Legionella Risk Assessment: A Complete Guide for London Landlords

What Is Legionella and Why Does It Matter for London Landlords?

Legionella pneumophila is a bacteria that occurs naturally in water. In the right conditions — warm water between 20 and 45 degrees Celsius, stagnant pipework, and nutrients such as scale and biofilm — it multiplies to levels capable of causing Legionnaires disease, a potentially fatal form of pneumonia. The disease spreads not through drinking water but through inhaling fine water droplets, making showers and spray taps the highest-risk outlets in a domestic property.

For most London landlords, the practical risk of Legionella in a well-maintained property is low. A combi boiler system with no stored water, regularly used outlets, and no dead legs presents a minimal risk. However, "low risk" is not the same as "no obligation" — and every London landlord is legally required to assess and document the risk regardless of the outcome of that assessment.

The Legal Basis: HSE L8 ACOP

The duty on landlords arises from the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health (COSHH) Regulations 2002. Landlords are defined as "duty holders" under these provisions. The HSE Approved Code of Practice L8 — "Legionnaires disease: the control of Legionella bacteria in water systems" — provides detailed guidance on how duty holders must fulfil this obligation.

Importantly, L8 ACOP applies to domestic rental properties let to tenants. The HSE has published supplementary guidance specifically for landlords of domestic premises, confirming that the duty extends to private residential lettings. The guidance makes clear that a written record of the risk assessment must be produced and retained, and that verbal assessments are not acceptable.

What a Legionella Risk Assessment Covers

A Legionella risk assessment for a domestic rental property examines every part of the water system that could create conditions for bacterial growth. The assessment begins with the cold water supply — specifically the temperature at the incoming mains, which should remain below 20 degrees Celsius. Cold water stored in a loft tank is a higher-risk scenario than a mains-fed system, because stored water can warm to the Legionella growth range during summer months.

The hot water system is assessed for storage temperature and distribution temperature. Hot water should be stored at 60 degrees Celsius and reach all outlets at above 50 degrees Celsius within one minute of running. A hot water cylinder thermostat set below 55 degrees Celsius creates the ideal growth condition for Legionella. The assessment checks the cylinder thermostat setting, the circulation temperature, and whether there is adequate insulation on the cylinder and distribution pipework.

Showerheads and spray taps are inspected for scale and biofilm accumulation. A heavily scaled showerhead contains exactly the warm, nutrient-rich conditions in which Legionella proliferates. Showerhead descaling or replacement is one of the most frequently recommended actions in a domestic Legionella assessment. Rubber shower hoses deteriorate with age and provide a surface for biofilm to establish — they should be replaced periodically.

Dead legs — sections of pipework that no longer serve an active outlet — are identified. Water in a dead leg does not circulate and quickly reaches the Legionella growth temperature range. Redundant pipework is commonly found in London properties where kitchens and bathrooms have been relocated, leaving capped-off pipe runs within the walls. These should be removed or capped as close to the active main as possible.

Low Risk vs High Risk Properties

Most domestic rental properties in London are classified as low risk after assessment. The key risk factors that elevate a property to higher risk are: a stored water system (hot water cylinder and/or cold water tank in the loft); complex pipework with dead legs or infrequently used outlets; occupants who are elderly, immunocompromised, or have chronic respiratory conditions; and properties that are regularly left vacant for extended periods.

A combi boiler property with no stored water, regularly occupied, with straightforward pipework and no dead legs will typically receive a low-risk assessment with minimal recommended actions beyond maintaining outlet use and temperature. A Victorian conversion with an indirect hot water cylinder, a cold water tank in the loft, and multiple bathrooms — some of which may be infrequently used — requires more detailed assessment and a more active control regime.

Thermostatic Mixing Valves

Thermostatic mixing valves (TMVs) blend hot and cold water to deliver a pre-set safe temperature at the outlet. In care home and NHS settings, TMVs are mandatory to protect vulnerable occupants from scalding. In domestic rental properties, TMVs may be recommended where the tenant includes elderly, very young, or disabled occupants. The outlet temperature from a TMV is typically set at 44 degrees Celsius — safe for vulnerable users while the underlying system maintains Legionella control temperatures of 60 degrees Celsius at the cylinder.

Flushing Procedures and Void Periods

Infrequently used outlets — a spare bathroom, a guest WC, an en-suite not in regular use — should be flushed weekly by running all taps and the shower for at least two minutes. This purges any stagnant water that has accumulated in the branch pipe and outlet. During a void period, either flush all outlets weekly or drain the system if the property will be empty for more than four weeks.

On re-letting after a void period, the Legionella risk assessment should be reviewed, all outlets flushed, and the hot water cylinder temperature checked before the new tenant moves in. Where a significant change to the water system has occurred during the void — new pipework, a new cylinder, extension works — a full re-assessment is required.

What Landlords Receive

A compliant Legionella risk assessment produces a written report documenting the water system, the risk factors present or absent, the control measures in place, and any recommended actions with suggested timescales. This report must be retained in the landlord compliance file and must be made available to the local authority on request — particularly relevant for HMO licence applications and renewals. The report is also evidence of due diligence if a tenant or visitor were to contract Legionnaires disease and make a claim against the landlord.

HMOs and Shared Water Systems

Houses in Multiple Occupation present an elevated Legionella risk because of shared water systems, higher water consumption, and more complex pipework. An HMO with multiple bathrooms, a communal kitchen, and potentially both a hot water cylinder and a cold water tank requires a more detailed assessment than a single-let flat. Annual re-assessment is recommended for HMOs, and the Legionella risk assessment should be a standard part of the HMO licence renewal documentation. Some London boroughs require evidence of a current Legionella risk assessment as a condition of HMO licensing.