What Is a Legionella Risk Assessment and Do You Need One?

What Is Legionella?

Legionella pneumophila is a bacterium that occurs naturally in freshwater environments — rivers, lakes, and groundwater — at low concentrations. It becomes a health hazard when it proliferates to high concentrations in artificial water systems — hot water cylinders, cold water storage tanks, cooling towers, and the pipework and fittings connected to them — and is then inhaled as a fine water mist or droplets.

Inhalation of water droplets containing Legionella at sufficient concentrations can cause Legionnaires' disease, a serious and potentially fatal form of pneumonia. The disease predominantly affects people over 45, smokers, and people with weakened immune systems. In the UK, there are approximately 500–600 confirmed cases of Legionnaires' disease per year, with a case fatality rate of approximately 10–12%. It cannot be transmitted from person to person — every case involves direct exposure to a contaminated water source.

A related, milder condition is Pontiac fever — a flu-like illness caused by inhalation of Legionella that does not develop into pneumonia. It typically resolves without treatment.

Conditions in Which Legionella Proliferates

Legionella bacteria multiply most rapidly in water at temperatures between 20°C and 45°C, with peak growth around 37°C (body temperature). Below 20°C, bacteria are dormant. Above 60°C, bacteria are killed within minutes. The risk is therefore concentrated in:

• Hot water systems where the stored water or pipework is not maintained at sufficient temperature (below 60°C in the cylinder)
• Cold water systems where the water temperature in the storage tank or pipework rises above 20°C — particularly relevant in warm loft spaces in summer
• Sections of pipework that are rarely or never used (known as deadlegs) where water sits stagnant at ambient temperature
• Shower heads, hose connections, and spray fittings — where water can accumulate in the internal passages and warm to ambient temperature between uses
• Water systems with scale, sediment, rust, or biofilm — these provide nutrients for bacterial growth

Legal Position for Landlords

The legal framework for Legionella risk management in landlords' properties is set by the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health (COSHH) Regulations 2002. These create a general duty for employers and self-employed persons to manage risks from biological agents — and Legionella qualifies as a biological hazard under COSHH.

The HSE's approved code of practice and guidance, known as L8 (Legionnaires' disease: The control of Legionella bacteria in water systems), sets out the framework for managing risk in water systems. For large commercial premises, L8 requires a formal, documented Legionella risk assessment and a written control scheme. For private landlords with simple domestic hot and cold water systems, HSG274 Part 2 (the supplementary guidance for domestic premises) applies a proportionate standard.

The HSE's position, confirmed in guidance for the private rented sector, is that landlords of residential properties are required to carry out a Legionella risk assessment, but for typical simple domestic systems — a combi boiler or hot water cylinder with a standard cold mains supply and no storage tanks, plus direct-fed hot and cold outlets — the assessment is straightforward and does not require a professional engineer. The landlord, or a competent person on their behalf, can carry out the assessment.

What this means in practice: a landlord with a simple flat or house where the water system is a conventional combi boiler (no cold storage tank, no hot water cylinder) serving direct-fed hot and cold taps needs to document that they have assessed the system, identified no significant risk factors, and established simple ongoing controls. This is not the same as the complex formal L8 assessment required for a hotel with cooling towers or a large purpose-built office building.

Higher Risk: Systems with Storage

The risk level increases significantly for water systems with storage components:

• Cold water storage tanks (header tanks in the loft): Common in older London properties. The stored water can warm above 20°C in summer, particularly if the tank is in an uninsulated loft. Tanks must be insulated, covered, and the water should not remain stored for extended periods.
• Hot water cylinders (unvented or vented): The cylinder must store hot water at a minimum of 60°C. The thermostat should be set to 60°C or above. Water at this temperature kills Legionella within two minutes. Cylinders where the thermostat has been turned down to save energy (a common cause of inadvertent risk) represent a significant Legionella concern.
• Combination tanks and thermal stores: Some older properties have combined hot and cold storage arrangements that are particularly prone to temperature overlap between the hot and cold zones.

What a Legionella Risk Assessment Covers

For a residential rental property, a risk assessment should systematically examine:

• The type of water system — combi boiler (lower risk), unvented cylinder, vented cylinder with cold storage tank
• Hot water storage temperature — is the cylinder thermostat set to maintain 60°C?
• Cold water storage temperature — if there is a header tank, is it insulated and does the water in it stay below 20°C?
• Any deadlegs in the pipework — sections that were capped off but not removed, pipework to rooms that are no longer used, or redundant connections from previous layouts
• Shower heads and flexible hoses — these should be removed and descaled, or replaced, at least annually; flexible hoses are a recognised source of biofilm that harbours Legionella
• Frequency of use — if a property is left vacant for extended periods or if some bathrooms are rarely used, infrequently flushed outlets present a risk that increases with time
• Tenant profile — elderly, immunocompromised, or very young tenants are at higher risk and may warrant more conservative control measures

Control Measures

For most standard London rental properties, the control measures following a risk assessment are straightforward:

• Maintain hot water storage at 60°C — check the cylinder thermostat setting
• Ensure cold water from the storage tank (if any) is kept below 20°C — insulate the tank and pipework in the loft
• Flush infrequently used outlets weekly — run taps and showers in spare rooms or unused bathrooms for two minutes weekly to prevent stagnation
• Descale and clean shower heads and replace flexible hoses annually
• Remove deadlegs — capped-off pipework sections should be cut out and removed rather than left in place
• Advise tenants to run taps after any period of property vacancy, and to report any concerns about water temperature or quality

HMO Requirements

Houses in Multiple Occupation present a higher Legionella risk than single-let properties due to the complexity of plumbing arrangements, higher occupancy, shared bathrooms with multiple users, and the potential for more complex pipework configurations. For HMOs, particularly those licensed by the London borough council, the risk assessment should be more thorough:

• A schematic of the water system should be documented
• Temperature checks should be carried out and recorded
• The assessment should be reviewed annually and whenever the system changes significantly
• Some London borough HMO licensing conditions explicitly require evidence of a Legionella risk assessment as a licence condition