How to Write a Legionella Risk Assessment Report: Structure and Requirements

The risk assessment report is the deliverable that your client receives, that the HSE may inspect, and that a court may scrutinise if something goes wrong. It needs to be clear, comprehensive, and defensible. Too many reports fail on one or more of these counts.

This guide covers what a compliant legionella risk assessment report should contain, how it should be structured, and the standards it should meet.

What the report must achieve

A legionella risk assessment report serves three purposes:

1. Inform the duty holder of the risks present in their water system and what actions they need to take
2. Demonstrate compliance with ACOP L8 and HSG274 by evidencing that a suitable and sufficient assessment has been carried out
3. Provide a defensible record that can withstand scrutiny by the HSE, insurers, or a court

Every section of your report should serve at least one of these purposes. If a section does not inform, demonstrate, or defend, it probably does not need to be there.

Report structure

A well-structured report follows a logical flow from context to findings to recommendations. The following structure is widely accepted as best practice:

1. Executive summary

A concise overview of the assessment, written for a non-technical reader (the duty holder, a building manager, or a board member). It should include:

• The date and scope of the assessment
• The overall risk level
• The number and severity of findings
• The most critical actions required

The executive summary is often the only section that senior decision-makers read. Make it clear, direct, and actionable.

2. Site and system information

Factual information about the premises and the water systems assessed:

• Site address and description
• Building type and use
• Number of occupants and any vulnerable groups
• Water system type (stored hot water, combi, mains-fed, etc.)
• Water supplier and mains supply details
• Description of all water systems including tanks, calorifiers, distribution, and outlets

3. System schematic

A diagram or schematic showing the layout of the water system, including:

• Cold water storage tanks and their location
• Calorifiers and water heaters
• Distribution pipework (hot, cold, and return)
• Outlet locations
• TMV positions
• Any treatment equipment

The schematic does not need to be an engineering drawing, but it must be accurate enough to understand the system layout. If an accurate schematic cannot be produced during the assessment, this should be flagged as a finding.

4. Assessment methodology

A brief statement of the methodology used, including:

• Reference to ACOP L8 and HSG274 Part 2
• The date and duration of the on-site assessment
• The assessor's name, qualifications, and organisation
• The conditions assessed and the risk rating scale used
• Any areas that were inaccessible or could not be assessed

5. Findings and risk ratings

This is the core of the report. Each risk condition should be listed with:

• The condition being assessed (e.g., "Is hot water stored at 60 degrees C or above?")
• The finding (what was observed or measured during the assessment)
• The risk rating (A, B, or C)
• Supporting evidence (temperature readings, photographs, observations)

Findings should be specific and evidence-based. Avoid vague statements like "the system is in reasonable condition." Instead, state what you observed: "The calorifier thermostat was set to 58 degrees C and the measured storage temperature was 56.2 degrees C at the bottom drain."

6. Recommendations

Each finding with a B or C rating should have an associated recommendation. Recommendations should be:

• Specific: State exactly what action is needed
• Prioritised: Indicate the urgency (immediate, within 1 month, within 3 months, etc.)
• Referenced: Link back to the relevant guidance (ACOP L8 paragraph number, HSG274 section)
• Practical: The recommendation should be achievable and proportionate to the risk

7. Monitoring programme

The report should include or recommend a monitoring programme that specifies:

• What tasks need to be carried out (temperature checks, tank inspections, TMV servicing)
• How often each task should be done (referencing Table 2.1 of HSG274)
• Who should carry out each task
• How results should be recorded

8. Appendices

Supporting information that provides evidence for the findings:

• Temperature readings recorded during the assessment
• Photographs of key findings
• Asset register of water system components
• Copies of any existing monitoring records reviewed

Writing standards

Beyond the structure, the quality of the writing matters:

• Use plain language -- the duty holder may not be technically qualified
• Be precise -- "the tank was dirty" is not as useful as "the cold water storage tank had approximately 5mm of sediment on the base and green biological growth on the internal walls"
• Avoid jargon without explanation -- if you use a technical term, explain it on first use
• Be consistent -- use the same terminology throughout the report
• Date everything -- every finding, measurement, and photograph should be dated

Common report failures

The most common reasons reports fail to meet the required standard:

1. Generic content -- boilerplate text copied from one report to the next without site-specific detail
2. Missing measurements -- temperature readings not taken or not recorded
3. Vague recommendations -- "improve the system" without specifying what, how, and when
4. No photographs -- findings that could be supported by photographic evidence but are not
5. No risk ratings -- findings listed without a clear assessment of whether they are adequately controlled
6. Outdated references -- citing the superseded edition of HSG274 instead of the current (2024) edition
7. No executive summary -- forcing the duty holder to read the entire report to understand the key issues

L8Pro generates a fully structured, HSG274-compliant PDF report directly from your on-site assessment data. Every finding is linked to a specific risk condition, every recommendation references the relevant guidance, and the report includes an executive summary, risk ratings, and action priorities.

Key takeaways

• Structure your report logically: context, findings, recommendations, monitoring
• Every finding should be specific, evidence-based, and rated
• Recommendations must be actionable, prioritised, and referenced to guidance
• The executive summary is critical -- it may be the only section decision-makers read
• Avoid generic boilerplate -- every report should be site-specific
• Include photographs, temperature readings, and a system schematic