itself being challenged as unfair.

Put the plan in writing, even briefly. It gives the investigation a clear boundary and something to refer back to if anyone later questions why certain lines of enquiry were or were not pursued.

Interviewing Complainants, Respondents and Witnesses Fairly

Investigation meetings are fact-finding, not disciplinary hearings, but they still need structure and fairness:

• Prepare questions in advance based on the allegation, but stay open to following up on what actually comes out in the room.
• Ask open, non-leading questions. "Tell me what happened" gets better evidence than "Did he shout at you?" Leading questions can taint the evidence and make findings easier to challenge later.
• Interview the complainant first, then the respondent, then witnesses — giving the respondent a fair chance to know, in general terms, what is alleged and to respond to it.
• Keep an accurate record of every meeting, ideally agreed or shared with the interviewee afterwards, and note who was present.
• Treat everyone with respect, regardless of which "side" they appear to be on. An investigator is not there to build a case against anyone — only to find out what happened.

Note that investigation meetings are not the same as a formal disciplinary or grievance hearing, so the statutory right to be accompanied does not automatically apply to them. Many employers still allow a colleague to attend as good practice, particularly for sensitive allegations — check your own policy and apply it consistently.

Gathering Evidence and Maintaining Confidentiality

Alongside interviews, an investigator should gather any relevant documentary evidence: emails, messages, CCTV, rotas, expense records, HR files or clocking data, as appropriate to the allegation. Evidence should be preserved promptly, since systems can auto-delete messages or overwrite CCTV footage within days or weeks.

Confidentiality has to be managed carefully throughout. Information should be shared strictly on a need-to-know basis, and everyone involved — complainant, respondent and witnesses — should be reminded not to discuss the matter with colleagues while it is live. That said, employers cannot usually promise absolute anonymity or confidentiality: the respondent generally has a right to know, in enough detail to respond fairly, what is alleged against them, even if a witness's identity is protected where possible. Investigation notes and evidence should be stored securely, and it is worth remembering that this material can later be requested under a subject access request, so records should be factual, professional and something you would be comfortable an individual eventually reading.

Avoiding Bias and Not Prejudging the Outcome

The single biggest risk to a fair investigation is an investigator who has already decided the outcome before the evidence is in. Signs of this include asking questions designed to confirm a theory rather than test it, only pursuing evidence that supports one side, or drafting disciplinary paperwork before the investigation has finished. To guard against this:

• Choose an investigator with no personal stake in the outcome and no prior involvement in the events.
• Follow the evidence wherever it leads, including lines of enquiry that might undermine the original allegation.
• Separate the investigation role from the decision-making role wherever possible — the investigator establishes facts and, usually, whether there is a case to answer; a different manager decides on any disciplinary outcome.
• Be alert to unconscious bias, especially where seniority, popularity or past history with either party could colour judgement.

An investigation that is, or even looks, one-sided is one of the fastest ways to convert a manageable HR issue into a costly tribunal claim.

Writing the Investigation Report

The report is the record that everything else — a disciplinary hearing, a grievance outcome, or a tribunal claim years later — will be judged against. A solid investigation report should include:

• The terms of reference and scope agreed at the outset.
• A summary of the allegation(s) and the process followed, including dates of meetings and who attended.
• A clear, factual summary of the evidence gathered from each interview and document, without unnecessary editorialising.
• Where evidence conflicts, an explanation of how that conflict was weighed and why.
• A conclusion on whether there is a case to answer and, where relevant, a recommendation on next steps — not, ordinarily, a finding of guilt, which is a matter for the disciplinary decision-maker.

Keep the tone neutral and evidence-based throughout. A report that reads as an argument for one outcome, rather than a summary of what was found, undermines the fairness of everything that follows.

Linking the Investigation to a Fair Overall Process

An investigation does not stand alone — it is one stage in a wider disciplinary or grievance process governed by the Acas Code of Practice on disciplinary and grievance procedures. The Code applies in England, Scotland and Wales (Northern Ireland has its own equivalent through the Labour Relations Agency), and while it is not itself legally binding, employment tribunals must take it into account and can increase or reduce compensation by up to 25% where an employer or employee has unreasonably failed to follow it. That makes a properly conducted investigation not just good practice, but a direct financial and legal safeguard.

Once the investigation is complete, the process should flow into a fair hearing: the employee should be told the case against them in advance, given a reasonable chance to respond, allowed to be accompanied where the statutory right applies, and given a right of appeal against any outcome. Investigations connected to a protected disclosure raise their own particular sensitivities — see Learnsignal's guide to handling whistleblowing disclosures for how those cases differ from a standard grievance or disciplinary investigation.

Frequently Asked Questions

How long should a workplace investigation take?
There is no fixed legal deadline, but the Acas Code expects investigations to be carried out without unreasonable delay. As a guide, aim to complete straightforward cases within a few weeks, and keep those involved updated if a more complex investigation needs longer.

Can the same person investigate and then chair the disciplinary hearing?
It is best practice to use different people for each role, and Acas recommends this wherever the organisation's size allows it. In very small businesses where this is not practical, be transparent about the limitation and manage it as fairly as possible.

Does the person under investigation have a right to be accompanied at an investigation meeting?
The statutory right to be accompanied applies to formal disciplinary and grievance hearings, not investigation meetings themselves, though many employers allow it anyway as good practice. Check and apply your own policy consistently.

What happens if we don't follow a fair investigation process?
Failing to investigate properly, or investigating in a biased way, can make any subsequent dismissal or disciplinary sanction unfair. Employment tribunals can also adjust compensation by up to 25% where the Acas Code has been unreasonably ignored.

A fair, well-documented investigation protects everyone involved — the complainant, the respondent, and the organisation itself. Building this discipline into your HR processes now is far cheaper than untangling a flawed one later; Learnsignal's CPD courses can help HR teams and managers build these investigation and employment law skills further.